WO2017046456A1

WO2017046456A1 - A method of treating water

Info

Publication number: WO2017046456A1
Application number: PCT/FI2016/050646
Authority: WO
Inventors: Markku SUUTARI; Ilmo KOLEHMAINEN
Original assignee: Pajupojat Oy
Priority date: 2015-09-18
Filing date: 2016-09-19
Publication date: 2017-03-23
Also published as: FI126642B; EP3350133A1; FI20155671A

Abstract

A method and a system for separation of impurities, especially nutrients, from wastewater. In the present method, water having a low nutrient content is brought to flow through at least two zones, which zones are arranged in the direction of the water flow in a desired order, and which comprise, independently of each other, material which is intended to separate impurities and which is selected from a group that is comprised of living willows with roots, willow chips, carbonised parts of willow, or a mixture of willow chips and carbonised parts of willow, in which case at least two zones comprise materials that differ from each other. This solution can be used to purify impure, typically nutrient-rich wastewaters, such as household and municipal wastewaters, as well as farming and agricultural leachates, for example surface water and storm water originating in fields, barns, pig houses, sheep barns, henhouses, stables and fur farms, and to recover nutrients

Description

Method of treating water

Field of Technology

The present invention relates to the recovering of nutrients and harmful substances in water and in other nutrient-rich liquids. In particular, the invention relates to a method, according to the preamble of Claim 1 , of treating impure water, especially nutrient-rich water. The present invention also relates to a water treatment system according to the preamble of Claim 23 and the use according to Claims 29-32.

Background of Technology

The treatment of wastewater using biological means is a typical way to exploit microbial chemical reactions, in which reactions the microbes "digest" organic waste and nutrients in the water and they take part in binding inorganic compounds. The bacteria also convert the nutrients into another form. All of this requires oxygen and nutrients for the microbes and a suitable breeding environment. There are also known methods in which microbes and their enzymes are not used - such methods are direct oxidation of wastewater, as well as direct chemical precipitation. Direct oxidation can be carried out with ozone (0₃), or peroxides (-OOH) or per-compounds, such as permanganates or hypochlorites. Various methods for treatment of wastewater by biological methods are described in the patent literature. Thus, FR application publication 2876047A1 describes a wastewater treatment in which the wastewater is allowed to flow through the roots of plants, in which case the water is alternately in an aerobic state and in an anaerobic state. The trees mentioned in the document are, among others, willows. CN application publication 101965785A1 describes how cadmium-containing water is purified by planting willow twigs as willow cuttings into cadmium-containing soil, in which case the growing willows bind the cadmium ions. EP application publication 2360123A1 describes how agricultural wastewaters can be purified by filtration through the soil, in which case they are finally evaporated by using rapidly growing plants, such as willow and bamboo.

CN application publication 103252216 A describes a method of purifying ammoniated wastewater, in which method wood chips which are impregnated with ferric hydroxide, i.e. Fe(OH)3 are used.

According to CN application publication 10343541 OA, biochar is used in the fertilisation of flowers and other plants, as part of the cultivation ground, in such a way that the carbon is allowed to absorb nutrients from the fertiliser solution, after which it later releases them slowly to the plants, as part of the cultivation ground.

CN application publication 102515363 A describes a mixture of biochar and peat, which mixture is floated, by using a frame, on the water surface of a lake or river. In this frame, plants grow and their roots purify the water.

EP application publication 2256091 Al describes how Petasites hybridus growth binds nutrients, after the phosphorus has first been precipitated from the wastewater, by using methods which are known per se. The nitrogen is evaporated by aeration.

Further background art is represented by JP 2008212843 A, US 2008197073A1, JP

2006015256A, CN 104556589A, JP 2000185294A, CN 100540482C and Christianson, L. E., et al. Suitability of three wood species for denitrification bioreactor fill, American Society of Agricultural and Biological Engineers Annual International Meeting 2014, Vol. 2, pp. 1447- 1457. The above-mentioned methods provide an insight into biological wastewater treatment which utilises plants in one way or another.

It has also been proposed that non-aborescent plants can be effective ion-exchangers for heavy metals, when the plant has first been carbonised to a certain degree, so that the ash content of the carbonised plant part is 15-50 %. The ion exchange takes place in water. The best result is achieved when the plants are harvested during the active growing season.

As will appear from the above, by using the background art solutions it is possible to bind individual impurity components present in water, such as wastewater. To date, no technically simple solution has been proposed, with which a significant part of all of nutrients, heavy metals and toxic, other non-desired compounds, which are contained in it and particularly which are soluble in it or which are dissolved in it, can be separated and, if desired, be recovered or transferred for utilisation.

