FR3112768A1 - BIOLOGICAL TREATMENT OF EFFLUENTS RICH IN CARBON AND NITROGEN MATTER WITH BIOGAS PRODUCTION - Google Patents

Abstract

BIOLOGICAL TREATMENT OF EFFLUENTS RICH IN CARBON AND NITROGEN WITH BIOGAS PRODUCTION The invention relates to a process for the biological treatment of an effluent, the effluent to be treated containing nitrogen in the form of ammonium and having a concentration of carbonaceous material measured by the chemical oxygen demand COD greater than or equal to 1g per liter of effluent, the process comprising: a) a step of anaerobic digestion of the effluent to be treated to produce biogas and a digestate constituting a first liquid effluent having a reduced carbonaceous material content, this step being implemented in a covered lagoon equipped with a biogas recovery device, b) a step of biological treatment of the digestate obtained in step (a) to produce a second liquid effluent with reduced nitrogen content. Figure for abstract: figure 1

Description

BIOLOGICAL TREATMENT OF EFFLUENTS RICH IN CARBON AND NITROGEN MATTER WITH BIOGAS PRODUCTION

Field of invention

The invention relates to the biological treatment for the elimination of nitrogen and carbon from effluents rich in carbonaceous matter and nitrogen, such as for example leachates, with the production of biogas, and in particular methane.

State of the art

Human activities produce huge amounts of solid waste. Landfilling is the most widespread solution despite material incineration and/or energy recovery solutions. The burial of municipal waste generates effluents - generally contaminated - called leachate. These leachates require appropriate treatment before being discharged into receiving environments (rivers, lakes, groundwater, sea, etc.) or into municipal wastewater treatment plants. This treatment by conventional processes consumes energy and potentially exogenous carbon sources. The main challenges associated with leachate treatment are the evolution of regulations with lower discharge limits for nitrogen and new environmental constraints, involving additional costs (consumption of energy and/or chemicals, in particular exogenous carbonaceous matter).

In this context, a biological treatment of leachates and more generally of effluents with a high concentration of carbonaceous matter and nitrogen requires the implementation of appropriate treatments.

Carbon treatment can use anaerobic digestion, also called anaerobic digestion, which is a process of biological degradation of organic matter under anaerobic conditions. It allows the degradation of soluble or particulate organic matter into biogas, an energy gas composed essentially of methane and carbon dioxide. In addition to biogas, a compound called “digestate” is generated. This compound is depleted in carbonaceous matter (more particularly in biodegradable carbonaceous matter), part of it being consumed during the methanation stage.

Anaerobic digestion is well known and typically comprises three main steps: a first step of hydrolysis/acidogenesis, a second step of acetogenesis and a third step of methanogenesis.

During the first stage, complex organic matter (sugars, proteins, lipids) is hydrolyzed into simple organic matter (oses, amino acids, alcohols) which are then assimilated by extremely diverse heterotrophic species (strict or facultative anaerobes) to end up with a mixture of volatile fatty acids (AGV, in particular acetic acid, lactic acid, propionic acid, butyric acid, etc.), simple alcohols (in particular ethanol), gaseous products (CO ₂ and H ₂ ) and ammonium. During the second stage, these compounds undergo acetogenesis by bacteria producing dihydrogen (H ₂ ), acetate and carbon dioxide (CO ₂ ). During the last stage, H ₂ , CO ₂ and acetate are transformed into biogas by methanogenic archaea.

Two major pathways of methanogenesis can be differentiated. One called hydrogenophilic which, from the H ₂ /CO ₂ couple, produces H ₂ O and CH ₄ , the other called acetoclastic, which degrades the acetate into CO ₂ and CH ₄ . Other carbon sources like methanol, formaldehyde and methylamines can also be used directly by methanogenic microorganisms.

The biogas produced contains methane, CO ₂ , and also water and volatile organic compounds, and can be recovered, possibly after purification.

Nitrogen treatment generally uses nitrification and then denitrification (N/DN) processes. Nitrification is an oxidation reaction by autotrophic bacteria, ammoniacal or ammonium nitrogen, noted as NH ₄ or NH ₄ ⁺ , in:

• nitrous nitrogen, also known as nitrite, NO ₂ or NO ₂ ^–

• then nitric nitrogen, also known as nitrate, NO ₃ or NO ₃ ^- .

Denitrification consists of a reduction in gaseous nitrogen, known as N ₂ , by denitrifying heterotrophic bacteria, of the nitrates produced during the nitrification reactions.

