WO2014060740A1

WO2014060740A1 - Bubble generation to strip components of a liquid

Info

Publication number: WO2014060740A1
Application number: PCT/GB2013/052687
Authority: WO
Inventors: William Bauer Jay ZIMMERMAN
Original assignee: Perlemax Limited
Priority date: 2012-10-15
Filing date: 2013-10-15
Publication date: 2014-04-24
Also published as: EP2906679A1

Abstract

Uses of a method of producing small bubbles of gas in a liquid include gas transfer in airlift bioreactors and anaerobic digesters, and particle separation. The method uses a source of the gas under pressure, a conduit opening into a liquid and oscillating the gas passing along the conduit. The oscillation is effected by a fluidic oscillator, comprising a diverter that divides the supply into respect outputs, each output being controlled by a control port, wherein the control ports are interconnected by a closed loop. Separation of algae from water involves delivering a laminar flow of microbubbles in the range 10 to 100 µm diameter. Such bubbles also deliver a laminar flow in bioreactors that delivers enhanced liquid flow despite the small bubbles, which improves mixing and also enhances efficiency of gas exchange and retention of the bubbles in the reactor. In an anaerobic digester, the microbubbles strip waste gas and deliver nutrients. In particle separation, a pH-adjusting gas adjusts the pH in the boundary layer surrounding each bubble to inhibit repulsion between bubbles and particles.

Description

BUBBLE GENERATION TO STRIP COMPONENTS OF A LIQUID

[01] This invention relates to the generation of fine bubbles and their application in processes, one being an anaerobic digester and another being a particle, in particular, an algal floe, separator, especially in the context of airlift loop bioreactors.

BACKGROUND

[02] Bubbles of gas in liquid are frequently required in many different applications and usually, but not exclusively, for the purpose of dissolving the gas in the liquid.

[03] WO2008/053174 describes method and apparatus for generating microbubbles in liquid, and suitable applications in which such microbubbles may be employed. These range from aeration and cleaning of water, oil lifting in wells, and bioreactors and fermenters.

[04] The application cites a number of prior art solutions for generating bubbles, including WO99/31019 and WO99/30812, EP1092541 , SU1616561 , GB1281630, US4793714, US5674433, GB2273700, DE4405961 , DE19530625 and JP2003-265939.

[05] Small bubble generation has application in the sewage treatment industry, in which it is desired to dissolve oxygen in the water being treated. This is to supply respiring bacteria that are digesting the sewage. The more oxygen they have, the more efficient the digestion process. However, a similar requirement exists in bioreactors and fermenters generally where they are sparged for aeration or other purposes. Specifically, the yeast manufacturing industry has this requirement, where growing and reproducing yeast bacteria need constant oxygen replenishment for respiration purposes. Another application is in the carbonisation of beverages, where it is desired to dissolve carbon dioxide into the beverage. A process not looking to dissolve the gas but nevertheless benefiting from small bubbles is in the extraction of hard-to-lift oil reserves in some fields which either have little oil left, or have the oil locked in sand. Indeed, much of the oil in Canada's oil reserves is in the form of oil sand. Bubbling gas up through such oil-bearing reserves has the effect of lifting the oil as the bubbles rise under gravity and bring the oil with them. The bubbles are formed in water and pumped into the well or reserve and the oil is carried at the interface between the gas and water of each bubble as it passes through the reserves. The smaller the bubble, the greater is the relative surface area for transport of the oil.

[06] Another application of bubbles is in particle separation from a liquid suspension of the particles, of which an example application is algal separation. This might be desired for one or both of two reasons. A first reason is to clarify water contaminated with algae. A second reason is to harvest algae grown in water. The process is not limited to algal separation; any mixer comprising solid particles can comprise the use of bubbles which attach to suspended particles and carry them to the surface from which they may be scraped, either to recover the particles or clarify the water. However, attachment of the bubbles to the particles is problematic, particularly if the particles are charged when they may simply bounce off bubbles and not attach to them. Dissolved air flotation (DAF) is a known technique where air is dissolved under substantial pressure into water which, when released into the separation tank immediately releases large quantities of small bubbles. However, there are two issues. The first is the substantial energy requirements to compress and dissolve air in water. The second is the turbulence of the bubbles released into the tank.

[07] It is an object of the present invention to develop the applications taught in WO2008/053174. It is also an object to provide process applications using small bubbles.

[08] An anaerobic digester is a processing unit in a wastewater treatment plant where organic matter is broken down via anaerobic bacteria in the absence of oxygen. The biodegradation of organic matter in an anaerobic digester takes place through four steps. The first step is an hydrolysis stage which converts the complex organic matter in to a simple state. The second step is the acidogenesis stage. In this stage, the product of the first stage converts into volatile fatty acids. Volatile fatty acids are converted into acetate in the third step by acetatogenic stage. Finally the acetate and carbon dioxide with hydrogen produced in second step convents into methane and carbon dioxide in the methanogenesis stage. Each stage is mediated by specific type of bacteria. Each bacteria requires a specific environment. Methanogenesic bacteria are more sensitive to change of operating conditions. However, there are general operating conditions, such as temperature, pH, Carbon-nitrogen ratio, and ammonia etc, appropriate for all the bacterial consortia.

[09] The hydraulic retention time of a mesophilic anaerobic digester is approximately 20 days. Then the sludge discharges as effluent. The digested sludge (effluent) contains organic matters (biodegradable), anaerobic bacteria and some dissolved gases, for instance carbon dioxide and hydrogen sulphide. The presence of these dissolved gases has negative impact on piping and the downstream processing units. Corrosion is one potential problem in piping metals. In addition, the generation of biogas continuously in digested sludge during transfer creates a gas-liquid mixture. Even if a small phase fraction of gas, it degrades the performance of pumps due to cavitation phenomena.

[010] Anaerobic digestion is an important source of methane in the search for green energy. It is UK Government policy to recycle 50% and recover energy from 25% of human waste by 2020. Anaerobic digestion breaks down food and plant waste to produce biogas, a mixture of methane and carbon dioxide which is burnt to produce electricity and a residual material which can be used as a soil improver. BRIEF SUMMARY OF THE DISCLOSURE

[011] In accordance with a first aspect of the present invention there is provided an anaerobic digester comprising a liquid fermenter tank for anaerobic microorganisms and a diffuser of a microbubble generator to introduce bubbles of non-oxygen containing gas into the digester whereby methane and acid gases produced by the digestion are exchanged across the bubble surface to strip such gases from the liquid when said bubbles connect with a header space of the tank.

[012] In one embodiment, at a commencement phase of said digestion, said gas is nitrogen or another inert gas that merely strips the fermenter of oxygen to promote the onset of anaerobic digestion.

[013] In another embodiment, during a growth phase of the microorganisms in the digester, said gas is or comprises the biogas generated and released into a head space of the fermenter tank. This typically comprises approximately 60% methane and 40% carbon dioxide. While reintroduction of bubbles of such gas into the fermenter does little to change the equilibrium of such gases in the liquid, it provides an escape route for the methane which in physical terms should be gaseous at room temperatures and the temperature of anaerobic digestion (typically 35°C) it typically clings to particles such as the microorganisms themselves. Locally positioned bubbles permit methane to escape the liquid phase. Inside the bubbles, which thus grow in this environment, the balance of carbon dioxide remains so that it is also stripped from the liquid, along with acid gases.

[014] Conventional anaerobic digestion does not need a nutrient gas. However, in yet another embodiment, during a growth phase of digester, said gas is or comprises carbon dioxide. Not only does this preferentially extract methane and hydrogen sulphide, but also provides "food" for the methanogenesic bacteria.

[015] In another embodiment, at an endphase of said digestion, said gas is also is nitrogen or another inert gas that serves in this event to strip methane and hydrogen sulphide. Not only does this reduce the acidity of the remaining liquid but also it promotes further growth of the methanogenesic bacteria enhancing the methane output.

[016] Thus, microbubbles introduced into the reactor increases methane yields, while at the same time 'sinking' waste C0₂.

[017] Such a process has the possibility to increase or even double the amount of methane extracted through anaerobic digestion. Methane tends to adhere to the microorganisms and biomass in the reactor, rather than be released to the gas headspace. By using carbon dioxide microbubbles, it is found that, in addition to removing the methane already produced more effectively, the production rate of methane increases. The dissolved carbon dioxide is taken up as food by methanogenesic bacteria, thus increasing their production of methane. Microbubbles speed up gas exchange by providing more food to the bioculture, but also by removing the methane, which has an inhibitory effect on metabolism.

[018] Also, fertilizer recovered from anaerobic digestion has hitherto been dried and trucked to farms, despite the fact that the digestate is about 90% water and would benefit from other transport mechanisms such as piping. This is necessary because the digestate is highly acidic and corrosive. By stripping out the acid gases, leaving the digestate substantially neutral, this barrier to transport via pumps and pipes is removed, improving the attraction of the anaerobic digestion process and the production of more environmentally sustainable electricity. It also supports sustainable agriculture by recovering the nutrients, particularly phosphates and potassium, which are not renewable without recycling.

[019] This is better explained by reference to the reactions taking place. There are two competing reactions, the desirable methanogenesis reactions and the competing sulphide reduction reactions (see equations Eq. 12 - Eq. 15 below). From these equations, it can be seen that the sulphide-reduction reactions have greater thermodynamic driving force than methanogenesis, so that methane production is adversely impacted by sulphide concentration. Moreover H₂S has a negative impact on the methane production bacteria. The concentration of H₂S can be taken as an indicator of inhibition of methanogenesic bacteria.

[020] Thus removal of dissolved H₂S from sludge prevents inhibition of methanogenesic bacteria and reduces odour from digested sludge. Normally, the removal of C0₂ and H₂S take place through displacement of biogas generated (CH₄ and C0₂) or by contact with head space in the top of sludge. But this is insufficient to remove the dissolved gases. Mixing of the digested sludge provides intimate contact between sludge and bubbles of biogas or headspace.

[021] Thus a method of operating an anaerobic digester as defined above comprises the steps of:

a) filling the tank with liquid comprising water and sludge, which sludge includes micro-organisms, organic matters, elements and suspended solids;

b) providing a head space above the liquid for the extraction of biogases evolving from the liquid;

c) rendering the tank anaerobic;

d) periodically, and on each occasion over a limited period of time, introducing bubbles of non-oxygen containing gas from the diffuser, at least a proportion of which bubbles have a size in the range 10 to 100 μηι. [022] The flow generated by the bubbles may be non-turbulent laminar, having a Reynolds number less than 2000, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser. Sufficient bubbles must be introduced, of course, and sufficient proportion of them must be small enough (in the range 10 to 30 μηι) to be retained in suspension to provide a site for microorganisms to attach and enable the transfer of methane from their surface into the bubble. There will be sufficient number of bubbles introduced, and sufficiently small, if the gas holdup of the bubbles in the tank one hour after at the end of the period provided in step d) is at least of 0.05%, preferably at least 0.5%.

[023] During the digestion, step d) may be repeated daily and the period of time in step d) may be between 30 and 120 mins. the bubbles introduced in step d) may comprise carbon dioxide, indeed, more than 90% carbon dioxide, or essentially pure carbon dioxide. When the yield of biogas from the digester falls below a predetermined amount, step d) may be repeated a final occasion with said bubbles comprising substantially only nitrogen gas, whereby remaining methane, carbon dioxide and hydrogen sulphide is stripped from the digestate to neutralise the pH of the digestate. After this step, the digestate can be pumped from the tank and transported by pipeline to an irrigation array for land fertilisation. This is much less energy consuming than drying the digestate and transporting it by road.

[024] While the anaerobic reactor has application in the digestion of waste, it also has application in yeast fermentation to (bio) ethanol and in anaerobic ammonium oxidation (anammox). In the latter, the carrier gas introduced into the digestion is nitrogen diluted by C0_2, and the bacteria uses the dissolved C0₂ as a carbon source, resulting in greater anammox efficiency, i.e. production of N₂ from nitrates and nitrates in an aqueous medium.

[025] The carbon dioxide may be sourced from one of: power production from combustion of the methane produced by said digester; and from sequestered carbon dioxide from another source, for example power station waste gas.

[026] The bubbles preferably have a size in the range 10 to 100 μηι. At least a proportion of the bubbles may have a size in the range 10 to 30 μηι accounting for at least a gas holdup of 0.05%. Preferably, at least a proportion of the bubbles have a size in the range 10 to 30 μηι accounting for at least a gas holdup of 0.5%.

[027] In the digester, the tank may have sides and a base and the liquid in the tank may have a top surface above which is a header space. The diffuser may be disposed in the liquid at the base of the tank and be arranged to inject bubbles of gas into the liquid in the tank whereby the apparent density of the liquid above the diffuser is reduced by the bubbles thereby creating a flow of the liquid, which flow is: up the tank in a riser section thereof,

turned sideways at the surface of the liquid, where at least a proportion of the bubbles either break at the surface, terminate passage through the liquid at the surface, or are reduced in diameter through dissolution of the gas in the bubbles into the liquid during their passage up the riser section;

turning down at the sides of the tank into a downcomer section of the tank; and turning sideways back into the riser section,

which flow, at least in the riser section, is non-turbulent laminar flow having a Reynolds number less than 2000, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.

[028] Indeed, while a physical divider is possible, comprising, for example, a draft tube in a cylindrical tank, a physical divider is not, in fact, absolutely necessary because the requisite circulation occurs naturally, when the flow is entirely laminar. The divider is simply the boundary separating the rising flow from the returning downcomer flow, which separate effectively from one another without turbulence or significant mixing in these conditions.

[029] By arranging for laminar flow, which is achieved by taking into account a number of different factors including the density and viscosity of the liquid as well as the rate of injection and size of the bubbles introduced, the energy required for the gasification and circulation of the fermenter can be reduced to very low levels. A most important aspect is the size of the bubbles that must have such a slow terminal velocity in the liquid that, to all intents and purposes, they are stationary in the liquid and primarily generate movement of the liquid through the apparent density change of the liquid and which results in mass transport of the liquid including the bubbles. Absolutely stationary bubbles (with respect to the liquid) are of course impossible because of the density difference between the gas injected and the liquid which will always cause some movement, however small the bubble. However, because the frictional resistance of a rising bubble is proportional to its surface area, whereas the gravitational force causing it to rise is proportional to bubble volume, the smaller the bubble the slower its rise in any given liquid. In an aqueous environment, a bubble of less than 100 μηι diameter rises sufficiently slowly not to create significant turbulence. Nevertheless, with sufficient number of such small bubbles per unit volume, the apparent density of the bubble- containing-liquid can be reduced to such an extent that the density difference can drive an entire column of liquid to rise, possibly at a much greater rate than the rise of the bubble with respect to the liquid. Indeed, the rate of injection of one bubble followed by a succeeding bubble can be close to the rate of rise of the liquid so that the injection itself generates little or no turbulence. [030] Depending on the particular disgestion/fermentation taking place, preferably a proportion of the bubbles are carried over the divider into the downcomer section whereby gas transport across the bubble surface also takes place in the downcomer section. This only occurs once the rate of fall of the liquid in the downcomer exceeds the rate of rise of the bubble in the liquid. Again this reduces wastage (of bubbles reaching the surface), if the object is the dissolve all the gas of the bubble in the liquid, which is frequently the case in fermenters. In aqueous fermenters, bubble size needs to be less than about 100 μηι in diameter.