Summary of the present invention

It is an aim of the present invention to eliminate at least some of the problems associated with the background art and to provide a novel solution for the purifying of wastewater, such as nutrient-rich water.

The present invention is based on the idea that the nutrients contained in the water can be separated by bringing the wastewater to flow through at least two zones that are arranged in optional order, which zones comprise growing willows, parts of willow or carbon made from willow.

Accordingly, the water treatment system comprises at least two zones, through which nutrient- rich water can be arranged to flow, and which zones are arranged in the direction of the water flow in a desired order, and which contain, independently of each other, material which is intended for separating impurities and which is selected from a group that comprises

- living willows with roots,

- willow chips, or other wood chips - carbonised pieces of willow, or other biochar that is activated, treated with catalysts or untreated, or

- a mixture of willow chips and carbonised pieces of willow, a mixture of chips and biochar, in which case at least two zones comprise material that differ from each other.

The zones can be separated from each other, or two or more zones can be combined into a single entity.

The carbonised pieces of willow may be chemically modified, or otherwise activated.

The method and the system can be used to purify impure, typically nutrient-rich wastewaters, such as household and municipal wastewaters, as well as farming and agricultural leachates, for example surface water and storm water originating in fields, barns, pig houses, sheep barns, henhouses, stables and fur farms, as well as water from underdrains.

More specifically, the method according to the present invention is mainly characterised by what is stated in the characterising part of Claim 1.

The system according to the present invention is characterised by what is stated in the characterising part of Claim 23, and use according to the present invention is characterised by what is stated in Claims 29-32.

Considerable advantages are achieved with the present invention. Thus, the present invention offers a solution for purifying water that has a low nutrient content by means of a new and versatile method in which the components used (wood chips, carbon, biomass of willow) can be further used as fertiliser and/or soil amendment.

This solution also facilitates the recycling of nutrients when the nutrients are bound to the biomass by the process of growing. Such nutrients and the carbon can in the future act as a nutrient source for the plants and increase the soil carbon content, and possibly act as a breeding ground for microbes. A nitrification process occurs in the chips made of willow in which process the nitrogen compounds are converted into a form suitable for plants. In this case, the ammoniacal nitrogen is converted into nitrate-nitrogen and also partly into elemental nitrogen (denitrification process).

Carbon derived from willow ("willow carbon") provides a variety of features. A growing willow uses and absorbs a lot of nutrients from the soil due to intense evaporation, and certain willow species also absorb heavy metals such as cadmium. In the following, the present invention will be examined in more detail with the help of a detailed disclosure, representative embodiments of the present technique being disclosed in the following description.

Brief Description of the Drawing

The figure shows a process flowchart of a water treatment arrangement according to one embodiment of the present invention.

Description of Embodiments

In the present context, "willow" means a wood species belonging to the Salix L. genus of the family Salicaceae. It is well known that there are numerous species of willows. They grow especially at the watersides of water systems, such as brooks, ditches and small rivers. Due to their rapid rate of growth and regenerating capacity, willows are well suited to be a source of energy wood. Their natural structural porosity is an advantage particularly in carbonisation. Examples of willows species are the following: Pale willow (Salix starkeana), Pale willow (Salix starkeana ssp. cinerascens), Bay willow (Salix pentandra), Creeping willow (Salix repens), Creeping willow (Salix repens ssp argentea), Violet willow (Salix daphnoides), Peking willow (Salix babylonica), Almond willow (Salix triandra), Swamp willow (Salix myrtilloides) , Halberd willow (Salix hastata), Crack willow (Salix fragilis), Dark-leaved willow (Salix myrsinifolia), and Polar willow (Salix polaris), Goat willow (Salix caprea), as well as Eared willow (Salix aurita). Examples of planted willows growing in Finland are, among others, White willow (Salix alba), Silver willow (Salix alba var. sericea 'Sibirica'), as well as Osier (Salix viminalis).

In the present technology, willow is particularly advantageous. This is for example because its root system on the one hand remains at or near the soil surface, and on the other hand that system seeks out nutrients and generates widespread root hairs. The willow root system does not generate a taproot like many other tree species. The willow is also resistant to very strong humidity changes in the soil. It is also possible to influence the willow roots by regenerating the willow. Thus, regenerating the willow growth every 2 or 3 years, for example by completely cutting down the growth aboveground, i.e. the growing willows, new growth is encouraged and the root system will regenerate. At the same time, the root system remains particularly in the surface layers of the soil. When a mycorrhiza is brought to the root system, either in the plants or as separately sprayed mycorrhiza, the mycorrhiza will improve the efficiency of the nutrient uptake. In association with the cutting down, part of the root mass dies, and this provides the bacteria and the microbes with more breeding ground and gradually produces humus.

