WO2016001563A1

WO2016001563A1 - Method for treating liquid effluents

Info

Publication number: WO2016001563A1
Application number: PCT/FR2015/051774
Authority: WO
Inventors: Mostafa Benmoussa
Original assignee: Lixival
Priority date: 2014-07-04
Filing date: 2015-06-29
Publication date: 2016-01-07
Also published as: FR3023280A1; FR3023280B1

Abstract

The invention relates to a method for treating liquid effluents, especially leachates, by means of the coupling of a physicochemical process based on a calcic product and a phytoremediation and/or evapotranspiration process.

Description

PROCESS FOR TREATING LIQUID EFFLUENTS

DESCRIPTION Technical field

The present invention relates to the treatment of aqueous effluents, in particular leachates. The invention more particularly relates to the coupling of a physicochemical process and a biological process.

The present invention finds its applications mainly in the treatment of household and / or industrial liquid effluents (leachates of discharge, industrial effluents, wastewater, rejection of composting platform or anaerobic digestion, etc.).

In the description below, references in brackets ([]) refer to the list of references at the end of the text.

State of the art

The degradation of organic matter, during its storage in landfills, landfills or on composting or methanisation plants, generates liquid juices that mix with rainwater form what are called leachates. These liquid effluents with a high organic and mineral load can cause significant pollution when they are sent directly to the receiving environment (water table, rivers, bodies of water, etc.). In addition, the temporary storage of these leachates emits odorous gas molecules and / or gaseous molecules contributing to global warming (greenhouse gases), in particular: volatile organic compounds (VOCs), volatile fatty acids (VFAs), molecules of hydrogen sulfide (H ₂ S), ammonia molecules (NH ₃ ), etc.

Leachate treatment regulations are becoming more demanding, and now require that their concentration of pollutants be extremely low before being released into the environment. Currently, there are different methods of treating these leachates: extraction of pollutants by evaporation, decantation, membrane filtrations, etc. However, these methods have a problem of storage of pollutants extracted and / or of rejecting said pollutants in the atmosphere. .

Several leachate treatment methods incorporate one or more filtration stages, in particular membrane filtration (eg by reverse osmosis). However all these methods have various disadvantages. Thus the high concentrations of organic substances leachate regularly cause clogging filters or membranes; which often requires prior treatments of leachates before the filtration stage. Thus, Slater et al. (J. Sci, Health, A20 (1): 97-111, 1985) [1] described the addition of lime to the leachate to be treated prior to decantation. The liquid supernatant (filtrate) obtained is treated by ultrafiltration and / or reverse osmosis, while the concentrate is returned to the discharge. However, said concentrate comprises the pollutants which are added to the pollutants of the waste; which generates a new leachate even more concentrated in pollutants so more difficult to treat. Patent FR 2904622 [2] has described the addition of lime to the leachate to be treated before subjecting it to one or more microfiltration stage (s). However, this process uses, on the one hand, lime in its liquid form (milk of lime); which eliminates the positive impact of the exothermic reaction that occurs during the extinction reaction of quicklime as well as the positive impact on the degradation of organic molecules by the thermophilic effect. The use of lime milk does not benefit either from the ammonia volatilization reaction which is generally favored by the rise in the temperature evolved by the exothermic reaction and results in the almost total elimination of this element which is considered highly toxic. On the other hand, the method uses a microfiltration technique by means of a vacuum rotary filter covered over its entire surface with a filter cloth covered on its outer surface by a layer siliceous admixture based on perlite or diatomite (products which are not available everywhere and which are relatively expensive) whose consumption must be excessive when one knows the very clogging nature of lime mixed with leachate. The presence of lime also makes it very difficult maintenance operations of the filter because of its very sticky and clogging characteristics.