More precisely, the nitrification consists of two sub-steps: a first step of nitritation in the presence of oxygen followed by a second step of nitration also in the presence of oxygen. Nitritation consists of the oxidation of ammonium to nitrite by nitriting autotrophic bacteria, known as AOB or "Ammonia Oxidizing Bacteria", the predominant genus of which is Nitrosomonas . Nitratation consists of the oxidation of nitrite to nitrate by other autotrophic bacteria, known as NOB or "Nitrite Oxidizing Bacteria", the predominant genus of which is Nitrobacter .

Denitrification can also be broken down into two sub-steps: a denitrification step which will transform the nitrates into nitrites, and a denitrification step which will transform these nitrites into gaseous nitrogen. Each of these two substeps is carried out by heterotrophic bacteria and requires large amounts of biodegradable carbon. Denitrification requires approximately 2.9 kilograms of carbon in the form of chemical oxygen demand (COD) to reduce one kilogram of N-NO3 to nitrogen.

To reduce the amount of energy and carbon used for nitrogen treatment, other metabolic pathways can be considered: nitritation-denitritation and de-ammonification.

The nitritation-denitritation process, also called “nitrate shunt”, seeks to stop the oxidation of nitrogen at the nitrite stage by avoiding the production of nitrates, hence the shunt of the “nitrate part” of the cycle. To implement nitritation-denitritation, it is therefore necessary to repress the NOBs in favor of the AOBs. According to the state of the art, this process saves 25% on the need for oxygen and only requires 1.7 kilograms of carbon in the form of COD to reduce one kilogram of N-NO2 to nitrogen. This represents a saving of approximately 40% on carbon requirements compared to a conventional nitrification-denitrification process.

Another process, called deammonification or partial nitritation/Anammox (NP/A) uses the nitritation reaction described above but then involves anaerobic autotrophic bacteria, called Anammox for "ANaerobic AMMonium, Oxidation", which consume ammonium and nitrite to produce N ₂ without the need for oxygen and biodegradable carbon.

The first step in deammonification is partial nitritation (NP). It consists of the oxidation of a fraction (57%) of the ammonium ion into nitrite. The second stage is carried out by Anammox anaerobic bacteria. In this reaction, approximately 11% of the nitrogen charge is transformed into nitrate, which brings the theoretical maximum elimination rate to 89%.

The same bacterial population, aerobic oxidizing bacteria (AOB), as that of N/DN, is implicated for partial nitritation. In this case, (i) only a fraction of the ammonium is oxidized, unlike the N/DN process which requires 100% oxidation of the NH ₄ and (ii) the level of oxidation is reduced because the targeted molecule is the NO ₂ and not NO ₃ . According to the state of the art, the oxygen savings for this treatment route reach approximately 50% compared to a conventional treatment of nitrification and denitrification.

Furthermore, since AOB and Anammox bacteria are autotrophic populations, the entire NP/A process can be performed without any biodegradable carbon. No addition of external (or exogenous) carbon is necessary to achieve the nitrogen treatment. This treatment therefore does not eliminate any carbon that may be present.

The complexity of the landfill leachate matrix induces technical challenges such as high salinity, biodegradable carbon content and potentially low temperatures not allowing the implementation of a deammonification process. The activity of the Anammox biomass can be reduced or completely inhibited by the matrix, such as a leachate type matrix, and may require periods of acclimation.

Composition of leachates

The composition and toxicity of leachates vary according to the composition of the waste (geographical area), the operation of the landfill (closed / open, dry grave, bioreactor, etc.) and the seasons, and mainly the state of stabilization garbage. Leachate is generally classified into three distinct groups, corresponding to the state of degradation of the organic waste and related to the average age of the buried waste: young, medium and mature leachate. In general, leachates are characterized by their high electrical conductivity (1 to 35 mS.cm ^-1 ) due to the large amount of dissolved salt (Ca ²⁺ , Mg ²⁺ , Na ⁺ , K ⁺ , NH ₄ ⁺ , Cl ^- , and SO ₄ ^2- ) and mainly to NH ₄ ⁺ which is the most concentrated ion. They can also be concentrated in metals (including heavy metals), which can be a problem when biological treatment is considered.

Mature leachates are characterized in particular by a low biodegradable carbon/reduced nitrogen ratio (BOD5/TKN < 0.5 gO ₂ /gN) compared to younger leachates (BOD5/TKN ratio that can reach 40 g O ₂ /gN) (Renou , S., Givaudan, JG, Poulain, S., Dirassouyan, F., Moulin, P., 2008. Landfill leachate treatment: Review and opportunity. J. Hazard. Mater. 150, 468–493. doi:10.1016/j .jhazmat.2007.09.077). In addition, compared to municipal wastewater, it seems that mature leachates generally have lower BOD5 concentrations (<0.2 gO ₂ /L) for higher reduced nitrogen concentrations TKN (>0.4 gN/ I).