In the present invention, willow chips, carbonised willow pieces or a mixture of willow chips and carbonised willow pieces, are used to purify water. In addition, willows or similar wood species of the Salix L. genus are grown in a place where they can utilise the nutrients in the wastewater by using their roots.

The figure shows a process flowchart of one embodiment, in which reference number 1 refers to a water flow retardation basin, reference number 2 refers to a chip field (chip zone), reference number 3 refers to a field (zone) which comprises porous carbon, and reference number 4 refers to willow plants. Reference number 5 refers to an optional post-filtration zone in which the nutrients which have passed through the preceding zones are attached, for example, to modified carbon.

Parts can be cut from the willows 4 by cutting off pieces from growing willows every year, or every few years or during a period of a few years. In one embodiment, "pellets" are made from the thinnest parts by cutting. These pellet-sized parts are torrefactioned and/or carbonised (resulting in "biochar") and used in the carbon field 3. The thickest parts of the willow are chipped into normal chips. These chips can be used in the chip field 2. All of the shoots can be chipped to the size range of 25-35 mm for the production of biochar, or coarser for chip use.

In one embodiment, willow thicket 4 is grown in such a way that by cutting willows at maximum every four years, especially at maximum every three years, it is possible to make the willow root system extend to a depth of at maximum 50 cm. This is advantageous, because, in the present technology, the water is conducted to the willow thicket primarily along the surface of the ground, or along ditches dug in the surface, in which case the water penetrates generally at maximum to a depth of 50 cm.

In one embodiment, mycorrhiza is brought to the growing willows, which mycorrhiza thrives especially in willows. Mycorrhiza improves the tree's capacity to absorb and use the nutrients in the soil. Depending on the place of growth and the application, it is possible to engraft the mycorrhiza or, correspondingly, infect the roots with mycorrhiza, which is present in the soil.

In particular, a mycorrhiza which is suitable for the willows and growth-promoting is used.

In the applications of the new technology, the wastewater to be purified or the nutrient-rich water is typically brought to pass, in a determined order, through the zone 2 formed of chipped willow parts, the zone 3 formed of carbonised "willow pellets" and, correspondingly, through a zone formed of the root system 4 of growing willows.

In order to regulate the water flow rate, in particular to slow down the water flow before the zones 2-4, it is possible to arrange a water flow retardation basin shown in the figure, the size of which basin is selected according to the volume of the water to be treated and the flow rate. By using the retardation basin, it is possible to slow down the water flow in such a way that the carbons, the chips and their microbes in the zones 2-4 have time to react and bind nutrients from the water. There may be one or more basins, typically the number is 1-15. The basins can be arranged side by side or in series, or both side by side and in series. In one

embodiment, one or more of the basins are equipped with a control valve and preferably with a bypass pipe, in which case the control valve will lead excess runoffs (flood situation) through the bypass pipe, passing the next zones or zone in the flow direction.

The waters to be treated, which typically have a low nutrient content and which are conducted into the retardation basin, are for example surface runoff water (storm water) of rain and meltwater coming from agriculture, swamps or industries and from construction areas. The waters to be treated are described in more detail below. In the retardation basin, part of the solids matter remains in the basin.

"Zone" generally refers to a region or a volume which comprises the said parts or root system and through which the water can be made to flow.

The zone can be uniform or divided into two or more parts. Thus, there may be several consecutive zones, or there may be several zones comprising the same material at different points of the present system.

The zones can be separate from each other or they may be combined into a single entity. Thus, two or more zones can be combined into a single entity. In one embodiment, which is described below in more detail, the chips and carbon are mixed with each other and the willows, i.e. the willow growth, is arranged in the immediate vicinity of this. In one embodiment, the willow, i.e. the willow growth, is arranged into the

superstructure of the zone which comprises chips and carbon, or such a superstructure is formed of them. Typically, the willows, i.e. the willow growth, grow in the superstructure and remain upright for example due to the superstructure. In one embodiment, the root system of the growth in the superstructure extends to the chips, from which it extracts nutrients and water.

In one embodiment, the said zones are arranged in the order shown in the drawing, in which case the water is first conducted through the chips 2, after that through the carbonised pieces 3, and finally through the root system of the growing willows 4. In another embodiment, the water is brought to flow first in the vicinity of the growing willows, or through their root system, which willow thicket may have been preceded by one or more other zones, for example a chip and/or a carbon zone. After that, the water is possibly conducted through the zone formed of the chips. Finally, the water is conducted through the zone formed of carbonised pieces.