The idea of using plants for sewage treatment is not a new concept. Indeed, 3000 years ago, humans already used the purification capacities of plants for the treatment of water. Since the 1970s, this practice has found renewed interest, particularly in the treatment of pesticides and metals. Phytoremediation consists of using plants to reduce, degrade or immobilize polluting organic compounds or various contaminants (metals, hydrocarbons, organochlorines, pesticides, etc.) from soil, water or air. It is essentially based on interactions between plants, soil and microorganisms. Thus US Pat. No. 4,415,450 [3] describes the treatment of wastewater through a filter of granular materials in three layers whose upper layer is planted with aquatic plants (Phragite communis). However, it turns out that phytoremediation does not allow to degrade or eliminate certain pollutants (eg metals, inorganic pollutants with low biodegradability); which led to the development of new phytoremediation processes. Thus, patent FR 2876047 describes a process for the depollution by introducing pollutants to be treated into a planted filter bed and then watering said filter bed so as to organize aerobic and anaerobic periods, possibly in combination with the addition of acidifying or acidifying products. natural acids to bring the pH of 7-9 before addition to a pH of 4-7 after addition. This process requires a succession of vertical flow filter stages and horizontal flow filter stages. However, a pH of 4 or 6 is harmful to any biological activity. Currently, therefore, there is no method of treating liquid effluents, particularly leachates, completely satisfactory. There is therefore a real need for a process for treating liquid effluents that overcomes the defects, disadvantages and obstacles of the prior art, in particular a method making it possible to reduce the pollutant load of the liquid effluents, to reduce as much as possible the discharges. treated liquid effluents, and allow their re-storage in situ at a lower volume; while significantly reducing treatment costs. Description of the invention

The inventors have quite unexpectedly demonstrated that the synergistic coupling of a physicochemical stabilization process and a biological process makes it possible to treat liquid effluents, in particular leachates, which are entirely satisfactory for the environment since the aquatic and atmospheric pollution risks and at a lower cost due to the energy-saving nature of these processes and simple installation and maintenance. The disadvantages and obstacles of the art are avoided, in the context of the process of the invention, because of the use of (i) a product that increases very strongly the pH, for example caustic soda, a calcium product, preferably a natural calcium oxide reagent, especially quicklime, which leads to a release of heat during the quenching reaction, and (ii) a planted filter bed and / or a soil-plant system to promote the evapotranspiration of treated liquid effluents, which allows:

- to increase the degradation of the organic matter contained in the liquid effluents (leachates) thanks to the thermophilic effect and to favor its decantation,

to promote the dewatering of the sludges which result from the decantation operation by virtue of the residual heat which is generated by the exothermic reaction, - Promote the volatilization of ammonia and reduce its toxicity to make supernat (liquid fraction) capable of biological treatment by a filter bed planted.

The physicochemical process comprises or consists of the addition of a product that greatly increases the pH of the liquid effluent, for example caustic soda, a calcium product, preferably a natural calcium oxide reagent. preferably lime, most preferably quicklime, to the liquid effluents to be treated. Said calcium product may be reformulated and enriched by products well known to those skilled in the art. These are, for example, cationic chemicals. This treatment results in a stabilization of treated effluents following the cessation of all fermentations under the effect of pH (10 - 13) and consequently the elimination of all the bad odors of the liquid effluents by neutralizing them, as well as volatilization of ammonia favored by the simultaneous increase in pH and heat released by the extinction reaction of quicklime. The pollutant load (organic and ammonia load) of the liquid effluents thus reduced allows the subsequent treatment by a biological process (volatilization of ammonia in an ultra-basic environment favored by high temperatures).

The biological process comprises or consists of the introduction of liquid fractions (supernatant waters) resulting from the physicochemical process for their purification in at least one planted filter bed, preferably reeds (Phragmite communis), and optionally filled with strong adsorbent capacity (eg apatite). During or at the end of the biological process, the treated water can be recovered and disposed of using the evapotranspiration potential (eg natural evaporation on a soil-plant exchange system) of plants until the total absence of rejection under optimal climatic conditions. To do this, it is possible to use the rehabilitated bin surfaces from landfills for the development of soil-plant systems for evaporation. treated water. These systems contribute to the creation of plant belts on the site as well as their integration into the landscape.

The present invention therefore relates to a process for treating at least one liquid effluent, said process comprising the following steps:

a) adding a calcium product to said at least one liquid effluent to be treated in an amount sufficient to induce a stabilization of the fermentations and a liquid-solid separation of said at least one liquid effluent;

b) decantation of the liquid effluent / calcium product mixture to separate the solid fraction from the liquid fraction;

c) separating the solid fraction and the liquid fraction of said settled mixture resulting from step b), and

d) introducing said liquid fraction from step c) into at least one planted filter bed.