[031] Conveniently, more than 90% of the bubbles having a size in the range 10 to 100 μηι diameter (preferably 20 to 40) and a bubble density of at least 50 million per cubic metre, optionally at least 100 million.

[032] Preferably, the gas holdup is less than 2%, optionally less than 1 %. Holdup is the overall volume of gas in bubbles per unit volume of liquid.

[033] A suspended particle separation tank may comprise: a floor having a floor area; liquid in the tank having suspended particles; and a diffuser of a microbubble generator, which diffuser is disposed at the base of the tank and is arranged to inject bubbles of gas into the tank to reduce the apparent density of the liquid above the diffuser, thereby creating a flow of the liquid and gas bubbles that is a non-turbulent laminar flow having a Reynolds number less than 2000, wherein more than 90% of the bubbles have a size in the range 10 to 100 μηι diameter and a bubble density of at least 50million per cubic metre, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.

[034] The Reynolds number may be less than 200. Optionally, more than 90% of the bubbles have a size in the range 20 to 50 μηι diameter. The bubble density may be at least lOOmillion per cubic metre.

[035] Preferably, the gas holdup is less than 2%, optionally less than 1 %. The diffuser may be disposed across substantially the entire floor area of the tank. A specific example of a suspended particle separation tank is an algal separation tank.

[036] A method of particle separation comprising the steps of providing a separation tank as defined above comprising an aqueous mixture of suspended particles to be separated from the water, adding a flocculant to cause the particles to coagulate in floes and adjusting the acidity of the mixture to a predetermined pH appropriate for the selected flocculant, and injecting bubbles into the tank, optionally across substantially the entire area of the floor of the tank, wherein more than 90% of the bubbles have a size in the range 10 to 100 μηι diameter (preferably 20 to 40) and a bubble density of at least 50million per cubic metre and wherein the flow created by the bubbles injected is a non-turbulent laminar flow having a Reynolds number less than 2000. Again, the Reynolds number is based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.

[037] The foregoing separation technique has application also in the minerals sector and for oil water emulsions. It is also feasible to separate yeast or, indeed, any microorganism.

[038] Preferably, the bubbles introduced create non-turbulent laminar flow having a Reynolds number less than 200. Preferably, the Reynolds number is less than 20. The benefit of a non-turbulent introduction of the bubbles is two-fold. Firstly, little energy is employed, so that the cost of the process is minimised, and secondly the flux rate of the bubbles is such that they gently approach floes of coagulated material without disturbing them. Indeed, the problem of attachment is avoided because the particles (floes) are much larger and so the bubbles can gently lift from underneath the floe rather than having to chemically attach and connect. This is not to suggest that chemical attachment does not occur, but rather that time is permitted for the attachment to occur through the gentle pressure caused by the constant, albeit small, force of gravity. The tendency of bubbles to bounce off or slip by particles is reduced.

[039] The method is continued until a desired level of clarity of the water remains and at least until the sludge layer on the surface of the water comprising the particles separated from the water has thickened and thinned.

[040] By "thickened and thinned" is meant that the solids density of the sludge layer has increased through compression from below by accumulated gas bubbles lifting the sludge against the surface and consequential squeezing out of water from the sludge layer, and whereby the thickness of the sludge layer reduces.

[041] In some embodiments, certainly in the case of algal separation, the flocculant may be a metallic inorganic coagulant, such as iron or aluminium salts, added in solution. Coagulation is achieved with the coagulants dissociating into Fe³⁺ and Al³⁺ respectively as well as other soluble complexes having varying high positive charges. Essentially, the rate and extent to which these trivalent ions and other complexing species adsorb onto colloidal surfaces is pH dependent. At room temperature, under acidic pH, trivalent species-Fe³⁺ and Al³⁺.are the dominant species in the continuous phase. These predominant trivalent species are the most effective in colloidal charge neutralization and attach to the negatively charged algal cell. The excess H⁺ present under low pH react with hydroxides of these metals to further release the trivalent metal species. As a consequence, more Al³⁺ and Fe³⁺ species become available again for charge neutralization, but the amount of hydroxide species is reduced. As pH shifts away from acidity however, H⁺ concentration becomes less than OH^" and the amount of trivalent ions present in solution reduces. These prevalent OH^" react freely with the available trivalent metallic species to form the corresponding metallic hydroxide species. As such, hydroxide species become predominant under alkaline conditions attaching to algal cells and precipitating as large gelatinous floes. Increased concentrations of hydroxide species for aluminium and ferric salts respectively has been reported as pH moves beyond pH 7 at room temperature. This explains the large floes generated under alkaline condition. It is for these reasons the recovery efficiency is observed to increase again under alkaline pH. Consequently, either acidic or alkaline conditions may be selected to enhance the flocculant effect. (Note that the terms "flocculation" and "coagulation" are used interchangeably for the same effect). Nevertheless, despite the above, acidic conditions are preferred. Despite large floes being good vehicles for sweep flocculation, the overall efficiency under alkaline conditions for aluminium and ferric sulphate coagulants is lower than under acidic conditions.

[042] This observation can be explained by the difference in charge density of species. The higher the size and charge of the species, the more effective the coagulation process will be. Because these charges increase with increasing acidity, recovery efficiency is highest under acidic pH. In addition, relatively larger floes are developed under alkaline state and given that as particle size increases the residence time of the rising microbubble-floc agglomerate also increases leading to a prolonged flotation time. Moreover, the lifting force of microbubbles diminishes with increased particle size.

[043] By contrast, the condition is quite different for FeCI₃ however. Whilst a similar tendency occurs under acidic conditions, FeCI₃ exhibits a rather different behaviour under basic pH. It is note-worthy to reiterate that ferric chloride produced the overall best recovery result so far discovered. This is because ferric salts are relatively less soluble than aluminium salts. In addition, hydroxides of aluminium are amphoteric - containing both basic and acidic functional groups. Furthermore, the addition of ferric salts decreases the solution pH and the closer the pH tends towards acidity, the concentration of trivalent species in the solution increases. Thus the optimum pH for algal separation ranges from 5-7 for ferric chloride but for aluminium and ferric sulphate, two ranges are effective- 5-6 and 8-9. Overall, the process governing these reactions is very complex and by no means easy to fully detail especially also as the growth medium itself may contain vital and very reactive chemical constituents.

[044] Preferably, said separation tank is also a fermenter tank comprising algae, which are grown through photosynthesis. The algae are harvested when required by introducing the flocculant into the acidity-regulated liquid so as to causes the algae to coagulate together in floes which are lifted to the surface by the microbubbles. With a laminar flow of the fermenter liquid, the bubbles do not disrupt the floes and effective clarification of the liquid can be achieved and the algae subsequently harvested by scraping from the surface.

[045] In systems where pH does affect coagulation and agglomeration of particles, such as those described above, as well as attachment of those particles and agglomerates to bubbles, it is clearly desirable to adjust the pH to optimise that agglomeration and attachment. This can be done in a number of ways including bubbling gas into the liquid which, when relevant molecules or radicals of the gas are dissolved in the liquid, affect the pH of the liquid. Using this method, it would be necessary to determine what quantity of gas will be sufficient to dissolve enough radicals to alter the pH of the bulk liquid to achieve the desired pH. The present invention recognises that pre-adjustment of pH is not required if the gas used for separation contains pH adjusting gas and sufficiently small bubbles are used to develop a suitable local pH environment around the bubbles.

[046] Thus, in another aspect, the present invention provides a method of separation of particles suspended in a liquid, the method comprising the steps of:

providing a suspension or colloid of particles in a liquid wherein the pH of the liquid requires adjusting to facilitate binding of the particles with the bubbles;

introducing microbubbles of pH-adjusting gas, which gas comprises molecules or radicals which, when dissolved in the liquid across the bubble/liquid boundary, adjust the pH in a desirable direction; wherein

the microbubbles are introduced into the liquid with sufficiently low energy, and the bubbles are sufficiently small so that they rise in the liquid sufficiently slowly under the influence of gravity, that a stationary boundary layer of liquid pertains and remains around the bubble as the bubbles rise in the liquid, and whereby a pH gradient develops across the boundary layer through dissolution of said molecules or radicals from said bubbles in the liquid; and

the boundary layer is sufficiently thick that particles approaching the bubble contact liquid in the boundary layer at a desirable pH of the liquid so that electrostatic repulsion between the microbubble and particle is minimised and microbubbles and particles collide and attach so that said particles are raised with the microbubbles and separate from the liquid more effectively than would be the case if the average pH of the liquid was pre-adjusted to the average value achieved by said introduction of said microbubbles.

[047] Thus the microbubbles not only lift the particles but they also adjust the pH of the liquid in the vicinity of the microbubble to prevent repulsion between bubble and particle and encourage attachment between them. The adjustment of the overall pH of the liquid is much less than traditionally understood to be required, because it is only in the vicinity of bubble that such adjustment is needed. This is only possible, however, where the bubbles are introduced in non-turbulent conditions, otherwise the boundary layer in which a gradient can be established to overcome electrostatic repulsion is too thin to prevent such repulsion. Bubbles can only be in this condition when they are small and when they are created with a low energy process.

[048] The net effect of employing the system of the present invention is that the overall pH is only marginally adjusted, minimising the amount of pH regulation required to effect efficient separation.

[049] The invention provides a corresponding particle separation system to separate particles from an aqueous suspension or colloid of the particles, the system comprising:

the injection into the liquid of microbubbles of a pH-adjusting gas, said microbubbles, wherein the microbubbles are small enough that flow of liquid around the bubbles as the bubbles rise in the liquid does not prevent a boundary layer of liquid remaining stationary around the bubbles;

dissolution in the liquid of molecules or radicals of the gas in the microbubbles across the bubble/liquid boundary, which dissolution establishes an acidity gradient in the liquid across the boundary layer, such that, at a distance from the bubble surface where repulsion of a particle by electrostatic forces would otherwise occur, the acidity of the liquid is such that said repulsion does not occur or is inhibited and binding to and lifting of the particle by the bubble is more effectively achieved than would be the case if the average acidity of the liquid was pre-adjusted by to the same extent as achieved by said introduction of microbubbles of said pH-adjusting gas, and the gas introduced by the microbubbles was not said pH-adjusting gas

[050] Preferably, the desirable pH is determined by the kind of a coagulant or flocculant added to liquid to facilitate coagulation or flocculation of the particles.

[051] Where the desired pH is acidic, the gas preferably is or contains carbon dioxide. The desired pH may be between 5 and 7. The coagulant may be ferric chloride.

[052] The system may comprise a suspended particle separation tank, the tank comprising: a floor having a floor area;

liquid in the tank having suspended particles; and

a diffuser of a microbubble generator, which diffuser is disposed at the base of the tank and is arranged to inject said microbubbles of gas into the tank to reduce the apparent density of the liquid above the diffuser, thereby creating a flow of the liquid and gas bubbles that is a non-turbulent laminar flow having a Reynolds number less than 2000, wherein more than 90% of the bubbles have a size in the range 10 to 100 μηι diameter and a bubble density of at least 50million per cubic metre, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.

[053] The tank may also be a liquid fermenter tank for microorganisms, wherein the tank has sides and said floor, the liquid in the tank has a top surface above which is a header space, and said flow is:

up the tank in a riser section thereof,

turned sideways at the surface of the liquid where at least a proportion of the bubbles either break at the surface, terminate passage through the liquid at the surface or are reduced in diameter through dissolution of the gas in the bubbles into the liquid during their passage up the riser section;

turned down at the sides of the tank into a downcomer section of the tank; and turned sideways back into the riser section,

which flow, at least in the riser section, is said non-turbulent laminar flow having a Reynolds number less than 2000.

[054] Particles are in suspension in a liquid, and fail to coagulate naturally and separate out without external influence, because they are or become charged, positively or negatively. They develop a Stern layer on their surface of identical but opposite charges to the charge of the particle itself. A shear or slipping plane of mixed charges develops around the Stern layer, and further from the particle a diffuse layer of mixed charges exists of potential dependent on the pH of the bulk liquid. The electrical potential difference in the shear plane between a colloidal particle and liquid bulk is known as the zeta potential, Z, and it decreases away from the particle. It is given by:

where

v_Q = electrophoretic mobility, ^m/s)/(V/cm) = v_E/E

v_E = electrophoretic velocity of migrating particle, μηι/s

E = electric field at particle, V/cm

K_z = a constant that is 4π or 6π

μ = dynamic viscosity of water, N.s/m²

ε = permittivity relative to a vacuum (ε for water is 78.54)

ε₀= permittivity in a vacuum, 8.854188 x 10^"12 C²/J.m, or N/V²

[055] The zeta potential is essentially exponential, but if it is sufficiently high, sufficiently far from the surface of the particle then another particle or a bubble cannot approach without being repulsed by electrostatic forces. However, as the pH is appropriately adjusted, an isoelectric point of zero zeta potential may be developed close to the particle so that the particle can be approached. By arranging for bubbles to carry with them their own local environment of liquid with appropriate acidity, the isoelectric point can be "squeezed" between the bubble and a particle so that they can contact and stick by virtue of the hydrophobic nature of the particle.

[056] In WO2008/053174, there is provided a method of producing small bubbles of gas in a liquid comprising the steps of:

providing a source of the gas under pressure;

providing a conduit comprising a diffuser opening into a liquid under pressure less than said gas, said gas being in said conduit; and

oscillating the gas passing along said conduit with a fluidic oscillator,

wherein the fluidic oscillator comprises a diverter supplied with the gas under constant pressure through a supply port that divides into respect output ports, and including means to oscillate flow from one output port to the other, and

whereby substantially monodisperse bubbles are provided in the liquid with sufficient gap between them to prevent coalescing.