An embodiment in which the last phase of the system and, correspondingly, of the method, is a zone which comprises carbonised pieces, can be used in situations where the water which has passed through the willow thicket still comprises nutrients or organic compounds, for example herbicides residues used in agriculture, such as glyphosates.

If needed, the carbon may be treated chemically to improve its adsorption capacity.

Such a zone comprising carbon may also be arranged as a separate additional zone after the other zones, as described in the embodiment shown in the drawing, see zone 5. The metal- bearing carbon can be separated from the purified water.

Chemically treated carbon binds the nutrients from the water also during winter. When there is no microbial activity, the roots do not absorb water and the growth does not evaporate water. In the above embodiments, the chips can be kept in sacks, in other holders, or the chips are arranged in such a way that a dam retains the chips but not the water.

Similarly, the carbonised pieces can be arranged in the same way as the chips, for example in sacks or other holders or they can form dams. Through all these zones, the water can be conducted in the same way as through the chips.

In the present solutions, the willows grow and the roots, which are mycorrhized or exposed to mycorrhizae or engrafted, collect nutrients which are not desired in the water system.

Impurities adhere to the willow pellets, the chips and the carbon. Thus, at the same time an active microbial activity is generated in these, which activity modifies the nutrients into a form which is usable by the plants. If necessary, the fresh chips may be pre-treated with a suitable microbial strain, in order to make it function immediately, before the microbial activity which is generated along with the water to be treated starts. The carbon absorbs nutrients from the water. For example, the phosphorus in the water is bound to the chips and, in particular to the carbon, by electrical bonds. Therefore, carbon can be modified in such a way that it is able to efficiently bind residual nutrients which have possibly passed through the preceding zones.

In one embodiment, carbon is treated in such a way that it is capable of binding phosphorus which is soluble, and possibly in a particulate form. The purpose of the treatment is in particular to bring the carbon into such a form that is able of binding phosphorus at least temporarily; in one embodiment the carbon is brought into such a form that it is able to bind phosphorus in particular outside the growth period. Then treatment of carbon can be carried out chemically or with a catalyst.

The two- or multiple-zoned purifying process according to the present technology can be achieved by planting and growing willows and using parts of the grown willows, in the form of chips and, correspondingly, carbonised pellets or pieces. However, it is obvious that the chips and the carbonised pellets or pieces can also be generated from sources other than the willows that are planted for the present technology.

In one embodiment, the willows are planted in a location where wastewater or other water to be purified flows. As mentioned, the willows are preferably engrafted with an appropriate mycorrhiza.

In one embodiment, in which surface waters are treated, their flow is first slowed down in a separate basin. This enables at least part of the solids to settle on the basin bottom. The runoff of the surface waters varies, in which case, especially in spring and autumn the runoff is particularly large, i.e. the spring and autumn runoff peaks occur. Correspondingly, in the spring and in the autumn the ability of the willow growth to evaporate water is low. For this reason, in a preferred embodiment of the present technology, excessive volumes of runoffs are led past the purification unit. This is to ensure the correct dwelling time of the water in the purification unit. In one embodiment, the water to be purified is condudcted into and through the purification zones in the surface draining field.

The water can be channelled on the surface or under the ground in such a way that the willow roots come into contact with the flowing water, or so that the willow roots are allowed to find their way into the flowing water, by using channelling, draining or a manifold.

In a preferred embodiment, the water is conducted to the root system area of the willows so that the water is in contact with the ambient air, i.e. the water travels (for example in open ditches, irregularly or in a zig-zag motion), in which case it is at the same time aerated and oxidised.

In one embodiment, the linear speed of the water is at least 1 metre per day, typically at least 5 metres per day, preferably at least 10 metres per day. Most suitably, the linear flow rate is less than approximately 1000 metres per day, especially less than approximately 500 metres per day, preferably less than 100 metres per day.

In one embodiment, from the willow thicket or from other willows, part is annually harvested, chipped or cut into pieces (hereinafter are also referred to as "pellets").

A significant share of the obtained pellets, for example approximately half, is carbonised. The amount carbonised and its subsequent use depend on the quality of the water to be purified, i.e. the amount varies depending on how many impurities the wastewater comprises. The percentages of impurities can be determined with analyses which are known per se. Another part of the pellets will be used uncarbonised, but most suitably they are first treated to increase their surface area. The share of carbon of the total amount of the uncarbonised and the carbonised material may be even 1-75 % by weight, for example 5-60 % by weight, for example even 10 % by weight. This is the case, for example, when the operation unit is large, i.e. a unit which deals with large volumes of water. Thus, in one embodiment, the pellets are compressed or otherwise broken up in order to increase the surface area and to allow as many bacteria and other microbes as possible to settle onto the surface of the wood. The bark is the most porous part of the willow. In one embodiment, the zones which comprise chips and carbonised pieces are arranged before the willow thicket zone. In this case, the water is first allowed to flow through the chips and their carbonised form, after which it is brought to flow through the growing willow root system, where nutrients are bound and converted into a form suitable for plants. The plant roots utilise the nutrients.