According to a particular embodiment of the process of the invention, said calcium product of step a) is quicklime.

According to a particular embodiment of the process of the invention, step a) further comprises the addition of at least one cationic chemical agent. For example, said at least one cationic chemical agent is selected from the group consisting of iron salts, alumina salts, polymers.

According to a particular embodiment of the process of the invention, the mixture, at the end of step a), has a pH greater than or equal to 10, preferably a pH of 10-13.

According to a particular embodiment of the method of the invention, said at least one planted filter bed comprises a planted filter system and / or a soil-plant exchange system.

According to a particular embodiment of the process of the invention, said process further comprises, before step d), an intermediate step c ") of neutralization of said liquid fraction resulting from step c). for example, step c ") is carried out by addition of carbon dioxide and / or acid to said liquid fraction, preferably phosphoric acid.

According to a particular embodiment of the process of the invention, said liquid fraction, at the end of step c "), has a pH of 7 to 8.5.

According to a particular embodiment of the process of the invention, said process further comprises an intermediate step c ') of adding a material with a high adsorption capacity after step c) and / or after the step d).

According to a particular embodiment of the process of the invention, said at least one filter bed of step d) comprises a material with a high adsorption power. For example, the material with high adsorption power is apatite. The treatment of the supernatant with apatite can be done either in batches before passing through the filters planted with reeds, or on a second stage of the filters planted with reeds, at least not before the liquid-solid separation step.

According to a particular embodiment of the method of the invention, step d) is repeated n times; n being an integer greater than or equal to 2.

Other advantages may still appear to those skilled in the art on reading the examples below, illustrated by the appended figures, given for illustrative purposes.

Brief description of the figures

FIG. 1 represents the general block diagram of a standard unit implementing the method of the invention.

Figure 2 (taken from a Carmeuse company document) represents the flocculation of suspended solids

Figure 3 (taken from a Carmeuse company document) represents the effect of thermolysis on bacterial enzymes.

Figure 4 (from a Carmeuse document) shows the effect of pH on the coexistence of ionic species (HS7S ^~~ ) (A) and the formation of ionic species (RS ^" ) (B). Figure 5 (taken from a Carmeuse company document) shows the effect of pH on the formation of the NH ₃ volatile form.

Figure 6 (MOLLE "Reed plant filters: evolution of research and current trends", SET Review, 9: 24-31, 2012) [4] is an example of a reed filtration system.

Figure 7 (MOLLE "Reed plant filters: evolution of research and current trends", SET Review, 9: 24-31, 2012) [4] represents a schematic cross section of a horizontal flow apatite filter .

EXAMPLES

EXAMPLE 1: Implementation of the Method According to the Synthesis of FIG. 1

I. PHASE 1: PHYSICO-CHEMICAL STABILIZATION

The different stages of physico-chemical treatment are as follows:

A. Storage of leachates

Leachate generated at landfill bins or composting or methanization platforms was collected and stored in a basin of appropriate capacity.

B. Stabilization of leachates by mixing with the reagent

The leachate treatment with the calcium reagent was done in an industrial mixer designed to treat very large volumes. This mixer was chosen in such a way as to allow a very intimate mixture between the leachate and the reagent. The reagent was stored, when it was stored in a suitable storage silo. The reaction sought by this mixture exploited the following principle: 1. Principle

The principle of the process consisted in exploiting the extinction reaction of quicklime (Calcium oxide: CaO), which by reaction with water resulted in the release of heat and the formation of a strong base (Calcium hydroxide: Ca (OH) ₂ ) responsible for a significant increase in the pH of the medium and the formation of an ultra-basic medium (pH> 10) according to the reaction: CaO + H ₂ 0 ==> Ca (OH) ₂ + 1155 Kj / Kg CaO

Mixing with the quicklime particles, increasing the temperature and raising the pH had a direct consequence on: a. Flocculation of suspended solids

The objective, using lime, has been to increase the efficiency of the solid-liquid separation of leachates "sludge", that is to say to remove the water incorporated in a structure in which the solid particles were dispersed (electrostatic repulsion of negative charges) and obtain on the one hand a supernat, more limpid which becomes capable of biological treatment thereafter and on the other hand to obtain slower, stable and hygienized sludge.