Table 1 below summarizes the usual characteristics of leachates according to their age (Tałałaj, IA, Biedka, P. & Bartkowska, I. Treatment of landfill leachates with biological pretreatments and reverse osmosis. Environ Chem Lett 17, 1177–1193 (2019).

Young leachate Medium leachate Mature leachate Age (years) <5 (<1)* 5-10 (1-5)* >10 (>5)* pH <6.5 6.5-7.5 >7.5 COD ( _mgO2 /L) >15000 4000-15000 <4000 BOD5/COD (gO ₂ /gO ₂ ) 0.5-1 0.1-0.5 <0.1 N-NH3 (mgN/L) <400 - >400 Total organic carbon (TOC)/COD (gC/ _gO2 ) (**) <0.3 0.3-0.5 >0.5 Total Kjeldahl Nitrogen (TKN) (gN/L) 0.1-2 - - Heavy metals (mg/L) >2 <2 <2 Biodegradability high mean weak

* value according to Kurniawan, Lo, Chan, 2006, Physico-chemical treatments for removal of recalcitrant contaminants from landfill leachate, J. Hazard. Mater., 129(1-3) pp. 80-100.

** total organic carbon can be measured by standards NF EN1484-July 97 or ISO8245:1999.

Nitrogen treatment in leachate

There is a wide variety of processes for treating leachate depending on their characteristics, site constraints (surface area, personnel), discharge limits, climate, cost (CAPEX (investment expenses), OPEX ( operating expenses)) and disposal of by-products. Table 2 below lists the nitrogen removal treatments in leachates usually encountered in the profession.

Process Objectives (elimination) Disadvantages Biological Aerated lagoon BOD & Nitrogen Large surface area required, little or no carbon beneficiation, large aeration energy, poor performance Activated sludge BOD & Nitrogen Energy consumption, sludge production, frequent exogenous carbon requirement, little or no carbon recovery Fixed Culture BOD & Nitrogen Management of suspended solids, frequent need for exogenous carbon, little or no carbon recovery Physico-chemical Precipitation Metals, MES, part of COD and TKN Low purification efficiency, use of chemicals Stripping Ammonia nitrogen Elimination of ammonia only, energy consumption, use of chemical products Membrane Nanofiltration Hard COD, metals and salts Energy consumption, concentrate management, high maintenance reverse osmosis Hard COD, metals and salts Energy consumption, concentrate management, high maintenance

Among these processes, the most common way of eliminating nitrogen is the biological process, in particular nitrification-denitrification (N/DN) well known to those skilled in the art and described above. N/DN requires the oxidation of ammonium to nitrite and then to nitrate. This represents significant quantities of oxygen, which has a strong impact on energy consumption and OPEX. In addition, a biodegradable carbon source is needed to reduce the oxidized N-NO ₃ nitrogen to gaseous N ₂ . The carbon must be in sufficient quantity, otherwise the nitrogen cannot be treated and unwanted nitrates or nitrites will be found in the effluent.

However, leachates, in particular mature leachates or those having undergone carbon pretreatment, are effluents with a high nitrogen concentration and a low biodegradable carbon content. There is not enough biodegradable carbon to process nitrogen through the N/DN process. For example, due to often significant spontaneous carbon degradation, carbon-laden effluents stored in an open lagoon come out with a biodegradable carbon content that is too low to allow nitrogen treatment by N/DN.

To allow this treatment by N/DN, either the hydraulic residence times in the lagoons are reduced (total or partial bypass of the storage structure), to the detriment of the potential recovery of carbon in the form of methane, or a source of exogenous carbon is brought into the biological pools, which leads to additional costs. The by-pass of storage structures, which is difficult to implement in a controlled manner with conventional techniques and does not make it possible to recover energy in a satisfactory manner, or the contribution of exogenous carbon is an economic nonsense, ecological and environmental. This carbon pollution leads to higher energy consumption, larger volumes of work, and more substantial sludge production.

By “exogenous carbonaceous material” or “exogenous carbon”, we mean carbonaceous material which is not present in the effluent to be treated and which is added to this effluent. In the context of nitrogen treatment, this carbon is usually supplied in the form of easily biodegradable molecules (methanol, glycol, etc.) to allow its rapid integration into the nitrification process. These are resources whose purchase can generate significant OPEX. Thus, the term "exogenous carbonaceous material" or "exogenous carbon" may be understood more generally to mean carbon outside the treatment site.

Treatments with activated sludge or aerated lagoons require significant energy requirements due to the necessary aeration.

Finally, physical nitrogen treatment solutions (such as nitrogen stripping) are energy-intensive and do not eliminate or recover carbon. Furthermore, the performance is impaired by the presence of particulate matter, in particular carbonaceous, and does not make it possible to treat it.