In one embodiment, wastewater which comprises harmful heavy metals is first conducted to a zone that comprises carbonised parts of willow, or a mixture of willow chips and carbonised willow parts, after which the thus treated wastewater is conducted into a zone which comprises chips. Following separation of the heavy metals, by using carbonised pellets or similar wood parts, the chips used in the next step can be used as a soil amendment.

Water is thus conducted, among others, through the "pellets" of carbonised willow. Normally, these absorb nutrient cations, which can be used as a fertiliser, when the carbon is later transferred to form the cultivation ground of the plants and a ground additive. It has been found that the carbon balances the moisture of the soil, depending on the porosity of the carbon.

When the willow is carbonised, the calorific value begins to change, as well as the density, when the temperature is above 250 °C and, at the same time, the wetting of carbon with water improves. A preferable carbonising temperature is approximately 300 °C. At this temperature, a sufficient number of pores is generated and the carbon is wetted with water. The wetting can also be improved with surface-active agents, such as isopropanol, when the carbon is wetted for the first time. Generally, by using different treatments, the carbon can be provided with properties which will improve the carbon's ability to bind pre-selected nutrients more effectively. Using the appropriate temperature programme, the carbon will be provided with a high porosity, which is a prerequisite for optimal nutrient binding. By bringing carbon into the C/H ratio required by the microbes, the growth conditions of the microbes will improve. The temperature programme and different surface materials give desired properties to various objects. In one embodiment, a material is generated which has (generally) a high porosity. In another embodiment, a material is generated in which the number of pores and their sizes are significant factors. In general, the willow parts can be carbonised to one, two or three different carbonising levels, for example at approximately 250 °C, 350-500 °C and, correspondingly, at approximately 700 °C. The treatment time is 10-600 minutes, especially approximately 15-360 minutes.

The lowest "torrefactioning" temperature is 250 °C, and the highest temperature

approximately 700 °C. The lowest temperature produces a roasted wood which does not rot or moisten easily. When the temperature exceeds 500 °C, many pores are generated, which adsorb metal cations.

In chips made of willows, a nitrification process occurs, in which the nitrogen compounds are converted into a form suitable for plants. In this case, ammoniacal nitrogen is converted into nitrate-nitrogen and partially also to elemental nitrogen (denitrification process). In this case, the conditions are anaerobic.

This nitrification event is generated on the surface of the chips, because the large surface area and the nutrient base promote the activities of different bacteria. It is advantageous to bring the water to flow in such a way that it sometimes receives oxygen from the air, in which case also denitrification can occur.

The carbonised willow pellets are burned if they have absorbed toxic heavy metals. Otherwise, they are used for soil amendment. As an ingredient of soil amendment, the carbonised willow breaks up the soil. In the carbon pores, typically many microbes grow, which in turn purify the water and neutralise many toxic chemicals. In particular, the carbon in the willow comprises a large number of 10-50 μ pores, which provide an area of adhesion for many microbes. Other smaller pores, in turn, adhere cations such as Ca, Mg and K ions to their surfaces.

For the water purification, it is advantageous that the water is brought to flow in such a way that the flow rate is distributed evenly, both through the willow roots and through the chips and the carbonised willow. This is achieved by passing both the chips and the carbonised willow through a screen to filter out fine material.

Willows grow best when they are in an optimal environment, and in the present technology the water flow is preferably conducted by evenly distributing it over at least one willow thicket.

For example, the willows can be planted in a desired order in a willow cultivating area, in which case, in the planting, both the row width and the spaces between the plants in the row are selected in advance.

In the present technology, the chips and the carbonised willow are placed appropriately in relation to the willow thicket. This can be achieved by using networks, screens, sacks, baskets or other similar structures, which hold the particles that perform the purification, but through which, for example through the coarse texture of which, the water can flow smoothly.

In one embodiment, the particles that perform the purification are placed together or separately in sacks, vessels, cages, which can be easily transported, put into water (therefore no

"floating" carbon) and taken out, i.e. easily replaced. The sack texture is preferably coarse, in which case it allows water to flow through.

By carbonisation or torrefaction of wood, such as willow, carbon is generated, hereinafter also referred to as "biochar".