[057] Thus the entire energy of the system is in oscillating the gas, and not the conduit through which it is passed, whereby the efficiency of the system can be maximised. Energy is not wasted in oscillating the conduit that will have a much greater mass and consequently will require more energy to oscillate. Despite any resonance, friction still accounts for a proportion of the energy employed.

[058] Sonic and ultrasonic vibrations, as suggested in GB2273700 and JP2003-265939 respectively, are high frequency and may not be as effective in generating bubbles. Although high energies can be imparted, the most effective detachment of bubbles is with longer stroke (higher amplitude) oscillations, rather than higher frequencies.

[059] Preferably, said oscillations effected by the fluidic oscillator are effected at said frequency between 1 and 3000 Hz, preferably between 5 and 500 Hz, more preferably between 10 and 30 Hz.

[060] Preferably, the bubbles formed are between 0.1 μηι and 100 μηι in diameter, more preferably between 30 and 80 μηι.

[061] Preferably, said oscillation is of the type that has between 10% and than 30% backflow of gas from an emerging bubble. Indeed, said oscillation preferably is of the type that has between 10% and 20% backflow of gas from an emerging bubble. This is preferably provided by an arrangement in which a fluidic oscillator divides flow between two paths, at least one of said paths forming said source. [062] Backflow here means that, of a net gas flow rate from said conduit of x mV¹, (x+y) mV¹ is in the positive direction while (-y) mV¹ is in the negative direction, 100(y/(y+x)) being defined as the percentage backflow. Some backflow is largely inevitable, particularly with the arrangement where flow splits between paths, since there will always be some rebound. Indeed, such is also a tendency with bubble generation since, with the removal of pressure, back pressure inside the bubble will tend to cause some backflow. Indeed, backflow here means at the conduit opening, because backflow may vary by virtue of the compressibility of the gas.

[063] Further preferred arrangements are also described in WO2008/053174.

[064] Such methods of bubble production as described above may be employed in the digesters, fermenters and separators as defined above.

[065] In connection with the algal and particle separators, as well as in anaerobic and other digesters, such methods of bubble production as described herein are especially beneficial because of their low energy consumption. To introduce bubbles into a tank, the pressure of the gas supply need only be about half a bar higher than the hydrostatic pressure of the liquid at the point of introduction. In a typical tank with a three metre height of liquid, traditional DAF (dissolved air flotation) injects liquid saturated with gas at about 6 bar pressure into the -1.3 bar clarifying tank. The turbulence caused by the liquid injection and release of the bubbles, despite their small size, is significant. The bubble production of the present invention injects at ~2 bar into the -1.3 bar tank, and it is gas, not liquid, that is injected so there is no liquid turbulence. The present invention achieves the onset of the bubble creation with little more than the energy to create the bubble and the momentum needed to overcome the head of liquid above it. Consequently, each bubble can be injected at not much more than its terminal rise velocity.

[066] While turbulent flow has in the past been seen as beneficial with regard to attachment in flocculation processes (because it increases the collision rates of bubbles and generalized particles (droplets included), the shear instability of turbulent flows also produces strong eddies that destroy floes.

[067] The present invention employs bubbles that rise in laminar flow. Moreover, at low gas flow rates, the gas phase holdup can be less than about 1 %. Given that the traditional levels of gas phase holdup in dissolved air flotation is in the order of 10-12%, there is no comparison with the bubble flux; that is, there is much less kinetic energy/momentum injected with the present invention, so that, on the face of it, there is much less lift force as well. It is therefore unexpected that the present invention achieves comparable separation performance and rates to traditional DAF. [068] Without being bound to a particular hypothesis, one theory is that the floes that form with a laminar flow of few small bubbles do not break up (because of the lack of the turbulent destruction mechanism) and therefore achieves comparable separation performance and rates. Additionally, because energy dissipation rates are typically proportional to the Reynolds number of the liquid flow, based on the diameter of the exit pore/nozzle, the present invention provides a Reynolds number in the range of 10-100, whereas conventional DAF has exit- nozzle Reynolds numbers of 10,000-100,000. It is therefore expected that the present invention dissipates -1000-fold less energy. The capital cost of equipment, due to working at pressures less than 2 bar, is substantially less than working at pressures of 6 bar for DAF.

BRIEF DESCRIPTION OF THE DRAWINGS

[069] Embodiments of the invention are further described hereinafter, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a plan view of a suitable diverter to oscillate gas in a method in accordance with the present invention;

Figure 2 is a graph of oscillation frequency plotted against feedback loop length for one arrangement of the diverter shown in Figure 1 ;

Figure 3 is a graph of bubble pressure against bubble volume for conduit openings of two different diameters;

Figure 4 is a bubble generator plate of an alternative arrangement of the present invention;

Figure 5 is an end view showing the relative dimensions of the liquid and gas conduits of the bubble plate shown in Figure 4;

Figure 6 is a schematic illustration of the overall arrangement employing the bubble plate of Figures 4 and 5;

Figure 7 is a schematic illustration of the overall arrangement of a preferred embodiment of the present invention;

Figure 8 is a cross section through a bubble generator of the system of Figure 7;

Figure 9 is a section through a bubble generator according to the fourth aspect of the present invention;

Figures 10 a and b are respectively a schematic perspective view of a diffuser employed in a method according to the present invention and a side section showing bubble pinch-off;

Figures 11 a and b are respectively side sections, (a) to (e), through an elastic membrane showing the development of a bubble, and a graph of differential gas/liquid pressure ΔΡ across the membrane at each of the stages of bubble formation shown in Figure 11 a; Figure 12 is a schematic representation of the experimental set-up of an algal floatation separation unit in accordance with an embodiment of the present invention;

Figures 13 a and b are histograms of (a) bubble size distributions and (b) bubble density from a stainless steel mesh diffuser used in the set up of Figure 12;

Figures 14i a to c are photographs of the flotation unit of the set up of Figure 12, showing the separation at three different key stages: (a) a few moments after flocculated algal cells were introduced into the unit; (b) after 12 minutes, clearly showing the algae sludge blanket minutes and where small floes are predominant; and (c) a third stage after 30 minutes, marked by much slower separation as relatively smaller floes but intense sludge thickening and thinning is observed;

Figures 14ii is a graph over the three stages referred to in Figure 14i, of residual biomass concentration in the tank of the set up of Figure 12, for three different coagulants (aluminium sulphate, ferric sulphate and ferric chloride)

Figures 15 a to c are graphs of recovery efficiency at 150mg/L coagulant dose against time at varying pH levels for all three metallic coagulants mentioned with respect to Figure 14ii;

Figures 16 a to c are plots of algae recovery efficiency as a function of pH at different coagulant concentrations;

Figures 17 a to c are graphs of algae recovery efficiency at pH 5 as a function of time at varying coagulant concentrations for the three metallic coagulant types;

Figure 18 is a schematic diagram of an airlift tank with internal concentric flow loop;

Figures 19 a to c are snapshots of simulated gas concentration in the tank (across a half section thereof) at (a) diameter 20 μηι and (b) after 100 μηι over a development period from 0, 40, 80 and 120 sec after commencement, and (c) after 120 sec for different bubble diameters (20, 60, 100, 120 and 140 μΓΤΐ);

Figures 20a and b are graphs showing (a) the mixture density at different bubble diameters (20, 100, and 200 μηι) and (b) the volume gas fraction across the downcomer section at different bubble diameters (20, 100,200 μηι);

Figures 21 a to d are graphs showing (a) the velocity gas profile in cross-section riser zone after 120 sec at different gas bubble diameter (20, 100, 200, 400, 600, 800, 1000 μηι), (b) the velocity profile at a certain point in the riser zone after 120 sec, (c) the velocity liquid profile across the riser zone after 120 sec at different gas bubble diameters (20, 100, 200, 400, 600, 800, 1000 μηι), and (d) the velocity profile in certain point in riser zone after 120 sec;

Figures 22a and b are graphs showing (a) the gas volume fraction at a certain point in downcomer zone at different gas bubble diameters (20, 60, 100, 120, 140 μηι) after 120 sec, and (b) the depth of penetration (hp) of microbubbles into downcomer zone at different bubbles sizes (20, 60, 100, 120 and 140 μι ι); Figure 23 is a schematic illustration of the experimental apparatus relating to an anaerobic digester in accordance with the present invention;

Figures 24 a and b are graphs showing (a) methane production from the digesters of Figure 23 with and without bubble injection, and (b) methane production per day before and after one hour nitrogen sparging in the digester of Figure 23;

Figures 25 a and b are graphs showing (a) carbon dioxide produced from the digester, with and without nitrogen injection, and (b) hydrogen sulphide produced from anaerobic digestion, again with and without nitrogen sparging;

Figure 26 illustrates the distribution of charges around a particle showing different layers of ions, and an approaching microbubble;

Figure 27 is an illustrative graph of zeta potential against distance from a particle surface; and

Figure 28 is a graph illustrating a typical variation of zeta potential against pH of a liquid containing the particle.

DETAILED DESCRIPTION

[070] In Figure 1 a fluidic diverter 10 is shown in section, comprising a block 12 in which passages indicated generally at 14 are formed. An inlet passage 14a has a supply 16 of fluid under pressure connected thereto by an inlet port 18. Two outlet passages 14b, c branch from the inlet passage 14a. Two control passages 14d,e oppose one another on either side of the inlet passage just in front of the branch 14f between the two outlet passages 14b, c. The control passages are supplied by control ports 20d,f which are interconnected by a closed loop conduit 22. When fluid passes along the inlet passage 14a and enters the diverging branch 14f it tends to cling to one side or the other under the influence of the Coanda effect, and preferentially enters one or other of the outlet passages 14b, c. In fact, the effect is so strong that, provided the pressure region upstream of the outlet passages 14b,c is favourable, more than 90% of flow in the inlet passage 14a will enter one or other of the outlet passages 14b, c. The outlet passages 14b, c are connected to respective outlet ports A,B.

[071] If the flow is predominantly into outlet passage 14b, for example, then the flow of fluid follows closely wall 14g of the inlet passage 14a and across the mouth of control passage 14d, reducing the pressure in the passage accordingly by virtue of the venturi effect. Conversely, there is not so much flow adjacent control passage 14e. Consequently, a pressure difference is created in the control loop 22 and fluid flows from control port 20f, around control loop 22, and enters control port 20d. Eventually, the flow out of the control passage 14d becomes so strong that the flow from inlet passage 14a to outlet passage 14b detaches from the wall 14g containing the mouth of control passage 14d, and instead attaches on the opposite wall 14h, whereupon such flow is switched to passage 14c. Then, the opposite condition pertains, and the pressure in control port 14e is reduced, and grows in control port 14d, whereupon the flow in control loop 22 reverses also. The arrangement therefore oscillates, in known manner, dependent on several factors including the length of loop 22, which length affects the inertia of the control flow and the speed with which it switches. Other factors including the geometry of the system, back pressure from the outlets and the flow through the diverter 10 also affect the frequency.

[072] The arrangement shown in Figure 1 conveniently comprises a stack of several Perspex™ plates each about 1.2 mm thick and laser cut with the outline shape of passage 14. Top and bottom cover plates close and complete passage 14 and hold the stack together, the bottom (or top) one being provided with the ports 18, 20d, 20f, A, and B. However, it has been shown experimentally that the arrangement scales up effectively and is within the ambit of the person skilled in the art.

[073] Figure 2 illustrates the variation of frequency of oscillation of one system employing air as the fluid in the diverter of Figure 1 , with a control loop of plastics material of 10 mm internal diameter and an airflow of 10 litres per minute. Frequencies between 5 and 25 Hz are easily achieved, but also in the range of a few Hz to 5000 Hz. Again, the arrangement is capable of being scaled-up to provide significant airflows in this range of oscillation frequency.

[074] When the outputs A,B of diverter 10 are connected to bubble diffusers 30 in an arrangement 100 such as illustrated schematically in Figure 7, finer bubbles are produced than when a steady flow rate of similar magnitude is employed. Several diffusers 30 are connected in parallel to each outlet port A,B by appropriate tubing 17. Moreover, because the bubbles are finer, fewer large bubbles are produced: they are detached sooner by virtue of the oscillating air supply.

[075] A suitable diffuser 30 is shown in Figure 8, which comprises a housing 32 of shallow, hollow cylindrical form and having a central inlet opening 34 for connection to the tubing 17. The chamber 36 formed by the housing 32 is closed by a porous disc 38, which may be ceramic, or a sintered metal. Such bubble diffusers are known and in use in the water treatment industry, and such products are available, for example, from Diffuser Express, a division of Environmental Dynamics Inc of Columbia, MO, USA.

[076] Indeed, as regards Figure 7, the only part not already employed in the present sewage treatment industry is the diverter 10, and the arrangement of the present invention provides the opportunity for retro-fitting the method of the invention into existing installations, simply by interposing a diverter 10 of appropriate size and configuration into the supply to an existing network of diffusers 30. Other forms of diffuser do, of course, exist and are applicable to the present invention.

[077] While described above with reference to sewage treatment, as mentioned above, the present invention may have application in numerous other fields in which a gas needs diffusing into a liquid. In the sewage treatment regime, other than in the search for efficiency, the equality of the bubble size or their absolute minimisation in size may not be imperative. Rather, the capacity to retro-fit the arrangement may be more important. However, in new installations, or in other applications where, for particular reasons, a very small bubble size, and very even bubble size distribution, is desired, the arrangement illustrated in Figures 4 and 5 may be employed.

[078] Referring first to Figure 3, two plots are shown of internal pressure against bubble size being formed from two apertures of different size (0.6 and 1.0 mm). The pressure increases substantially linearly with increasing volume until the bubble reaches a hemispherical shape. Thereafter, however, pressure decreases as the bubble grows further. Thus, at any given pressure, a bubble can have two sizes. More importantly, however, if two bubbles are growing from two ports that are supplied by a common source in parallel with one another then as the pressure increases with growing bubble size, the growth of the two bubbles in parallel is stable. However, once the bubble reaches hemispherical the stable growth ends and as one bubble continues to grow its pressure diminishes. Consequently, if there should be any imbalance between the growth of the two bubbles so that one reaches hemispherical and beyond first, the pressure in the one whose growth is slower will be higher, rather than lower, than the bubble whose growth is faster. Consequently, what occurs is that faster growing bubbles grow larger and slower growing bubbles are smaller and may never detach.