By adding lime, the particles were neutralized, destabilized and formed micro-flocs, then flocs that decanted.

This effect could be enhanced by using iron or alumina salts or polymers as shown in FIG. 2A. Combined with lime, salts of Fe 3+ and Al ₃₊ are converted by increasing the pH Fe amorphous gels (OH) ₃ and Al (OH) ₃ which behaved in inorganic polymers linking the particles, as shown in Figure 2B. b. Stabilization of leachates by stopping fermentations and their hydrogenation.

The goal has been to destroy live microorganisms leachate as well as pathogens, biological agents causing diseases or acute problems to their host: bacteria, viruses and parasites.

Lime has destroyed living microorganisms and pathogens by the effect of the ultrabasic pH it has created (alkaline hydrolysis) and by the effect of the temperature released by the exothermic reaction (thermolysis). This resulted in a total cessation of all fermentations which led to the complete stabilization of organic matter and neutralization of all bad odors. The mechanisms involved in these processes are: i) Alkaline hydrolysis (pH increase)

Destruction of protein cell membranes and enzymes

Destruction of cellular components: lipids and carbohydrates i) Thermolysis (temperature increase)

Denaturation of certain pathogen enzymes

The results are shown in Figure 3. vs. Neutralization of bad odors

By increasing the pH, the lime has transformed odor-based sulfur and nitrogen compounds into their precipitated rather than volatile form. Only the volatile form is fragrant.

Organoleptic properties of leachates

Threshold

Quality of detection

Odorous compounds

odors g / m ³ )

Down up

H ₂ S Rotten eggs 0.7 14

C ₂ H ₅ SH Rotten cabbage 2.5 50.8

sulphides

(CH ₃ ) ₂ S ₂ Sulfur 0.1 346

(CH ₃ ) ₃ S ₃ Sulfur 6.2 6.2

NH ₃ Acute 26.6 39600

compounds

(CH ₃ ) ₂ NH 84.6 84.6 Nitrogen fish

(CH ₃ ) ₃ N Poisson 0.8 0.8

CH2O2 Acre 45 37800

CH ₃ COOH Vinegar 2.5 250000

Fatty acids

C ₂ H ₅ COOH Rance 84 60000

C ₃ H ₇ COOH Rance I ¹ 9000

^* Liming prevented the release of H ₂ S

At pH> 12, the ionic species (HS7S ^~~ ) coexisted, as shown in Figure 4A. pKa (H ₂ S / HS ^") = 7" pKa (HS7S ^"") = 13 <pKa (H ₂ O / OH ^") = 14 H ₂ S + OH ^" ¾ HS ^" + H ₂ O HS ^" + OH ^" ¾ S ^" + H ₂ O

* Liming prevented the release of mercaptans

At pH> 12, ionic species (RS ^- ) were formed as shown in Figure 4B.

C2H ₅ SH + OH ^" ¾ C2H ₅ S ^" + H ₂ O d. Volatilization of ammonia NH3 Calcium hydroxide reacted with ammonium salts (NH4)

(that leachates contain in large quantities and which are toxic for living organisms and microorganisms) to form ammonia (NH3), calcium chlorides and water:

2 NH4Cl + Ca (OH) 2 → 2 NH3 + CaCl2 + 2H2O)

Ammonia (NH3), a very volatile form, could be captured by air suction systems installed at the mixer and led, for treatment, at an air washer. It was thus avoided to send it to the atmosphere to pollute it.

At pH> 12, the volatile form NH ₃ was formed, as shown in FIG. 5. pKa (NH ₄ NNH ₃ ) = 9.25 "pKa (H ₂ O / OH ^" ) = 14 NH ₄ ⁺ + OH ^" ¾ NH ₃ + H ₂ O At the mixer, the transformation of the NH and the formation of NH ₃ were promoted by increasing the temperature evolved by the exothermic reaction.

K = exp (-AG RT)

VS. decanting

After a certain time of mixing the leachates with the quicklime, the mixture was sent to two decanters which operate in rotation to separate, naturally and without mechanical centrifugation, the liquid phase (supernat) of the solid phase (sludge).