The biochar works in several ways. It absorbs both cations (such as P, Ca, K and Mg ions) and vegetable toxins and other products, such as hydrogen sulphide and possibly other mercaptans, in other words it binds all types of odours. It is possible to modify the carbon in such a way that it is capable of reacting with selected nutrients. When an active microbial strain is formed in the carbon, it also serves as a biologically active component, in addition to that the carbon adsorbs cations and toxic molecules in a known manner.

Biochar can be used as such or as further activated.

The activation can be carried out using an alkali treatment or using superheated steam, or using a Zn salt or an organic or inorganic acid or a mixture thereof. By activating the carbon with an alkali it is possible to reduce the water repellency of the carbon. The metals in the solution can be easily bound to the carbon surface. Activated or otherwise modified carbon can be used in the post-filtration zone 5 to bind those impurities or nutrients which have passed through the preceding filtration zones and the willow thicket. When activated carbon is prepared of the willows, it is preferable to break the willows into a length of approximately 1 -2 times the thickness of the wood material, and to use willow parts which have almost the same thickness, preferably 5-10 mm. The product thus obtained is easy to keep in a basket, net or similar structures without the particles blocking the water passage. Both the willow carbon and the willow chips can be combined for example to form a sand filter component, or a component of a peat filter, a wetland or an evaporation field. Willow stems, peat, sand, gravel and straw can be used as the retention layer of the filter, which layer holds the particles in place. Also, different fabrics and non-woven fabrics can be used.

Naturally, these retention layers prevent the solids flowing in the water, and the carbon retains the dissolved humus.

In applications in which there are many solids in the water, the solids are separated by using different known methods.

Regarding carbon and activated carbon it is known that at various activation levels they adsorb various components and ions onto their surfaces and into their pores. Thus, according to our invention, it is advantageous but not necessary to carry out the carbonising for this purpose in such a way that the carbon is recovered at various temperatures.

The carbon generated is generally hydrophobic, i.e. it does not moisten easily. Consequently, it is advantageous, but not necessary, to treat first the carbon with water comprising a small amount of surface-active agent, preferably for example isopropanol.

The chip and carbon sacks used in the embodiments described above have a diameter of 10- 250 cm and a height of approximately 0.1-5 m, in which case their volume is generally approximately 0.1-12.5 m . The sacks can be arranged next to each other or at some distance from each other (for example a distance of 0.5-5 m).

The distance between the chip and carbon zones can vary widely, typically it is approximately 0.5-50 m, especially approximately 1-25 m. In one embodiment, a basin dug into the ground or a similar open storage space is arranged, which is filled with wood chips, especially willow chips. In a more preferred embodiment, also carbon is added into the basin, most suitably carbonised chips, described in the present applications. In this embodiment, the carbon and the chips are separated from the surface soil, preferably by a cloth. In the surface soil it is possible to plant a willow thicket which has part of its root system in the surface soil layer, which, among other things, ensures that the roots are not continuously in water. The plant, i.e. willow, grows in the surface layer, but part of its root system grows through the intermediate filter cloth, in which case the plant is actively seeking nutrients. The mycorrhizae which are possibly in the root system further intensify the nutrient uptake.

The biochar works as a nutrient binder, and also as a breeding ground and a refuge for the bacteria and mycorrhizae during periods of changing conditions which are especially unfavourable to the willow growth. These changing conditions which are non-favourable for the willow growth occur in cool periods and in dry seasons.

The importance of the willow thickets is most important also in this embodiment. A dense thicket which is renewed for example every 2-3 years will renew its root system and keep the root system in the surface layers. When a mycorrhiza is brought to the root system, either in the plants or as separately sprayed mycorrhiza, the mycorrhiza will intensify the nutrient uptake. In association with the cutting down, part of the root mass dies, and this renders to the bacteria and the microbes more breeding ground and gradually generates humus.

In a more preferred embodiment, the active root system of the willow and the mycorrhizae in it utilise the "nutrient bank" formed of the chips and the carbon. In one embodiment, the chips and the carbon in the basin are uniformly mixed, in particular without separate phasing.

In the present technology, typically the willow thicket has an area of 1-10,000 m², especially

2 2

10-5,000 m , most suitably approximately 25-4,000 m ; and the willow thicket is located 0.5- 100 m, especially 1 -20 m from the preceding zone.

The typical area of a willow thicket is approximately 1000-2500 m for small runoffs. For large runoffs the area is typically 5-10 % of the runoff area (i.e. the area from which the water to be purified in the purifying system is collected).

The numerical values presented above represent examples and do not restrict the present invention. Industrial Applicability

The present invention can be used for purifying nutrient-rich wastewaters, such as household and municipal storm waters and wastewaters, and farming and agricultural leachates, for example surface waters originating in fields, barns, pig houses, sheep barns, henhouses, stables and fur farms. The solution can also be utilised for the treatment of grey waters and overflow waters from purification plants. Also nutrient-rich grounds of forestry, peatland forests, peat production areas may be sources of the water to be treated. In addition, carbon can be used in filters, such as in recovering harmful elements in air and gases.