[079] In Figures 4 and 5, a diffuser 50 comprises a plate 52 having a top surface 54 in which a right-angled groove 56 is formed, with each of its sides 58,60 being angled at 45° to the top surface 54. Under the surface but parallel thereto are two supply passages 62,64 also lying parallel, and disposed one on either side of, the groove 56. Rising up from each passage are a plurality of ports 62a, 64a. Ports 64a are relatively narrow and open in the middle of the face 60 of the groove 56. Ports 62a are relatively broad and open at the base of the groove 56. There are as many ports 62a as there are ports 64a, and each port 62a is arranged opposite a corresponding port 64a. Moreover, the passage 62 and the ports 62a are arranged so that the direction of discharge of fluid from port 62a is parallel the face 60 of the groove 56.

[080] Passage 62 may be larger than passage 64, but the ports 62a are certainly larger than the ports 62b. The reason for this is that the passage 62 is arranged to carry liquid, the liquid in which the diffuser 50 is sited. The passage 64, on the other hand, carries gas. The arrangement is such that the diameter of the gas port 62b is small, according to the desired size of bubble to be formed, and possibly as small as 0.5 mm or less depending on the technique employed to form the port 64a. In Perspex™-type material, the holes can be drilled mechanically to about 0.5 mm, but other methods exist to make smaller holes if desired.

[081] Turning to Figure 6, a tank 80 of liquid 82 has a diffuser 50 in its base. A gas supply 16 supplies gas under pressure to a diverter 10 of the kind shown in Figure 1 , and whose two outputs A,B are connected to passages 64,62 respectively by lines 86,88 respectively. However, while outlet connection A and line 86 are closed, connection B has a bleed 84 to the environment above tank 80, so that its pressure is substantially ambient. Consequently, line 88 fills with liquid to the height of the liquid in the tank 80. Indeed, when the air supply 16 is turned off, so does the outlet A and consequently the diverter 10 is located above the level of the liquid in the tank.

[082] However, when the air supply 16 is turned on the pressure in branch A grows, albeit oscillatingly, to half the supply pressure, and this is arranged to be greater than the hydrostatic pressure at the bottom of the tank 80 so that air ultimately passes along the passage 64 and exits the ports 64a forming bubbles 90 in the liquid 82. When a pulse of pressure arrives in outlet B, the level of liquid in the line 88 drops, since the bleed 84 is controlled by a valve 94 transmitting the pressure pulse into a flow of liquid into the passage 62 and out of respective ports 62a. However, when the diverter switches flow back to outlet A, the hydrostatic pressure in the tank 82 returns the liquid through ports 62a refilling the line 88. Whether the line 88 is refilled entirely, so that the pressure outlet 88 is ambient by the time flow is switched again to outlet B is purely a design matter. It can be arranged that only when the pressure in the line 88 is substantially at the hydrostatic pressure near the bottom of the tank 80 is there sufficient pressure in the line 88 to bleed enough gas through the valve 94. In any event, the liquid level in the line 88 must be arranged at some point between the top and bottom of the tanks, and to oscillate above and below that level as gas supply is switched to and from the output B.

[083] The ports 62a are larger simply because of the increased resistance of the liquid to flow, but also because a large flow pulse, rather than a narrow flow jet, is better at knocking off bubbles.

[084] The back pressure regime in outputs A,B is arranged such that it does not adversely interfere with the oscillation of diverter 10, and each pulse into output A is arranged such that a hemispherical bubble forms at the mouth of each port 64a. When the pulse switches to output B, a jet of water issues from the mouth of each port 62a and is directed against the side of the bubble on the ports 64a and knocks them off. The bubbles 90 so formed are therefore very small, or at least much smaller than they would otherwise be, and of very even size distribution.

[085] When the arrangement described above is employed with a liquid of relatively low viscosity such as water, it works very well. However, when it is employed with more viscous liquids, such as oil, a different mechanism is observed which gives rise to an alternative arrangement of the present invention (shown in Figure 9 and described further below) and possibly an alternative explanation as to why the oscillation of the gas in a retrofit situation described with reference to Figures 7 and 8 may work, or indeed how the arrangement described with reference to Figure 4 to 6 may be working.

[086] Figure 9 illustrates a bubble generator 1000, in which a plate 12' has a conduit 64' having a plurality of ports 64a' connecting the conduit 64 with the liquid 82 in which bubbles are to be formed. The conduit 64' is connected via tube 86' to a source of gas under pressure greater than the pressure of the liquid in the ports 64a', so that there is a net flow of gas along the conduit 64'. However, at the same time, the gas is also oscillating by virtue of a fluidic mechanism (not shown in Figure 9) such as the diverter 10 of Figure 1.

[087] With high viscosity liquids such as motor oil as the liquid 82, the oscillations can be seen to permit introduction of some of the liquid into the conduit 64' through some of the ports 95. The exact mechanism is not yet explained, although could be through the venturi effect of high flow of gas periodically through the conduit 64' drawing liquid through certain of the ports (eg ports 95a), or it may be due to the low pressure phase of the oscillations and the relatively higher pressure in the liquid at this point in the gas pressure cycle. In any event, despite there being a net flow of gas through the conduit 64' and out of the ports 64a', nevertheless, plugs 97 of liquid appear in the conduit and progress along it, driven by the net flow of gas. As they travel along the conduit, they progressively close off mouths 98 of the ports (eg port 95b) and liquid enters the ports behind the gas already in the port. When the plug liquid contacts the main body of liquid 82 at the open end 99 of the port, the gas/liquid interface in the port completes the gas/liquid interface of bubble 101 presently being formed by the gas. Consequently it is easily detached from the port 95b and released into the liquid body 82.

[088] With this mechanism, an inclined series of bubbles rise from the ports 64a'; and possibly several such streams, if several plugs 97 form (as shown for example at 103 where the plug is almost exhausted having pushed off a series of bubbles 105 and losing some of its volume to the main body of liquid 82). Also a new plug 107 is shown being drawn into the conduit 64'.

[089] If such a mechanism is working with lower viscosity systems, (where the mechanism is more difficult to observe by virtue inter alia of the greater frequency of operation of such systems), then the above described mode of operation of the knock off system shown in Figures 4 to 6 may not be complete, or even entirely correct. However, the skilled person can find an arrangement that suits the particular requirements of a given application. Indeed, if the theory described above with reference to Figure 9 is correct, it may explain why the oscillating gas produces fine bubbles. They may be produced not because the of the oscillations per se causing inertial movements of the liquid that pull off bubbles from the open end of the exit ports, as described above and pinching the bubbles off, but rather that plugs of liquid get entrained into the system and push off bubbles from behind.

[090] In Figure 10, a glass diffuser 150 is constructed from two sheets of glass 152, 154 adhered face to face, in which, on one sheet 154, channels 156, 158 have been etched, so that, when connected as shown, a large conduit 156 is formed from which several smaller conduits 158 depend and emerge at surface 160 of the diffuser 150. In use, when connected to one branch of a diverter (such as that shown in, and described above with reference to, Figure 1), bubbles are formed at the openings 162 of each conduit 158. If the channels 158 are approximately 60 microns in depth and width, bubbles of a corresponding diameter are pressed from the conduits 158. If the gas flow is oscillated as described above, bubbles of that size break off. However, if the face 160 is rendered horizontal, it is, in fact, possible for bubbles much larger than that to be formed, circa 500 microns diameter, with surface tension managing to adhere the bubble to the opening and it merely growing, albeit oscillatingly, until finally the mass of liquid displaced detaches the bubble. However, when the face 160 is oriented vertically, as shown in Figures 10a, b, the rebounding bubble in the first or second oscillation does not fit squarely against the opening but is distorted upwardly by gravity, and this results in the bubble pinching off much sooner. This is particularly the case if the material of the diffuser 150 is non-sticky, as far as the gas, is concerned, and this is the case for glass where the gas is air. Likewise for non-stick materials such as Teflon®. Thus, with nothing else, bubbles of the order of 50 to 100 microns can be produced.

[091] Turning to Figure 1 1 , some existing diffusers employed in waste water cleaning, such as those illustrated in Figures 7 and 8, have a membrane (38, in Figure 8 and in Figure 11 a) which has a number of slits cut through it. The mode of operation is already oscillatory to some extent, even with a steady gas flow, as the pressure distends the membrane, opens the slits and, as bubbles pinch off, there is a certain rebound of the lips of the slit before a new bubble begins. However, with reference to Figure 11 a and an oscillating gas pressure, the differential pressure ΔΡ across a slit 170 increases from zero as shown at (a). In (b), the gas begins to deform the membrane 38 and it is forced through the slit commencing the formation of a bubble 90. As the pressure continues to increase, the membrane deforms further, as shown in (c) accelerating the growth of the bubble. However, at this point the pressure differential begins to decrease so that the natural rebound of the elastic membrane is facilitated, closing off the bubble 90 as shown at (d). Finally, with zero pressure the membrane returns to the position shown at (a), and (e) but in the latter with the bubble 90 released.

[092] By matching the oscillation of the gas flow to the elastic resonance of the membrane the formation of small bubbles is possible with little energy expenditure. Figure 1 1 b shows a preferred form of square wave pressure development that is potentially the result of both the fluidic arrangement and slitted membrane, and shows the potential pressure positions at each stage of bubble development illustrated in Figure 11 a.

1. Application in Algal Separation

[093] Flotation has become the mainstay for colloidal particle separation from an aqueous solution. In essence, the key subprocess is the generation of microbubbles that attach to hydrophobic particles, resulting in buoyant aggregates which then rise to the surface of the flotation cell, where following bubble rupture, the particles are recovered (Dai et al., 2000). Recovery of valuable end-products has been the centre of attraction in flotation separation. A large body of experimental evidence show the reclamation of products such as oil (Al- Shamrani et al., 2002b, Al-Shamrani et al., 2002a, Hosny, 1996, Li et al., 2007, Zouboulis and Avranas, 2000), minerals (Englert et al., 2009), algae (Teixeira and Rosa, 2006, Teixeira et al., 2010) and in cases where water scarcity is the challenge, potable water (Kitchener and Gochin, 1981 , Edzwald, 1995) can be achieved by flotation separation.

[094] Algae, in particular, are a reasonable target for flotation separations for biomass processing, but as yet untried with the dense solutions produced from algal cultivation. Pienkos and Darzins (2009) highlight harvesting and dewatering operations as a key challenge for economic algal biofuels processing. The density can reach 10g/L of dry biomass, which is substantially higher than DAF removal of fine particles in water purification. Gudin and Thepenier (1986) estimated that harvesting can account for 20-30% of the total production cost. Molina et al. (2003) present possibly the closest technique to microflotation for algal harvesting - flocculation and bioflocculation followed by sedimentation. Flotation is often viewed as "inverted" sedimentation. The Jameson Cell (Yan and Jameson, 2004) is an induced air flotation process which also achieves high separation performance for microalgae (98%) and phosphorus. When present in effluent water, algae could be a pernicious contaminant in potable water treatment otherwise, but could be regarded as a raw material given the numerous products obtainable from the unicellular organism such as β- carotene (Borowitzka, 1992) glycerol, biomass and in particular, biofuel from lipid (Chisti, 2007). While most previous works have focused on the production of biomass from algae (Zimmerman et al., 2011 b), only few researchers have been concerned with harvesting biomass and lipid from algae. Whether it is for potable water treatment or recovery of algae for biofuel, flotation separation is a viable means for harvesting algae.

[095] However for flotation to be successful, it is vital for particles to be hydrophobic (Gochin and Solari, 1983) and ultimately attach to gas bubbles. Chemical coagulation is employed to aid this process. Through the suppression of the electrical double layer of particles, particle- particle interaction is facilitated, leading to the formation of larger colloidal structures or floes. Following collision particles adhere to the surface of gas bubbles forming a strong stable particle-bubble union (Dai et al., 2000). Removal of floes is hugely dependent on the coagulation pH but another important factor is the bubble size and flux. Bubbles enhance particle recovery by providing the lifting force necessary for transport and separation. Separation efficiency varies inversely with bubble size (Dai et al., 1998, Dai et al., 2000).

[096] Application of gas bubbles in liquid is gaining extensive application across many fields. Generally, these processes entail efficient ways of facilitating bubble-particle interaction in the liquid rather than merely passing the bubbles through the liquid without it actually adhering and lifting the particles out of solution. Best practices however, require that the particles in the aqueous solution attain optimum collision, attachment and stability efficiencies respectively (Derjaguin and Dukhin, 1993) with the gas bubble for complete capture prior to reaching the liquid surface. As such, one of the most efficient ways of achieving this is miniaturising the bubbles. Due largely to their high surface area to volume ratio, particle flotation by small bubbles occur more rapidly and efficiently. Ahmed and Jameson (1985) estimate a 100-fold enhancement in separation performance for fine particles with bubble size reduction from approximately 700 to 70 microns. Further, small bubbles have gentle convective force relative to large bubbles by reason of their low rise velocity (Schuize, 1992), resulting in tender contact with fragile floes. To this end, several microbubble generation techniques have been thus developed for flotation applications. Examples include: turbulent microflotation (Miettinen et al., 2010), Induced Air Flotation (IAF) (El-Kayar et al., 1993), Dissolved Air flotation (Edzwald, 2010), and Electroflotation (Hosny, 1996). Of the several techniques available, dissolved air flotation and dispersed air flotation are the most widely developed. Specialist microbubble separations have been achieved in minerals processing with colloid gas aphrons, which are charged microbubbles (Cilliers and Bradshaw, 1996; Waters et al. 2008).

1.1 Dissolved Air Flotation

[097] Dissolved air flotation (DAF) in particular is the most efficient and widely employed flotation option. According to Henry's law, the process essentially requires dissolving air in water at very high pressure. By so doing, the solution becomes supersaturated; leading to nucleation of microbubbles as soon as pressure is reduces at the nozzle. Unfortunately, this process is energy intensive, due to the high pressure required for air dissolution in water as well as the work done by the pump in feeding the saturator with clarified water.

1.2 Dispersed Air Flotation

[098] Traditional dispersed air flotation involves the supply of continuous air stream directly into a porous material (usually a nozzle or a diffuser) from where bubbles are generated. By comparison with other microbubble generation methods, this technique is less energy consumptive. However, the natural problem associated with this method is the difficulty in small bubble production.