The attack by quicklime to favor (by the mechanisms described previously) this natural decantation and to increase the efficiency of the solid-liquid separation of the leachates; that is to say, to remove the water incorporated in a structure in which the solid particles are dispersed (electrostatic repulsion of the negative charges) and to obtain on the one hand a clear supernatant which is ready for a biological treatment thereafter and on the other hand to obtain slower, stable and hygienic sludge.

The action of quicklime allowed partial or total decarbonization (depending on the quicklime doses used) leachate forming a precipitate of calcium carbonate (CaCOs) which decanted with sludge.

D. Sludge treatment

After the settling phase, the sludge was sucked up and taken to a sloped storage buffer tank to promote liquid flow to the bottom of the pond. A pump has allowed to suck up the juices and return them to the decantation tank # 2.

E. Sludge dewatering

The dewatered sludge was in the form of conglomerates mainly composed of calcium carbonates (CaCOs), co-precipitated organic acid molecules of the humic acid type and, to a lesser extent, precipitates of metal hydroxides and magnesium.

A very interesting solution for the future of this sludge is to be stored directly on the site of the Center of Landfill

(THIS).

F. Neutralization of the supernatant

After a few hours of settling, the suspended solids settled at the bottom of the tank. The supernatant was then sucked by a pump to a second tank where it underwent a neutralization treatment with a natural product (carbonic acid or phosphoric acid) to find a neutral pH favorable to biological developments.

The technique of neutralization pursued was itself an innovation insofar as it used natural products which made it possible to carry out an additional reduction while neutralizing the excess alkalinity of the supernatant.

The supernatant was strongly bubbled, initially, by carbon dioxide (which is sought in the landfill site where will settle the unit of leachate treatment by this process). The carbon dioxide mixed with water was converted to carbonic acid (HCO3) which reacted with the strong base Ca (OH) ₂ which was obtained after the extinction of quicklime to form a precipitate of calcium carbonate (CaCOs) which by precipitating to the bottom resulted in an additional amount of pollution (about 10%).

Neutralization was continued when the objective was to induce the abatement of certain pollutants, such as sodium chlorides, with phosphoric acid (H ₃ PO ₄ ). The latter reacted with calcium hydroxide and chlorides to form chlorapatite (Ca ₅ (PO ₄ ) 3 Cl), which precipitated to the bottom and removed with it the chlorides.

The yields of the physicochemical treatment: during this first phase, the treatment of leachates with the calcium reagent at very precise doses allowed a very significant reduction in the pollutant load, which later authorized the installation of biological treatment by filters planted with reeds.

II. PHASE 2: BIOLOGICAL TREATMENT BY FILTER PLANT OF ROSE

After the neutralization step, the supernatant was then directed to the biological treatment station. This station was composed of a series of stages, each consisting of several filters planted with reeds that rotate. The principle used in the filters was as follows:

A. Principle

The reed plant filter treatment (Figure 6) was based on the use of their root system which is highly developed and very dense. These roots, specialized in the absorption of water and minerals in the soil, accumulated reserves and allowed the plant to attach to the substrate.

During the absorption processes, the roots released carbohydrates, enzymes and other nutrients, usable by microorganisms. The intense root network has therefore favored the fixation of purifying bacteria on the rhizomes. They therefore harbored a large bacterial flora, which fed on the effluents and degraded the organic matter.

A whole population of bacteria, fungi and other microorganisms has thus concentrated around the root: it has been estimated that bacteria in this privileged zone were 20 to 10,000 times more numerous than in bare soil. These microorganisms promoted the mineralization of nitrogen and phosphorus, which were then available to the plant. This has created a close cooperation between plants and micro-organisms.

This part of the soil where living organisms have been associated is called the rhizosphere. The microbial activity in this rhizosphere depends on various factors, such as water and oxygen content. The bacteria attached to this rhizosphere are aerobic: they need oxygen to degrade organic matter.

In addition to their involvement, through their root system, in the degradation of organic matter, the reeds had a mechanical action: with the wind, they broke the crust that formed at their feet; which allowed to limit clogging phenomena and to guarantee the permeability of the surface filter. This protection was possible thanks to the very fast growth mode of the roots. The rhizosphere generated a de-clogging system thanks to the tubular roots (rhizomes tracing) and the new stems that pushed through the filtering mass. Reeds colonized all traps from the second year of operation.