In general, nutrients, such as phosphorus and nitrogen, occur in soluble form in the soil water and also they adhere onto the surfaces of soil particles. In addition, nutrients form part of the structure in the degradable organic matter in the soil. Phosphorus may be present dissolved in the soil water in organic and inorganic form; it can also be adsorbed onto oxide surfaces, and form part of the structure in organic matter, from which it is released during the process of degradation. Similarly, nitrogen is present in organic form, both as nitrates and nitrites.

Most suitably, the waters treated have a total phosphorus content of at maximum 100 ug/1, for example 15-100 ug/1, especially 15-25 ug/1, and the total nitrogen content is at maximum 1500 ug/1, for example 400-1500 ug/1, especially 400 -600 ug/1.

The biological oxygen consumption BOD_MII is typically at maximum 500 mg/1, especially at maximum 250 mg/1, most suitably at maximum approximately 100 mg/1.

With the present solution, it is possible to reduce the abovementioned total percentages of phosphorus and nitrogen and the BOD by at least 25 %, most suitably by at least 30 %, for example by at least 40 %, especially by at least 50 %, preferably by at least 60 %.

More preferably, the present invention is used for treatment and purification of such nutrient- rich wastewaters which do not comprise solids or which comprise only a small amount of solids matter. Thus, in one embodiment, the water to be treated has a solids content of less than 5000 mg/1, especially less than 2500 mg/1, for example less than 1000 mg/1.

Reference Numerals List

1 Water flow retardation basin

2 Chip zone

3 Carbon zone

4 Willow growth

5 After-filtration zone

Citation List Patent Literature

FR application publication 2876047A1

CN application publication 101965785A1

EP application publication 2 360 123A1

CN application publication 103252216A1

CN application publication 10343541 OA

CN application publication 102515363 A

EP application publication 225 6091 Al

JP 2008212843 A

US 2008197073 Al

JP 2006015256 A

CN 104556589 A

JP 2000185294 A

CN 100540482 C

Non-Patent Literature Christianson L E et al. Suitability of three wood species for denitrification bioreactor fill, American Society of Agricultural and Biological Engineers Annual International Meeting 2014, Vol. 2, pp. 1447-1457.

Claims

Claims: 1. A method of separating impurities, such as nutrients, from wastewater, characterized in that the water is brought to flow through at least two zones, which zones are arranged in the direction of the water flow in a desired order, and which contain,

independently of each other, material which is intended for separation of impurities and which is selected from a group that comprises

- living willows with roots,

- willow chips,

- carbonised parts of willow, or

- a mixture of willow chips and carbonised parts of willow, in which case at least two zones comprise material that differ from each other.

2. The method according to Claim 1, characterized in that the roots of living willows are infected with mycorrhizae which are specific to the willow species.

3. A method according to Claim lor2, characterized in that the roots of living willows are infected with mycorrhizae which are already present in the ground.

4. A method according to any of the preceding claims, characterised in that all different separation materials are separated from each other, in different zones, in order that they can be separately employed for further use, for example as fertiliser.

5. A method according to any of the preceding claims, characterized in that the carbonised willow parts and the uncarbonised chips, or parts of these, are mixed together.

6. A method according to any of the preceding claims, characterized in that at least one zone contains living willows with roots and at least one zone contains willow chips, carbonised willow parts, or a mixture of willow chips and carbonised willow parts.

7. A method according to any of the preceding claims, characterized in that both the willow chips and the carbonised willow chip pieces are placed in the same area as the growing willows.

8. A method according to any of the preceding claims, characterized in that the carbonised willow parts are carbonised to one, two or three different carbonising levels, the temperatures of which are approximately 250 °C, approximately 350-500 °C or approximately 700 °C, for a period of 10-600 minutes, especially approximately 15-360 minutes.

9. A method according to any of the preceding claims, characterized in that at least part of the carbonised willows are activated using methods which are known per se, such as by using superheated steam, an alkaline solution, a Zn salt or an organic or inorganic acid or a mixture thereof.

10. A method according to any of the preceding claims, characterized in that the water to be purified is channelled on the surface or under the ground, in such a way that the willow roots come into contact with the flowing water, or so that the willow roots are allowed to find their way into the flowing water, by using channelling, draining or with a manifold.