1.3 Example of the Invention in use

[099] A schematic representation of the bench scale dispersed air flotation unit is shown in Figure 12. The main rig components comprise: a flotation cell 170, microbubble generator comprising a fluidic oscillator 172 and 40mm stainless steel baffle distributor diffuser 174. The fluidic oscillator 172 (Tesar et al. 2006,(Tesaf and Bandalusena, 201 1) measures: 10cm x 5cm x 5cm in length, height and width respectively while the flotation unit measures: 50cm by 9cm in height and diameter respectively. The tests were conducted with the diffuser placed at the bottom of the flotation cell 170. The oscillator mid-port 176 was linked by a 0.5m feedback loop 178. The supply in the form of a compressor 179 delivered air at a pressure of 0.8bars and a supply flowrate of 85L/min through the oscillator. Microbubbles were generated under oscillatory flow by connecting the diffuser to the outlet of the fluidic oscillator.

[0100] Three inorganic metallic coagulants were used: aluminium sulphate, ferric III chloride and ferric sulphate (Sigma Aldrich, UK), while hydrochloric acid and sodium hydroxide (Sigma Aldrich, UK) were used for pH adjustment. The tests were conducted across five pH ranges and five coagulant concentrations.

[0101] Dunaliella salina 19/30 obtained from the Culture Centre of Algae and Protozoa (CCAP), Oban, Scotland was previously pre-cultured in a 250 L airlift Loop Bioreactor containing 248 L of Dunaliella salina growth medium (Zimmerman et al., 2011 b) for 2 weeks. Following that, the microalgae from the laboratory scale 250 L airlift loop bioreactor was transferred to an outdoor 2200 litre ALB for field trials at Scunthorpe, UK. The microalgae were grown with waste C0₂ from steel plant exhaust gas. After - 17 days, the cultured microalgae from the ALB were emptied into several drums and delivered back to the laboratory for harvesting. Several months after growth the microalgae culture was taken for further processing. Two litres of microalgae sample at room temperature (20°C) was mixed to break lumps and disperse the cells homogenously in solution following sedimentation and clustering of cells as a result of prolonged storage. Coagulation and flocculation followed for 4mins and 10mins respectively following pH adjustment. Immediately after flocculating with a mechanical stirrer at 70rpm, the broth was gradually introduced into the flotation column to a height of 30cm above diffuser before the microbubble generator was turned on. The diffuser used in this study was made of Perspex material and measures 40mm in diameter and overlaid with a stainless steel mesh (Plastok, UK) with pore size of 38μηι and an open area of 36%. Broth samples were collected every three (3) minutes from sample ports SP1 ,2,3 and 4, and measured with the calibrated spectrophotometer DR 2800 (HACH Lange) to assay absorbance at 663 and 640 nm wavelength. Recovery efficiency (R) was determined using the formula:

where C, and C_f are the initial and final algae concentrations respectively.

1.4 Bubble size Distribution Measurement

[0102] There are two main methods for measuring the size of bubbles generated in a liquid: namely optical and acoustical techniques.

[0103] The acoustic bubble sizer (Dynaflow, Inc.) was developed to meet challenges in the optical method caused by cloudy liquid. By exploiting the ability of bubbles to affect acoustic propagated waves, bubble size and population can be extracted at varying frequencies (Wu and Chahine, 2010). The device consists of a pair of transducer hydrophones 177, made of piezoelectric materials inserted in a polyurethane material to prevent contact with water. Both hydrophones are connected to a computer 175 via a control box. The transmitting hydrophone generates short bursts of sound signals within a set frequency which are then received by the second hydrophone after travelling through the liquid. The signals are then analysed by special in-built software for processing the phase velocity and attenuation within the desired frequency range to estimate the size distribution of bubbles. The acoustic bubble sizer (ABS) was used in this study for bubble characterisation. The two sets of flat hydrophones 177 used (measuring: 7.5x 7.5 x 2.5cm, optimal operating frequency range from 70 ~ 200 kHz and corresponding bubble size of 34-100 μηι) were mounted vertically (9cm apart) on either side of the flotation column 170. Three (3) runs were undertaken to determine bubble size distribution under oscillatory conditions.

1.5 Bubble Size Distribution

[0104] Microbubble generation is an essential part of flotation separation. Figure 13a presents the distribution of bubble size generated under oscillated air supply conditions. The single peak graph shows a positive skew of bubble size distribution which reveals the dominance of 24μηι sized bubbles. The smallest bubble produced was 24μηι, while the largest size measured was 260μηι. However, average bubble radius was 86μηι with 60% of the bubbles approximately 74μηι. The average bubble size generated with the fluidic oscillator is approximately twice larger than the diffuser 174 pore size (which is 38μηι). By contrast, without the oscillator, the average bubble size achieved was approximately 28 times larger than the diffuser pore size (ie over 1 mm).

[0105] The bubble density graph presented in Figure 13b was determined by measuring the population of bubbles in the column and the results showed that 20-40 μηι bubbles made up 95% of the total bubble density, while 5% consisted of bubbles greater than 40 μηι in a bubble size distribution of 20-260 microns. The narrow distribution range of bubble size not only strongly suggests the production of largely non-coalescent but more particularly, relatively uniformly sized microbubbles.

[0106] The difference in bubble size is simply attributable to the fluidic oscillator. The bistable device facilitates microbubble production by oscillating a stream of the continuous air supply. The pulse generated due to the oscillation helps to knock-off bubbles at the developmental stage. Without oscillation, bubbles tend to move irregularly, leading to increased bubble- bubble interaction and coalescence leading to larger bubbles. Regular detachment results in less coalescence because the bubbles are more uniformly spaced and sized. The level of inertial force in the pulse can be tuned so that bubbles emerge with little excess kinetic energy over the terminal rise velocity (Parkinson et al., 2008).

1.6 Algal Recovery

[0107] Understanding the step-wise processes prevalent in a multi-floc system between particle-bubble interaction in a flotation column is both interesting and informative. See Figure 14i for photographs of three stages of separation. Initially, a sludge blanket immediately begins to form. Larger floes are preferentially collected first before smaller floes. The residual biomass of algae in the tank decreases exponentially with time in a first stage. However, removal efficiency decreases as the gradient of biomass Vs time in Figure 14ii. Similar behaviour is found with each of the three flocculants tried.

[0108] Stage 1 is simply attributable to the large surfaces of floes which readily render them susceptible to bubble collision and adhesion, bubble formation at particle surface, microbubble entrapment in aggregates and bubble entrainment by aggregates. (Edzwald, 2010) reported these bubble-particle interaction mechanisms in the review of flotation as a wastewater treatment. These large floes also engage in sweep flocculation as they travel upwards under the lift of microbubbles; hence the exponential biomass recovery efficiency recorded at the early stage. [0109] After half the separation time (being the sum of Stages 1 and 2 together), the amount of large floes decreases markedly. During the next, straight-line, phase (Stage 2), smaller floes become prevalent in the flotation unit. Biomass concentration only reduces slightly and as such recovery efficiency hardly changes. In the second stage, surface sludge build-up continues, thickening the sludge blanket. As more bubbles rise to the top, these bubbles compress the sludge layer from underneath, reducing the water content of the sludge.

[0110] The third key stage is primarily characterised by intensive sludge thickening and thinning. By that is meant increasing density of the sludge layer, and hence reducing depth, which makes separation of the sludge easier to achieve. At this stage, the majority of the particles have been separated, ending the separation phase, whereby microbubble rise velocity is increased, since very few particles are present to cause rise retardation. The rate of water removal from the sludge is thus high as it is compressed. The sludge layer is reduced to almost a quarter of the initial size.

1.7 Coagulant and Effect of pH

[0111] Chemical pre-treatment is essential in decreasing the effect of repulsive charge between bubbles and floes (except see below). The success of chemical pre-treatment depends on pH, because pH determines the solubility of chemical constituents of nutrient and metals in solution and influences the form and quantity of ions produced. Optimum pH and coagulant dosing reduces the charge on particles to about zero causing particles to be more hydrophobic (Edzwald, 2010). To investigate the effect of pH on separation, trials were conducted across different pH levels and results reported in Figure 15.

[0112] Figures 15 and 16 present the flotation results for three metallic coagulants. The effect of pH on algal removal efficiency from Figure 15(a) showed that with aluminium sulphate coagulant, efficiency increases with decrease in pH to the lowest at pH 7 before rising again as pH increases to 9. Optimum recovery result of 95.2% was obtained at pH 5 with efficiency gradually decreasing to 71.9% at pH6 and 50.6% at pH 7. At pH 8 however, a sudden increase to 74.6% was obtained and 81.5% at pH 9 indicating the other peak of result with aluminium sulphate. Data from Figure 15(b) (ferric sulphate) can be compared with Figure 15(a) which showed a similar trend in the effect of pH on algal recovery efficiency. Again two peaks were observed on either side of the pH range experimented in this study. Best results were obtained at pH 5 with 98.1 %, followed by 91.6% at pH 6. The drop in performance continued to 83.2% at pH 7 before hitting the lowest with 80% at pH 8. At pH 9 however, the performance was observed to rise sharply to 85.5%.

[0113] From the graphs in Figure 15(c), however, it is apparent that the result with this coagulant (ferric chloride) was different. Algal recovery efficiency dropped monotonically and nearly linearly with pH decrease. Optimum result of 99.2% was achieved at pH 5 and then 93.1 % at pH 6. The recovery result further decreased to 90% for both pH 7 and pH 8 respectively and finally to 86.4% at pH 9. Graph 15(c) is quite revealing in several ways. First, unlike the first two graphs, overall efficiency was higher. The least efficiency at pH 9 was higher than the 80% mark. Thus with this coagulant, efficiency ranged from 86.4%- 99.2%.

[0114] In general, the optimum cell recovery result in these experiments was found at the lowest pH studied. Figure 16 reveals a trend in recovery efficiencies for the different coagulants studied with aluminium sulphate exhibiting a non-monotonic tendency across all concentrations studied, followed similarly by ferric sulphate. Recovery efficiency with ferric sulphate nonetheless shows a fairly monotonic response as pH drops. One explanation for the non-monotonic behaviour observed for ferric sulphate is contactless flotation (Jiang et al. 2010). One would infer that isoelectric points for all three coagulants are achieved with acidic conditions, so the alkaline high separation with ferric chloride would not naturally be achieved by zeta potential neutrality. By adding metallic inorganic coagulants such as iron and aluminium salts in solution, coagulation is achieved with the coagulants dissociating into Fe³⁺ and Al³⁺ respectively as well as other soluble complexes having varying high positive charges. Essentially, the rate and extent to which these trivalent ions and other complexing species adsorb onto colloidal surfaces is pH dependent. At room temperature, under acidic pH, trivalent species-Fe³⁺ (Wyatt et al., 2011 ) and Al³⁺ (Pernitsky and Edzwald, 2006). are the dominant species in the continuous phase These predominant trivalent species are the most effective in colloidal charge neutralization and attach to the negatively charged algal cell. The excess H⁺ present under low pH react with hydroxides of these metals to further release the trivalent metal species. As a consequence, more Al³⁺ and Fe³⁺ species become available again for charge neutralization but the amount of hydroxides species is reduced. As pH shifts away from acidity however, H⁺ concentration becomes less than OH^" and the amount of trivalent ions present in solution reduces. These prevalent OH^" react freely with the available trivalent metallic species to form the corresponding metallic hydroxide species. As such, hydroxide species become predominant under alkaline conditions attaching to algal cells and precipitating as large gelatinous floes. Pernitsky and Edzwald, (2006) and Wyatt et al. (201 1) reported increased concentrations of hydroxide species for aluminium and ferric salts respectively as pH moves beyond pH 7 at room temperature. This explains the large floes generated under alkaline condition. It is for these reasons the recovery efficiency is observed to increase again under alkaline pH.

[0115] Considering that large floes are good vehicles for sweep flocculation, one might wonder why, despite the relatively large floes formed at pH greater than 7, the overall efficiency under alkaline condition recorded for aluminium and ferric sulphate coagulant was still lower than results under acidic state. Under the same operating conditions of flowr ate, bubble size and flux, this observation can be explained by the difference in charge density of species. The higher the size and charge of the species, the more effective the coagulation process will be. Because these charges increase with increasing acidity, recovery efficiency is highest under acidic pH. In addition, relatively larger floes are developed under alkaline state and given that as particle size increases the residence time of the rising microbubble-floc agglomerate also increases leading to prolonged flotation time. Moreover, the lifting force of microbubbles diminishes with increased particle size (Miettinen et al., 2010).

[0116] By contrast, the condition is quite different for FeCI₃ though. Whilst a similar tendency occurs under acidic condition, FeCI₃ exhibits a rather different behaviour under basic pH. It is noteworthy to reiterate that ferric chloride produced the overall best recovery result. The justification for this is that ferric salts are relatively less soluble than aluminium salts. This observation corresponds with the findings of Chow et al. (1998) on the concentration of iron speciation in solution. Their results showed that the soluble ion concentrations were less than 1 % of the total iron chloride amount initially added. In addition, hydroxides of aluminium are amphoteric - containing both basic and acidic functional groups. Furthermore, the addition of ferric salts decreases the solution pH and the closer the pH tends towards acidity, concentration of trivalent species in the solution increases. Wyatt et al. (2011) observed the same occurrence in their study of critical conditions for ferric chloride-induced flocculation of freshwater algae. The optimum pH for algal separation ranges from 5-7 for ferric chloride but for aluminium and ferric sulphate, two ranges are effective- 5-6 and 8-9. Overall, the process governing these reactions is very complex and by no means easy to fully detail especially also as the growth medium contains vital and very reactive chemical constituents.

2. Application in Gaslift Bioreactor

[0117] Airlift bioreactors have many advantages over stirred tanks. For instance, there are no moving parts inside the reactor, low cost of installation and maintenance, and low energy consumption. However, bio-reactors would benefit from increased efficiency of mass and heat transfer rate in gas-liquid processes. Enhancement of mass transfer rate in gas-liquid interface has been traditionally dependent on increasing interfacial area between gas and liquid phases. The use of microbubbles not only increases surface area to volume ratio, but, also, increases mixing efficiency through increase in the liquid velocity circulation around a reactor. The mixing process in bioreactors is an important and critical factor in determining the efficiency of fermentation process and the nature of design which plays an active role in providing a suitable environment for micro-organisms. The traditional mixing method (i.e. stirred tanks) may yield better performance in the degradation process, yet when the process energy requirement is weighed against the energy obtained from biogas produced, these processes become economically unviable. Therefore, the reduction of the energy required for mixing is one the most challenging targets that is faced by advanced developments of bioprocess applications.