The filters have been filled with different materials depending on the desired treatment and performance objectives. B. Plant filtering with reeds garnished with apatite

1. The reed-planted filter of apatite consisted of basins planted with reeds and garnished with apatite. This phosphate rock has favored the elimination by adsorption and precipitation of orthophosphates (MOLLE "The filters planted with reeds: evolution of research and current trends", Revue SET, 9: 24-31, 2012) [4], certain metals heavy (Pb, Cu, Zn, ...) (Chafki et al., Science Lib., Ed. Mersenne, ISSN 21 1 1 - 4706, 3 (1 10804): 1-9, 201 1) [5] and humic acids (Thesis supported by El Asri, 19 November 2009, University Mohammed V - Agdal, Faculty of Sciences, Rabat) [6] responsible for the coloring of leachates.

This type of filter (Figure 7) has provided perennial retention on a solid phase (phosphate rock) by adsorption and precipitation mechanisms, in order to optimize the contact between the water and the material used and to have the greatest possible residence time with respect to the reactor volume. The choice was to work on a horizontally flowing water saturated filter system. The sizing of this type of filter is based on two dissociated aspects, on the one hand a hydraulic aspect to ensure an under-surface flow and, on the other hand, a reaction aspect to guarantee a sufficient residence time allowing the expected performance of the system. In this type of filter, the permeability of the medium, like the retention kinetics of the pollutants, are often affected by the development of biomass and the accumulation of organic matter within the filter. This justifies the implantation of this first treatment stage just downstream of the physicochemical stabilization. The high pH of the supernatant limited the proliferation of biomass and thus provided very interesting performances in terms of adsorption and precipitation.

The treated water was subsequently oriented, for a final purification, on a second stage consisting of two filters planted with reeds with horizontal flow and garnished with gravel of different grain sizes.

List of references

1. Slater et al., J. Environ. Sci. Health, A20 (1): 97-111, 1985

2. Patent FR 2904622

3. US Patent 4,415,450

4. MOLLE "Reed plant filters: evolution of research and current trends", SET Review, 2012, 9: 24-31, 2012

5. Chafki et al., Science Lib., Ed. Mersenne, ISSN 2111-4706, 3 (110804): 1-9, 2011

6. Thesis submitted by El Asri, November 19, 2009, Mohammed V University - Agdal, Faculty of Sciences, Rabat

Claims

1. A method of treating at least one liquid effluent, said method comprising the steps of:

e) adding a calcium product to said at least one liquid effluent to be treated in an amount sufficient to induce a stabilization of the fermentations and a liquid-solid separation of said at least one liquid effluent;

f) decanting the liquid effluent / calcium product mixture to separate the solid fraction from the liquid fraction;

g) separating the solid fraction and the liquid fraction of said settled mixture resulting from step b), and

h) introducing said liquid fraction from step c) into at least one planted filter bed.

2. The method of claim 1, wherein said calcium product of step a) is quicklime.

The process of any one of claims 1 or 2, wherein step a) further comprises adding at least one cationic chemical agent.

4. The method of claim 3, wherein said at least one cationic chemical agent is selected from the group consisting of iron salts, alumina salts, polymers.

5. Process according to any one of claims 1 to 4, wherein the mixture has a pH greater than or equal to 10, preferably a pH of 10-13, at the end of step a).

The method of any one of claims 1 to 5, wherein said at least one planted filter bed comprises a planted filter system and / or a soil-plant exchange system.

7. Method according to any one of claims 1 to 6, said method further comprising prior to step d) an intermediate step c ") of neutralization of said liquid fraction from step c).

8. The process according to claim 7, wherein step c ") is carried out by addition of carbon dioxide and / or acid to said liquid fraction.

The process of claim 8, wherein the acid added to said liquid fraction is phosphoric acid.

The method of claim 7 or 8, wherein said liquid fraction has a pH of 7 to 8.5 at the end of step c ").

1 1. Process according to any one of claims 1 to 10, said process further comprising an intermediate step c ') of adding a high adsorption material after step c) and / or after step d ).

The method of any one of claims 1 to 10, wherein at least one filter bed of step d) comprises a high adsorption material.

13. The method of any one of claims 1 1 or 12, wherein the high adsorption material is apatite.

The method of any one of claims 1 to 13, wherein step d) is repeated n times; n being an integer greater than or equal to 2.