11. The method according to Claim 10, characterized in that roots of the willows extend to a depth of at maximum 50 cm.

12. A method according to any of the preceding claims, characterized in that the particles that perform the purification are placed, together or separately, in sacks, having a coarse texture which allows the water to flow through it and which sacks can be easily replaced and further utilised.

13. A method according to any of the preceding claims, characterized in that the willow thicket is formed of precultured plants, in which a desired mycorrhiza is already engrafted, or the mycorrhiza is separately brought to the root system area.

14. A method according to any of the preceding claims, characterized in that the zones which comprise chips and carbonised parts are arranged in the direction of the water flow, before the zone which comprises living willows, in which case the water is first allowed to flow through the chips and their carbonised form, after which it is brought to flow through the root system of the growing willow thicket.

15. A method according to any of the Claims 1-13, characterized in that the zone which comprises living willows is arranged in the direction of the water flow, before the zones which comprise chips and possibly carbonised wood pieces.

16. A method according to any of the Claims 1-13, characterized in that wastewater which comprises harmful heavy metals is first conducted to a zone which comprises carbonised willow parts or a mixture of willow chips and carbonised willow parts, after which the wastewater thus treated is conducted to the zone which comprises chips.

17. A method according to any of the preceding claims, characterized in that the nutrient-rich water is first conducted into a basin which regulates the water flow, from where it is conducted to the first treatment zone.

18. A method according to any of the preceding claims, characterized in that the total phosphorus content of the water to be treated is at maximum 100 ug/1, for example 15-100 ug/1, especially 15-25 ug/1, and the total nitrogen content is at maximum 1500 ug/1, for example 400-1500 ug/1, especially 400-600 ug/1.

19. A method according to any of the preceding claims, characterized in that two or more zones are combined, most suitably they are combined to a single entity.

20. A method according to any of the preceding claims, characterized in that the nutrient-rich water is conducted into a basin which is comprised of a layer which comprises wood chips, especially willow chips, and possibly, mixed with these chips, carbonised chips, in which case soil is arranged onto the layer, which soil is possibly separated from the chips and the carbon by a cloth, especially a filter cloth.

21. A method according to Claim 19 or 20, characterized in that willow is planted in the surface soil, at least part of the root system of which willow is in the surface soil layer, in which case possibly part of the willow root system grows, however, through the filter cloth.

22. A method according to any of the preceding claims, characterized in that nutrients are separated from such nutrient-rich waters which comprise solids less than 5000 mg/1.

23. A system of separating impurities from wastewater, characterized in that it comprises at least two zones, through which the water can be arranged to flow and which zones are arranged in the water direction in a desired order and which comprise, independently of each other, material that is intended for separation of impurities and which is selected from a group that comprises

- living willows with roots,

- willow chips,

- carbonised parts of willow, or

24. The system according to Claim 23, characterized in that it comprises three zones, of which, in the flow direction

- the first comprises willow chips,

- the second comprises carbonised willow parts, and

- the third comprises living willows with roots.

25. The system according to Claim 23, characterized in that it comprises three zones, of which, in the flow direction

- the first comprises willow chips,

- the second comprises living willows with roots, and

- the third comprises carbonised willow parts.

26. A system according to any of the Claims 23-25, characterized in that two or more zones are combined to form a single entity.

27. The system according to Claim 26, c h a r a c t e r i s e d in that it comprises a basin with a layer that comprises wood chips, especially willow chips, and possibly mixed with this carbonised chips, in which case, on the layer is arranged soil, which possibly is separated from the chips and the carbon by a cloth, especially a filter cloth, and in which case willow is planted in the surface soil, the root system of which willow at least partly is in the surface soil layer.

28. A system according to any of the Claims 23-27, c h a r a c t e r i z e d in at least one basin, which is arranged before the first treatment zone and which regulates the water flow, from which basin the water can be conducted to the first treatment zone.

29. The use of a method according to any of the Claims 1-22 or, correspondingly, of a system according to any of the Claims 23-28, for purifying nutrient-rich wastewaters, such as household and municipal wastewaters, as well as farming and agricultural leachates, for example surface waters and storm waters originating in fields, barns, pig houses, sheep barns, henhouses, stables and fur farms.

30. The use of a method according to any of the Claims 1-22 or, correspondingly, of a system according to any of the Claims 23-28, for treatment of grey waters and overflow waters from purification plants.

31. The use of a method according to any of the Claims 1-22 or, correspondingly, of a system according to any of the Claims 23-28, for treatment of waters originating in nutrient-rich grounds of forestry, and in peatland forests and peat production areas.

32. The use of a method according to any of the Claims 1-22 or, correspondingly, of a system according to any of the Claims 23-28, for separation of nutrients from such nutrient-rich waters which comprise solids less than 5000 mg/1.