2.1 Bioreactor

[0118] The airlift reactor (ALR) has been used in several industrial applications, and it has been the most appealing option for any gas-liquid contacting process. It has been noticed that using airlift reactor intensifies the efficiency the process compared to stirred tanks. According to their structure, airlift reactors can be classified into two main types: airlift external loop reactor, in which the circulation takes place in separate conduits; and, airlift internal loop reactor, which is provided with a tube or a plate to create the conduit (channel) inside a single reactor for circulating the liquid inside the reactor. The latter is shown in Figure18 and comprises a tank 180 containing a biological liquid medium 182 providing a head space 184. A gas diffuser 186 is provided at the floor 188 of the tank supplied with an oscillating supply 189 of gas from a source (not shown) whereby bubbles 190 of gas may be introduced. A baffle or draft tube 192 divides the tank 180 into a riser section or region 194, immediately above the diffuser 186, and a surrounding annular downcomer region 196.

[0119] The tank may be circular cylindrical, with a diameter D, and the draft tube may be likewise circular cylindrical with a diameter d, each centred on the axis A of the tank at which the diffuser is also positioned. The draft tube has a top edge 198, spaced from the surface 200 of the biological liquid medium 182, and a bottom edge 202 spaced from the diffuser 186 and bottom 188 of the tank. A toroidal path is thus established comprising the riser section 194, over the top edge 198 of the draft tube, down the downcomer section 196, and under the bottom edge 202 of the draft tube back into the riser section.

[0120] Bioreactor design requires accuracy in choosing the dimensions and materials required for manufacturing due to the complexity of the medium. The biological medium is a multiphase mixture, which consists of solid, liquid and gas, as well as having different microorganisms that need suitable environmental conditions. It is conceivable under such situations to provide reliable control systems for pH and temperature monitoring, in addition to maintain the process under anaerobic conditions (if required). A cylindrical bioreactor shape as airlift gas injection was used in the current study. The ratio (D/d) of the diameter (D) of the bioreactor to the draught tube diameter (d) was 0.7. The volume of reactor was 15 litres, while 8-9 litres were working volume leaving 6-7 litres in the head space.

2.2 Flow modelling of the gaslift digester [0121] A simulation process of an airlift bioreactor was carried out using COMSOL Multiphysics software (Version 4.1 ). The properties of the process are:

• Range of microbubble diameter between 20-1000 μηι

• Low gas concentration

• Low flow rate (300 ml/min)

• Liquid phase is water

• Gas phase is air

• The temperature is 298.15 K, the pressure is 1 atm.

[0122] A laminar bubbly flow model interface was used for modelling of the two-fluid flow regimes (e.g. mixture from gas bubbles and liquid), driven by gravitation through the density difference between gas-bubble-containing liquid in the riser section 194 and depleted-gas- bubble-containing liquid in the downcomer section 196.

[0123] A laminar bubble flow model interface was used for modelling of the two-fluid flow regimes (e.g. mixture from gas bubbles and liquid). Thus, the momentum transport equation is given by:

Where # _; is liquid volume fraction (m³/ m³), p is density of liquid, _} the velocity of liquid phase (m/s), t is time (sec), P is pressure (Pa), % is dynamic viscosity of liquid phase (Pa.s) and g the gravity(m/s²).

For low gas concentration, the liquid holdup coefficient (0_;) is about unity. Therefore, the change of & can be neglected in the following equation.

<?0:

Eq 4

The equation of the gas phase is illustrated as follows:

Eq.5

Where p. is the density of gas phase (kg/ m³), gas volume fraction (m³/ m³), is velocity of gas and --- i_§j the mass transfer rate (kg/m³.s).

For the purposes of approximation, there is considered no mass transfer between gas and liquid phases. Thus the =0. Therefore, the continuity equation can be arranged for two phases (e.g. gas and liquid) but without mass-transfer terms as follows:

Eq.6 The ideal gas law was used to calculate the density of gas (<¾):

p ,_;.

*** ^_ RT Eq.7

Where M_m is the molecular weight of the gas bubble, J? is the ideal gas constant (8.314 J/(mol.K) and T the temperature of gas (K).

The gas volume fraction is estimated by the following equation

¾ = - O; Eq.8

The gas velocity can be determined as ₉ = ¾_¾, since ¾½*_¾. is relative velocity between two-phases fluid (gas and liquid).

Pressure-drag balance, obtained from slip model, was used to calculate the t½_SS9. The assumption of this model suggests that there is momentum balance state between viscous drag and pressure forces on the gas microbubble:

3Cd _f

¾¾ ^{*■ 4 '} - ^f _{Eq 9}

where Cd is the viscous drag coefficient (dimensionless), d¾ is bubble diameter (m). Given that the microbubble diameters used in the simulation are equal to or less than 1000 μηι, the Hadamard-Rybczynski drag law was used, and hence:

_^ . _ 16

Eq.10 where:

M si»?,: — and where δ¾ is Reynolds number

[0124] On the draft tube 192 and internal airlift bioreactor walls, no slip ( = &} was used in boundary conditions (BCs) for the liquid phase, whilst no gas flux values were used for the gas bubble phase, hence the values of ¾ and n u₉0_s equal to zero. On the other hand, the "Gas outlet" and the slip (¾.- = Q) BCs were used at the top of liquid phase for both liquid phase and gas phase, respectively. On the top of the diffuser 186, no slip boundary conditions were used for liquid phase and the "gas flux" boundary conditions for the gas phase.

[0125] The gas concentration and arrows streamlines of the liquid velocity at different bubble diameters (20, 100, 200, 400, 600, 800, and 1000 μητι) over 120 seconds were determined. The results for 20 and 100 μηι bubbles are shown in Figures 19a and b, while Figure 19c is a snap shot after 120 sec development at 20, 60, 100, 120 and 140 μηι diameters. Figures 20 a and b show responses of the mixture bulk density and the volume gas fraction in the "downcomer" zone in gaslift bioreactor at different bubbles diameters (e.g. 20 100, and 200 microns). The responses for other diameters (i.e 400, 600, 800 and 1000 microns) have not been shown in this figure, as there were no changes in response for diameter values over 200 microns. When the diameter of the produced bubbles exceeded 200 μηι, both mass transfer and heat transfer were confined within the riser region, because the "downcomer" region, which is equivalent to about 30% of overall volume of the reactor, remained free from gas bubbles. The reason for this observation could be that the liquid circulation is unable to force big bubbles to make further circulations due to their high buoyancy forces, which is proportional to the height of the reactor, and inversely proportional to the diameter of the bubble. Therefore, all gases would leave the reactor beyond this region.

[0126] This situation was different when the produced micro-bubbles diameter was less than 200 μηι. Figure 20a shows clearly that the density of the mixture decreases with decrease in the produced bubbles diameter. For example, with bubbles diameter of 20 μηι, the density of the mixture was recorded less than that for 100 μηι diameter. The reason of this finding could be because of the presence of gas in this region as shown earlier in the Figures 19a and b, which is dependent on the diameter of the bubbles. The smaller the micro-bubbles introduced the more likely to penetrate into the bottom of the reactor, via the liquid circulation in the downcomer around the draft tube, due to their decreased buoyancy force and the increased drag force produced, as explained further below.

[0127] Therefore, the retention time of the gas bubbles increases dramatically (e.g. doubled, if it is assumed that the rotation of these bubbles has only been for one cycle). Moreover, the residence time of gas micro-bubbles in the "downcomer" zone would be longer than that for the riser zone, if the gravity force is considered. In addition, in certain areas in the downcomer zone, the buoyancy force of the gas bubbles is balanced with their drag force caused by flowing liquid; thus, leading to stationary states of the bubbles velocity, which cause the residence time of these bubbles to increase. Hence, the controllable size of micro-bubbles generated by fluidic oscillation would add another advantage to gaslift bioreactor system by increasing the mass and heat transfer not only in the riser region but also in the downcomer region.

2.3 Liquid and Gas Velocity Profile

[0128] The goals of the mixing system in biological processes include prevention of the formation of thermal stratification, maintaining uniformity of the pH, increase of contact between feed and microbial culture, and preventing fouling and foaming. As described earlier, some of biological media are viscous liquids, of high density, and contain solids, grits etc, thus, mixing of these materials thoroughly in order to achieve the desired objectives requires a great effort and energy. In fact, using a bubbling system for mixing of such media is inefficient at certain flow rates. Owing to generation of foams at the top of the culture surface, an increase of induced gas flow rate becomes necessary, perhaps rendering the entire bioprocess uneconomical.

[0129] Changing this scenario is possible, however, if micro-bubbles are used in this process, because, the rising velocity of gas bubbles is dependent on the pressure drag-coefficient and bubble diameter. For example, a decrease of bubble diameter causes a corresponding decrease in the Reynolds number, and the pressure drag-coefficient also increases. Consequently, a rise in velocity of the microbubbles decreases the drag coefficient. Figure 21 a shows a gas velocity profile in the riser zone, at 0.12 m height level and between 0 and 0.06 m radius from the centre of the tank at different bubble sizes. Figure 21 b shows the gas velocity in the Y (vertical) direction at different bubble sizes in certain points in the riser zone. On the other hand, for purpose of comparison, Figures 21 c and d show the liquid velocity profile at different bubbles sizes within similar areas, rise times, and distances mentioned above. Collectively, these figures can be discussed together in order to illustrate the benefits of micro-bubbles.

[0130] The simulation data showed that below a certain constant flow rate (e.g. 300 l/min), the gas velocity decreases with decrease in bubble size, due to increased drag force against the buoyancy force as shown in the Figure 21a and Figure 21 b. At similar conditions and flow rates, liquid velocity increases with decrease in bubble size (Figure 21c and Figure 21 d). Therefore, micro-bubbles would have enough power to move the liquid upwards even at low gas flow rates, and hence, decreasing energy required for mixing.

2.4 Penetration of the microbubbles

[0131] Penetration depth (hp) of the micro-bubble into the downcomer zone was also investigated in the present study. Depth of penetration represents, and can be an indicator, of enhanced efficiency of the mixing system in an airlift bioreactor as a result of the increased residence time in this region. Greater transfer rates of heat and mass would be achieved by higher residence times. The simulation data illustrated that the depth of penetration of the microbubbles increases with decreasing the bubbles size due to a bigger downward drag force compared to the buoyancy force. As mentioned above, that gas volume fraction increases in the downcomer zone with decreasing the bubbles size. Figure 22a presents the gas volume fraction in downcomer region at various bubble diameters (20, 60, 100, 120 and 140 μηι). The penetration of the microbubbles into downcomer depends on their diameter. For example, the depth of penetration of microbubbles with diameter 20 μηι was more than was observed for the microbubbles diameter of 100 μηι as shown in Figure 22b, and the snapshots of gas concentration in Figure 19c.

[0132] Microbubbles of smaller sizes penetrate deeper; however, the position of the gas diffuser in reactor has applied an important role in this situation. The gaslift bioreactor was simulated with four different locations of gas sparger 186. Dead zones were experienced with reduced circulation as the gas sparger was raised in the tank. Hence it is better to locate it low in the tank. Likewise the diameter d of the draft tube 192 in the gaslift bioreactor is relevant to circulation. The ratio of draft tube diameter to bioreactor diameter (d/D) was varied from 0.6 to 0.9. The effect of varying d/D ratio on velocity of gas and liquid was investigated. Two bubble diameters (20 μηι, 400 μηι) were used for investigating effect the draft parameter on mixing efficiency. The simulation data showed a maximum velocity of liquid in Y-axis (along axis A) that could be achieved with a ratio of 0.6 (m/m) is higher than that observed with ratio of 0.7, 0.8, and 0.9 (m/m). A narrow entrance between the diffuser and draft tube also contributed in increasing the velocity of liquid phase in the riser region.

[0133] The above considerations apply to any bioreactor including the anaerobic digester discussed below. However, different circumstances call for different requirements and while the above has purely considered the mixing and circulating effects of microbubbles, there is also the case of mass and heat transfer in the contribution of nutrients as well as the exhaustion of waste or product. In the case of algal growth, access to light is essential and, again the circulation and mixing of the tank contents reduces dead and stagnant zones and ensures access of all the entire algal population to light, as well as reducing the tendancy of adhesion to the tank surfaces.

3. Application in Anaerobic Digestion

[0134] Anaerobic digestion of already digested sludge by processing in an airlift bioreactor is used for nutrient and energy recovery from biomass. It is used to breakdown organic matter into methane (CH₄), carbon dioxide (C0₂), hydrogen sulphide (H₂S). Digested sludge is dried and used for fertilizer. There are four biodegradation stages. The rate of gas generation through mesophilic anaerobic digestion is generally high, yet the remaining dissolved gases in a digested sludge have a pejorative effect on the environment when they are eventually released, as well as causing operational difficulties. The generation of biogas in an already digested sludge causes cavitation phenomena in pumps. An airlift bioreactor (ALR) is used as anaerobic digester in the present invention to remove the produced gases from digested sludge, with a resultant reduction in pathogens and odour, as well as improvement digested sludge for fertilizer. As already discussed above, ALRs have many valuable benefits in comparison with stirred tanks for instance: there are no moving parts inside the reactor, low cost of installation and maintenance, and low energy required. In addition, using an airlift reactor enhances the mixing efficiency. The process is preferable to agitation by stirring in conventional tanks on power consumption grounds. The experimental data discussed below shows that the cumulative methane production of an airlift anaerobic digester is about 30% more than the observed in the conventional anaerobic digester, and even greater efficiency is achieved, as discussed further below, by nutrient supply.

3.1 Penetration of the microbubbles

[0135] In a duplicate procedure, two reactors D1 ,D2 as shown in Figure 23 were arranged, one (D2) acting as a control, and the other D1 being subjected to bubble injection. Each reactor comprises a digester tank 280, 282, with the tank 280 of the reactor D1 being arranged as the tank described above with reference to Figure 18. The sparger 286 was supplied with gas from a nitrogen generator 220. Each tank 280,282 was monitored for pH and temperature through probes 222, 224 and respective controls 226,228. Gas evolved from each tank was collected in collector tanks 240, which each was connected through a selectively engageable gas analyser 242.

[0136] Fresh sludge, taken from a wastewater treatment plant, has physical, chemical and biological properties. These properties change for several reasons, for instance, the type of wastewater, time of sampling and storage, handling and transfer from the wastewater treatment plant to the laboratory, weather conditions and seasonal variation of water treatment equipment design and operating conditions. Biologically, there are many types of anaerobic bacteria exist in wastewater. The activity, type and number of these bacteria depend on the characteristics of the wastewater and weather conditions at the time of collection. This will affect strongly the production of biogas and the efficiency of biodegradation of the organic matter. Since chemical and physical properties for the sewage sludge are variable, this may cause difficulties in linking the results of experiments that are carried out with different sludge batches. Therefore, the present study used samples of the same sludge, which was taken from the same wastewater treatment plant and distributed it to both reactors D1 ,D2 at the same time with the same operating conditions.

[0137] The introduction of nutrients or trace metals into sludge is usually not required because wastewater generally comprises lipids, polysaccharides, protein and nucleic acids which are bio-degraded by anaerobic bacteria to produce biogas and effluent, which used as fertilizer.

[0138] For the successful operation of anaerobic digestion, facultative anaerobes, including methanogenic bacteria and organic particulates should be present in the sludge. The primary clarifier in waste treatment provides particulates and many anaerobes including methane- produce bacteria, whilst the secondary clarifier provides many facultative anaerobes. In the present research, the digested sludge was collected from the outlet stream of a full-scale mesosphilic digester from a wastewater treatment plant in Sheffield city, UK. Digested sludge has methanogenesic bacteria but with low concentration of substrates.

3.2 Results and discussion

[0139] Figure 24a shows that during 170 hours, the cumulative methane production from the airlift anaerobic digester 280 was about 29 % higher than observed in the control anaerobic digester 282. A large amount of methane obtained from the airlift anaerobic digester occurred during the sparging with nitrogen for one hour daily as shown as in the Figure 24b. This indicates that the biogas produced by the biological process remains unreleased in digested sludge due to characteristics of sludge that prevents biogas rising. Therefore any contact with gas bubbles will strip due to the concentration gradient across them. Poor solubility of methane gas enhances gas stripping from digested sludge.

[0140] After sparging, the overall Gibbs free energy becomes more negative, and hence the reaction becomes thermodynamically favourable and moves towards the formation of more products. However, due to the use of digested sludge in this experiment, the substrate composition was minimal and limiting for the methanogenesic bacteria. Thus, during the hydraulic retention time, the production of methane decreased each day as illustrated in the Figure 24b. That reduction occurred in both digesters.

[0141] The essential ingredient in the biological medium is water with a composition of 90- 95% depending on the type of bioprocess. For instance, water content in sludge is around 95%, while 5% consists of micro-organisms, organic matters, elements and suspended solids. Micro-organisms feed on the organic matter and elements to produce gases by metabolic processes. Carbon dioxide, methane, and hydrogen are highest composition gases produced from fermentation process. The ability of these gases to stay in the liquid phase is related to their relatively solubility, which is 1.45 g gas/kg water in the case of carbon dioxide, 0.0215 g gas/kg water in the case of methane and 0.00155 g gas/kg water in the case of hydrogen. It can be seen that C0₂ is relatively highly soluble compared with CH₄ and H₂. Thus, it will be stay in the liquid phase longer as dissolved aqueous gas (C0_2(aq))_. Released carbon dioxide reacts with water to produce carbonic acid. Kinetically, the conversion to carbonic acid is very slow, just 0.2% of carbon dioxide converts to carbonic acid and its ions, while 99.8% remains as dissolved gas. Carbonic acid is a diprotic acid, dissociating into bicarbonate and carbonate ions, and producing two hydrogen atoms ionisable in water.

[0142] From the above, it can be noted that the presence of dissolved carbon dioxide in the liquid phase will produce a hydrogen ion that would be expected to lead to a lowering of the pH. However the pH observed in the airlift anaerobic digester 280 and the conventional anaerobic digester 282 approximately stabilized during the experimental work, (except that a slight change in the airlift digester was observed during sparging of nitrogen). This means that the carbonic acid produced from dissolved carbon dioxide is treated immediately by ammonia produced from biodegradation of protein. The low solubility of the methane contributes to its transfer from the liquid phase to the gas phase. Most of the carbon dioxide remains in the sludge as "dissolved gas" until a suitable opportunity for it to transfer is provided through the driving force of sparging with nitrogen.

[0143] Figure 25a shows the production of carbon dioxide from the anaerobic digester with and without nitrogen sparging. The figure shows that the bubbling system in anaerobic digestion contributes to increasing the carbon dioxide in biogas production. The efficiency with the bubbling system was 350% more than with the control digester. Experimentally, the complex characteristics of the sludge have played an important role in stripping of all gases.

[0144] The same thing happens with H₂S. The high solubility of hydrogen sulphide contributes to remaining in the sludge as H₂S(aq). When H₂S dissolves in sludge, the pH, also, would drop due to releasing a hydrogen ion and forming a weak acid. Indeed, the behaviour of the solubility of hydrogen sulphide is very similar to carbon dioxide because both gases form a diprotic acid in water.

[0145] Sulphate dissolved with a high concentration, can inhibit generation of biogas produced from the anaerobic digestion of wastewater. The most important reason leading to this inhibition is that the sulphate dissolved in wastewater encourages growth of sulphate-reducing bacteria, which consume acetic acid and hydrogen that would otherwise be consumed by methanogenesic bacteria. This competition between the sulphate-reducing bacteria and the methane-producing bacteria for the consumption of the hydrogen and acetic acid can be illustrated thermodynamically through the equations:

Methanogensis:

C0₂ + 4H₂ CH₄ + 2H₂0 ΔΘ = -135 kJ Eq.12

CH₃COOH + 4H₂ CH₄ + C0₂ AG = -28.5 kJ Eq.13 Sulphate reduction:

S0₄ ^2" + 4H₂ 2H₂0 + 20H^" ΔΘ = -154 kJ Eq.14

S0₄ ^2" + CH₃COOH H₂S + 2HC0₃ ^" AG = -43 kJ Eq.15

[0146] From above equations, it can be seen that the sulphate-reduction reactions have greater thermodynamic driving force than methanogenesis. Therefore methane production is inversely related to sulphate concentration. It has also been discovered that H₂S has a negative impact on the methane production bacteria. It is also suggested that the concentration of H₂S can be taken as an indicator of inhibition of methanogenesic bacteria (GERARDI M. H., 2003).

[0147] Removal of dissolved H₂S from sludge prevents inhibition of methanogenesic bacteria and is desirable also to reduce odour from digested sludge. Normally, the removal of C0₂ and H₂S take place by biogas generated (CH₄ and C0₂) or by contact with the head space in the top of sludge. But this is insufficient to remove the dissolved gases. Mixing of the digested sludge provides intimate contact between sludge and bubbles of biogas and headspace. However; the characteristics of digested sludge require high energy to make it. Using an airlift digester with low energy requirement helps to remove most of hydrogen sulphide generated.

[0148] Figure 25b shows the hydrogen sulphide removal from digested sludge during nitrogen bubbling. The figure indicates that with one hour of nitrogen sparging with fine bubbles, there is a stark increase in the removal of hydrogen sulphide compared to a conventional digester.

[0149] The benefits of the airlift bioreactor are illustrated through the above results. Low energy, good mixing, and enhancement of stripping gases are the most important of the characteristics of airlift bioreactor that were utilized in this study. More methane and more stripping of carbon dioxide and hydrogen sulphide were obtained from this utilization.

[0150] However, while bubbling nitrogen during the initial, growth and end phases of anaerobic digestion enhance methane production for the reasons explained above, a further development is in recycling biogas produced by the digestion during the growth phase. This appears counterproductive, because biogas has a composition of approximately 60% carbon dioxide and 40% methane, which broadly balances the concentrations in solution. Consequently, bubbles of gas with this constitution do not alter the composition of the digester through chemical imbalance, (where the bubble surface acts as a membrane across which a concentration gradient exists). Nevertheless, it is found that bubbles of biogas increase in size and volume following injection. The reason for this is that methane is "strongly" gaseous at room and warmer temperatures (anaerobic digestion is exothermic and can elevate temperatures of digesters to circa 35°C). However, methane is also "sticky", with respect to particles and bacteria surfaces, and therefore does not easily escape the liquid phase after its release by the producing bacteria. Collisions with biogas bubbles however provide an opportunity for methane to escape the liquid environment and return to the gas phase and thus enlarge the bubbles cycling through the digester. As a result, the concentration of carbon dioxide, and other gases such as hydrogen sulphide, therefore inevitably also decreases within the bubbles as methane is absorbed. By that means, a concentration gradient is restored across the bubble surface driving more dissolved gases into the bubble. This saves having to employ nitrogen. [0151] In a final development, it is found that pure carbon dioxide can be injected during the growth phase. This also has the counterintuitive effect, not of affecting methane extraction, but of course of adding to the carbon dioxide loading of the digester. However, it is found that the reaction-limiting factor of methanogensis is not a lack of hydrogen in equations Eq.12 and Eq.13 above (and Eq.12 in particular), but firstly the inhibiting effect of retained methane AND a lack of appropriate fuel (carbon dioxide) for the bacteria. The bubbles have therefore also comprised pure carbon dioxide (optionally exhaust carbon dioxide from other processes, for example, the combustion of biogas already produced and used in the generation of electricity). Consequently, in this context "pure" carbon dioxide may well contain impurities. Nevertheless, not only do the bubbles of such gas extract the methane, just as biogas or nitrogen does, but also they provide additional fuel to encourage growth of the methanogenesic bacteria. Consequently, the yield of methane is yet further enhanced.

[0152] At the end of the growth phase, it is still preferred to return to nitrogen purging, for the purposes explained above, to both remove vestigial dissolved gases and neutralise the final digestate. The result is a digestate that does not require drying, but instead can be distributed in liquid form without damaging pumps (through cavitation problems) or unprotected pipelines (through corrosive acid attack). Indeed, in appropriate situations, the digestate can be pumped from the digester through pipelines directly to irrigation arrays and ditches on agricultural land for fertilisation purposes. This saves significant energy costs in otherwise drying and transporting digestate by road.

[0153] The bubbling of gas into the anaerobic digester may be undertaken once or twice daily over a period of perhaps one hour on each occasion. If the bubbles are small, in the order of 10-30 μηι then they have such a slow rise rate in the tank that they will remain in place for up to 24 hours. During that period they are resident in the digester and, if sufficient quantity of bubbles are injected, and the bubbles are small enough, microorganisms throughout the digester can access bubbles to shed themselves of the inhibiting methane.

[0154] As mentioned above, the same anaerobic processes can be employed in the anammox process, for the digestion of nitrates by producing nitrogen. The application of the anammox process lies in the removal of ammonium in wastewater treatment and consists of two separate processes. The first step is partial nitrification (nitritation) of half of the ammonium to nitrite by ammonia oxidizing bacteria:

4NH₄ ⁺ + 30₂ 2NH₄ ⁺ + 2N0₂ ^" + 4H⁺ + 2H₂0 Eq.16

[0155] The resulting ammonium and nitrite are converted in the anammox process to dinitrogen gas and circa 15% nitrate (not shown) by anammox bacteria: NH₄ ⁺ + Ν0₂ ^' Eq.17

[0156] Both processes can take place in a single reactor.

4. Particle Separation

[0157] Returning to particle separation, usually, the surface charge on particles may develop from several sources, as most particles have complex surface chemistry. Nevertheless, studies have shown that the electrical charges found on the surface of particles develops from four main pathways namely: isomorphous replacement (also known as crystal imperfection), structural imperfection, preferential adsorption of specific ions and ionization of inorganic groups on particulate surfaces.

[0158] Regardless of the mode of development of electrical charges on colloidal particles, distribution of these charges on their surface affects the dispersion of ions in the surrounding interfacial region, leading to a rise in the concentration of counter ions (oppositely charged ions) near the surface of the particle to satisfy electro-neutrality. In Figure 26, a negatively charged colloid particle 300 has closely and firmly packed opposite ions 310 (positively charged) surrounding the particle surface, referred to as the Stern layer. This is followed by a layer 320 of relatively less strongly held ions, found just away from the particle surface. These two arrangements of charges are referred to as the double layer 315. Further away from the double layer 315 there exists loose ions 330 that result in the formation of a diffuse layer. A shear plane extends from the Stern layer to the diffuse layer. The shear plane is loosely attached to the particle relative to the Stem layer, but is unsusceptible to an external velocity gradient in the liquid, and is therefore bound to the particle as the particle moves within the liquid continuous phase.

[0159] The electrical potential difference between the colloidal particle in the shear plane and the liquid bulk is known as the zeta potential and decreases away from the particle as shown in Figure 27. In essence, the zeta potential is a measure of the electrical charge of a colloidal particle. A denotation of the potential stability of the colloidal system can be given by the magnitude of the zeta potential and it can be mathematically expressed as in Eq.1 above.

[0160] The dispersion of a solid in a continuous fluid results in a colloidal system. The higher the magnitude of the zeta potential, the higher the repulsion between particles and consequently, the more stable is the dispersion, and the lower is particle-particle agglomeration. However, if the zeta potential of the particles is low then there is no force preventing them from agglomerating and flocculating.

[0161] In general, the differentiating factor between a stable and an unstable suspension can be taken as +30 mV or -30 mV. Mean zeta potential for colloidal particles in wastewater ranges from -12 to +40 mV. A crucial factor influencing the particle zeta potential however is the medium pH. Figure 28 illustrates a typical zeta potential in different acid/alkaline conditions. Usually, under alkaline pH, the magnitude of the zeta potential increases as pH increases. Conversely, as pH tends towards acidity, this magnitude reduces until a point is reached where neutrality is attained (zeta potential = zero). This point is referred to as the isoelectric point (IEP) and results in the presence of multiple counter ions close to the particle body. Particles have the highest potential for agglomeration at the isoelectric point. Beyond this point towards acidity, the net charge becomes positive and electrostatic repulsive forces again become an issue preventing agglomeration and flocculation.

[0162] Obviously, one of the rate limiting factors in separation by flotation is the agglomeration of particles. Until the repulsive force existing between particles is neutralized, particle agglomeration will not occur. Several methods for enhancing particle size have been explored and reported. In general, the similarity of these techniques is to induce particle-particle attraction by overcoming the repulsive force. The known agglomeration processes are: selective flocculation, hydrophobic agglomeration and coagulation.

[0163] The process of selective flocculation involves the formation of floes by bridging between target particles. Long chain polymers are added to the dispersion which adsorb onto the surfaces of mineral particles by electrostatic forces before bridging with other particles to form loose floes. This technique is widely used in the mineral industry where selective mineral separation is required, but as yet to be fully explored in other fields such as potable water treatment, waste water treatment etc.

[0164] Hydrophobic aggregation is similar to froth flotation where particles are held in close proximity to be selectively hydrophobised. The particles undergo strong agitation. Non-polar oil could be an additive to improve aggregate strength. Other types of hydrophobic aggregation include: emulsion flotation, shear flotation, oil-extended flotation, spherical agglomeration, carrier flotation and two liquid extraction.

[0165] Coagulation differs from selective flocculation in that the addition of an electrolyte causes a decrease in electrostatic repulsion between particles. The energy barrier between particles that prevents agglomeration is overcome by coagulant addition. The disadvantage associated with this method of particle agglomeration is that it produces heterocoagulation, and so it is mainly employed in fields other than the mineral industry. Nonetheless, aggregation by coagulation is still the most widely applied technique of the three sorts but choice of technique ultimately depends on the recovery process as well as the desired end product. [0166] Particle destabilization by the addition of a coagulating or flocculating agent occurs by four (4) known mechanisms viz: the compression of the electrical double layer, adsorption and charge neutralization, adsorption and inter-particle bridging and the enmeshment in a precipitate. Broadly, there are two main categories of coagulant and flocculants viz: organic and inorganic coagulants and organic flocculants

[0167] Metal salts are the most common coagulants available and are still widely employed in water purification with aluminium salts being the most commonly used. These cations hydrolyse rapidly in the liquid medium and interact with particles, neutralising their net surface charge. When aluminium salts are added to an aqueous solution a rapid hydrolysis reaction occurs to form other dissolved Al ions. The main Al-hydroxide precipitates that result following dissolution of the metal salts are: Al³⁺; AI(OH)²⁺; AI(OH)^{1 2+}; AI(OH)^{1 4"} and the amorphous AI(OH)_3(am). Al species distribution in an aqueous solution is however pH dependent. In acidic pH, Al³⁺ is the predominant species present. But with increase in pH, Al ions with lower positive charge become dominant. As pH exceeds 6.5, the most active species are the AI(OH)^{1 4}\

[0168] Similarly, the presence and concentration of Fe³⁺ species increases under acidic pH when ferric salts undergo dissolution, but the concentration decreases with a shift in pH towards neutrality with the formation of more Fe(OH)²⁺ and Fe(OH)₂ ⁺ species.

[0169] Speciation of coagulants can also be temperature dependent. In cold water, positively charged Al species dominate, but this is less important than pH. However, what has not been appreciated in the past is that it is not the pH in the bulk liquid that matters but only the pH in the region between particles. A bubble is just another particle, although with different zeta potential compared with particles in suspension in a liquid. A bubble carrying a gas that permits active species to cross the boundary between the bubble and liquid in which it is immersed will affect, first, the pH in the immediate environment of the bubble. If that permits a bubble to approach a particle before electrostatic repulsion takes place, the attraction of particle to the gas phase by virtue of its hydrophobic nature permits attachment of the particle to the bubble before it is electrostatically repelled. The zeta potential (or isoelectric point) is squeezed between the bubble carrying the pH-adjusting gas and the particle.

[0170] Where the particles require a more acidic pH, therefore, a bubble carrying carbon dioxide, for instance, will release H⁺ ions and reduce the pH as it dissolves in the aqueous environment. Whereas, in the more unusual case where particles require more alkaline conditions, a gas comprising ammonia will release OH^" ions increasing pH.

[0171] However, once the bulk liquid is fully mixed, the pH is different from what would be expected to result in efficient separation. The result is that the liquid is not rendered more or less acidic than necessary which may have its own environmental benefits. In addition, it means that less flocculant/coagulant needs to be added, or less pre-adjustment of the pH is required.

[0172] Returning to Figure 26, a microbubble 400 is seen approaching the particle 300. Because the bubble is so small (circa 40μηι in diameter) and has been injected into the liquid with little energy, it has surrounding it a boundary layer 410 that is, to all intents and purposes, stationary with respect to the bubble. Boundary layers are usually defined as the radial position relative to the bubble centre where 99% of the free stream velocity is achieved. Within the boundary layer, it is well known that diffusion dominates over convection of mass. The bubble contains carbon dioxide that establishes, by diffusion of ions from the bubble, a pH gradient across the layer 410, between a minimum pH at the bubble surface 420, towards the bulk liquid pH in the diffuse layer 330 around the particle 300. Given the external pH and the internal concentration of C0₂ as controllable parameters, a position with attractive electrical potential can be engineered to occur. In the absence of the bubble 400, the zeta potential from the particle surface 312 may be as shown in solid line 500 in Figure 27. However, as the bubble approaches, the zeta potential between the particle and bubble is squeezed towards that shown by dotted line 510 in Figure 27, as H⁺ ions surrounding the bubble in the boundary layer 410 invade the diffuse layer 330. As a consequence, the electrostatic repulsion of the bubble by the particle that would pertain if the bubble contained a non-pH-adjusting gas is inhibited, possibly sufficiently for the bubble and particle to attach. Given that the particle is generally hydrophobic, once the double layer 315 is breached, the particle 300 attaches to the bubble 400 and thereby can be lifted with it, along with other particles, and ultimately be taken to the surface of the liquid in the tank.

[0173] The particles to which the above application applies may be residual waste particles in an anaerobic digester as described above or algal species grown in bioreactors as described above, but the invention is not limited to such particles.

[0174] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

[0175] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

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Claims

1. An anaerobic digester comprising a liquid fermenter tank for anaerobic microorganisms and a diffuser of a microbubble generator to introduce bubbles of non-oxygen containing gas into the digester whereby methane and acid gases produced by the digestion are exchanged across the bubble surface to strip such gases from the liquid when said bubbles connect with a header space of the tank.

2. An anaerobic digester as claimed in claim 1 , wherein, at a commencement phase of said digestion, said gas is nitrogen or another inert gas that strips the fermenter of oxygen to promote the onset of anaerobic digestion.

3. An anaerobic digester as claimed in claim 1 or 2, wherein during a growth phase of the microorganisms in the digester, said gas is or comprises the biogas generated and released into the head space of the fermenter tank.

4. An anaerobic digester as claimed in claim 3, wherein the biogas comprises approximately 60% methane and 40% carbon dioxide.

5. An anaerobic digester as claimed in any of claims 1 to 4, wherein, during a growth phase of digester, said gas is or comprises more than 90% carbon dioxide.

6. An anaerobic digester as claimed in any of claims 1 to 5, wherein, at an endphase of said digestion, said gas is nitrogen or another inert gas that serves to strip methane and hydrogen sulphide from the fermenter to neutralise the acidity of digestate liquid remaining.

7. An anaerobic digester as claimed in any of claims 1 to 6, wherein the carbon dioxide is sourced from one of:

power production from combustion of the methane produced by said digester; and from sequestered carbon dioxide from another source, for example power station waste gas.

8. An anaerobic digester as claimed in any of claims 1 to 7, wherein the bubbles have a size in the range 10 to 100 μηι.

9. An anaerobic digester as claimed in claim 8, wherein at least a proportion of the bubbles have a size in the range 10 to 30 μηι accounting for at least a gas holdup of 0.05%.

10. An anaerobic digester as claimed in claim 9, wherein at least a proportion of the bubbles have a size in the range 10 to 30 μηι accounting for at least a gas holdup of 0.5%.

11. An anaerobic digester as claimed in any of claims 1 to 10, wherein the tank has sides and a base and the liquid in the tank has a top surface above which is a header space; and the diffuser is disposed in the liquid at the base of the tank and is arranged to inject bubbles of gas into the liquid in the tank whereby the apparent density of the liquid above the diffuser is reduced by the bubbles thereby creating a flow of the liquid, which flow is:

up the tank in a riser section thereof,

12. A method of operating an anaerobic digester as claimed in any of claims 1 to 11 comprising the steps of:

c) rendering the tank anaerobic;

d) periodically, and on each occasion over a limited period of time, introducing bubbles of non-oxygen containing gas from the diffuser, at least a proportion of which bubbles have a size in the range 10 to 100 μηι.

13. A method as claimed in claim 12, wherein step d) is repeated daily.

14. A method as claimed in claim 12 or 13, wherein the period of time in step d) is between 30 and 120 mins.

15. A method as claimed in claim 12, 13 or 14, wherein the gas holdup of the bubbles in the tank one hour after at the end of the period provided in step d) is at least of 0.05%.

16. A method as claimed in any of claims 12 to 15, wherein the step of rendering the tank anaerobic comprises introducing bubbles of nitrogen gas from the diffuser.

17. A method as claimed in any of claims 12 to 16, wherein the bubbles introduced in step d) comprise carbon dioxide.

18. A method as claimed in claim 17, wherein the bubbles introduced in step d) comprise more than 90% carbon dioxide.

19. A method as claimed in any of claims 12 to 18, wherein, at the end of the digestion when the yield of biogas from the digester falls below a predetermined amount, step d) is repeated a final occasion with said bubbles comprising substantially only nitrogen gas, whereby remaining methane, carbon dioxide and hydrogen sulphide is stripped from the digestate to neutralise the pH of the digestate.

20. A method as claimed in claim 19, wherein the digestate is thereafter pumped from the tank and transported by pipeline to an irrigation array for land fertilisation.

21. A method of separation of particles suspended in a liquid, the method comprising the steps of:

22. A method as claimed in claim 21 , in which the desirable pH is determined by the kind of a coagulant or flocculant added to the liquid to facilitate coagulation or flocculation of the particles.

23. A method as claimed in claim 21 or 22, in which the desired pH is acidic, preferably between 5 and 7, and the gas is or contains carbon dioxide.

24. A method as claimed in claim 21 , 22 or 23, comprising the steps of:

providing a suspended particle separation tank comprising: a floor having a floor area; liquid in the tank having suspended particles; and a diffuser of a microbubble generator, which diffuser is disposed at the base of the tank and is arranged to inject bubbles of gas into the tank to reduce the apparent density of the liquid above the diffuser, thereby creating a flow of the liquid and gas bubbles that is a non-turbulent laminar flow having a Reynolds number less than 2000, wherein more than 90% of the bubbles have a size in the range 10 to 100 μηι diameter and a bubble density of at least 50million per cubic metre, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser;

filling the tank with the aqueous mixture of suspended particles to be separated from the water, adding a flocculant to cause the particles to coagulate, and injecting said microbubbles into the tank; and

wherein the acidity of the mixture appropriate for the selected flocculant is adjusted locally around each bubble by said bubble introduction.

25. A method as claimed in claim 24, wherein the bubbles are injected into the tank across substantially the entire area of the floor of the tank.

26. A method as claimed in claim 24 or 25, in which said bubbles are injected until a desired level of clarity of the water remains and until a sludge layer on the surface of the water comprising the particles separated from the water has thickened and thinned.

27. A method as claimed in claim 24, 25 or 26, in which the flocculant is a metallic inorganic coagulant.

28. A method as claimed in claim 27, in which the flocculant is a salt of one of iron and aluminium salts, added in solution, said salts dissociating into Fe³⁺ and Al³⁺ ions respectively, as well as soluble complexes having varying high positive charges.

29. A method as claimed in claim 28, in which the flocculant is FeCI₃, the pH is adjusted to between 5 and 7 and the bubbles comprise carbon dioxide.

30. A method as claimed in any of claims 24 to 29, in which the bubbles have a size in the range 20 to 40 μηι diameter.

31. A method as claimed in any of claims 24 to 30 in which the particles are algae and the separation tank also serves as an airlift bioreactor.

32. A method as claimed in any of claims 24 to 31 , wherein the Reynolds number is less than 200.

33. A method as claimed in any of claims 24 to 32, wherein more than 90% of the bubbles have a size in the range 20 to 50 μηι diameter.

34. A method as claimed in any of claims 24 to 33, wherein the bubble density is at least lOOmillion per cubic metre.

35. A method as claimed in any of claims 24 to 34, wherein the gas holdup above the diffuser is less than 2%.

36. A method as claimed in claim 35, wherein the gas holdup is less than 1 %.

37. A particle separation system to separate particles from an aqueous suspension or colloid of the particles, the system comprising:

dissolution in the liquid of molecules or radicals of the gas in the microbubbles across the bubble/liquid boundary, which dissolution establishes an acidity gradient in the liquid across the boundary layer, such that, at a distance from the bubble surface where repulsion of a particle by electrostatic forces would otherwise occur, the acidity of the liquid is such that said repulsion does not occur or is inhibited and binding to and lifting of the particle by the bubble is more effectively achieved than would be the case if the average acidity of the liquid was pre-adjusted by to the same extent as achieved by said introduction of microbubbles of said pH-adjusting gas, and the gas introduced by the microbubbles was not said pH-adjusting gas.

38. A system as claimed in claim 37, comprising a suspended particle separation tank, the tank comprising:

a floor having a floor area;

liquid in the tank having suspended particles; and

39. A system as claimed in claim 38, which is also a liquid fermenter tank for microorganisms, wherein the tank has sides and said floor, the liquid in the tank has a top surface above which is a header space, and said flow is:

up the tank in a riser section thereof,

40. A system as claimed in claim 39, further comprising a physical divider located in the tank to divide and define the riser and downcomer sections of the tank.

41. A system as claimed in claim 40, wherein the divider is a draft tube.

42. A system as claimed in claim 41 , wherein the draft tube and tank are cylindrical in section.

43. A system as claimed in claim 42, wherein the ratio of the diameter of the draft tube to the diameter of the tank is between 0.5 and 0.8.

44. A system as claimed in claim 43, wherein the ratio of the diameter of the draft tube to the diameter of the tank is between 0.6 and 0.7.

45. An anaerobic digester as claimed in any of claims 1 to 11 , a method of operating an anaerobic digester as claimed in any of claims 12 to 20, a method of separation of particles suspended in a liquid as claimed in any of claims 21 to 36, a particle separation system as claimed in any of claims 37 to 44, wherein the method of producing said microbubbles of gas in the liquid comprises the steps of:

providing a source of the gas under pressure;

oscillating the gas passing along said conduit with a fluidic oscillator,

46. A method as claimed in claim 45, wherein said oscillations effected by the fluidic oscillator are effected at a frequency between 1 and 2000 Hz.

47. A method as claimed in claim 45 or 46, wherein the bubbles formed are between 0.1 μηι and 100 μηι in diameter.

48. A method as claimed in claim 47, wherein the bubbles formed are between more preferably between 30 and 50 μηι in diameter.

49. A method as claimed in claim 46, 47 or 48, wherein said oscillation is of the type that has between 10% and than 30% backflow of gas from an emerging bubble.