The increasingly strict discharge constraints for effluents, in particular on carbon and nitrogen content, imply the implementation of appropriate treatments, which can prove to be complex in rehabilitation contexts where the carbon is no longer available, because treated upstream, but residual nitrogen still requires treatment.

There are also processes for treating effluents rich in COD and nitrogen. Document CN20374111U thus describes an anaerobic biological treatment in a UASB reactor (upflow anaerobic sludge blanket) followed by nitrogen treatment by deammonification for the treatment of industrial wastewater with high COD and high ammoniacal nitrogen content. However, this process requires precise design and know-how, and dedicated monitoring.

Document US2016052811A1 also discloses a treatment for removing nitrogen from wastewater comprising an anaerobic digestion step followed by a deammonification step. On leaving the deammonification reactor, the sludge is separated into granular sludge (containing Anammox and AOB bacteria) and non-granular sludge (NOB and heterotrophic bacteria), with reinjection of the granular sludge into the reactor. It follows, according to the document, an enrichment of the desired microbiological populations and the possibility of carrying out the deammonification at temperatures below 25°C. The field of application of this process is however limited to feedstocks for which the ratio of BOD5 to nitrogen is less than 2 gO ₂ /gN with a nitrogen and BOD5 concentration both less than 100 mg/L. It will also be noted that the proposed method does not allow treatment of the carbon downstream of the anaerobic digestion.

There is therefore a need to effectively, at low cost, biologically treat effluents containing nitrogen in the form of ammonium and having a high COD, in particular greater than 1g/L. There is also a need to treat this type of effluent at low energy cost and low environmental footprint by recovering the carbon present in the effluent. Such processing will preferably be robust so as to limit the need for human intervention, and is suitable for processing a wide range of situations, and in particular for various repository structure profiles.

A first object of the invention relates to a process for the biological treatment of an effluent, the effluent to be treated containing nitrogen in the form of ammonium and having a concentration of carbonaceous matter measured by the chemical oxygen demand (COD) greater than or equal to 1g per liter of effluent, the method comprising:

a) a step of anaerobic digestion of the effluent to be treated to produce biogas and a digestate constituting a first liquid effluent having a reduced carbon content, this step being implemented in a covered lagoon equipped with a recovery device biogas,

b) a stage of biological treatment of the digestate obtained in stage (a) to produce a second liquid effluent with a reduced nitrogen content.

In the context of the present invention, anaerobic digestion, also called anaerobic digestion, is carried out by lagooning and aims at the treatment of particulate and carbonaceous matter. Thus, in the context of the present invention, the covered lagoon constitutes a methanizer with low energy (no agitation), great robustness (long residence time) and great rusticity (structure simple to operate). Nitrogen treatment is done by a biological treatment which can be chosen according to the characteristics of the effluent after anaerobic digestion.

This particular sequence of steps, and in particular the use of a covered lagoon to produce biogas, allows the treatment of effluents, in particular the treatment of carbon and nitrogen pollution, at low energy cost and with recovery of the carbon in biogas. The recovery and recovery of biogas from the covered lagoon allows the production of renewable energy without greenhouse gas emissions.

Advantageously, the anaerobic digestion step can be carried out under conditions maximizing the production of biogas, and optionally minimizing the residence time of the effluent to be treated in the covered lagoon.

Advantageously, during step a), the covered lagoon can be fed continuously and the digestate can be extracted continuously. This facilitates the preservation of the biological balance of the lagoon.

Advantageously, the digestate can be stored in at least one storage capacity before its treatment in step b), for example an open lagoon, for easier management of hydraulic flows.

Advantageously, the treatment of step b) can be chosen as a function of the BOD5/N—NH ₄ ratio of the digestate and target values of the residual nitrogen and carbonaceous matter contents of the second liquid effluent.

Advantageously, the covered lagoon may comprise at one of its ends a first inlet for the effluent to be treated and at its other end an outlet for the digestate, and the effluent to be treated is introduced into the lagoon covered in part or in whole by the first inlet and at least one other effluent to be treated, or another part of the first effluent to be treated, is introduced into the covered lagoon by at least one other inlet of the covered lagoon located between the first inlet and the outlet.

Lagoons are usually storage structures, aerated, placed upstream of a biological treatment. Their operation is carried out according to the volume of effluents to be contained and the residual carbon requirements for downstream treatment. This second condition can come to constrain the first, by forcing the operator to reduce the hydraulic residence times in the lagoon by means of bypasses: the flow entering the lagoon does not circulate in it but is directly pumped towards the biological treatment of nitrogen downstream, to supply the carbon it contains. The present invention makes it possible to remove this second constraint: given that a nitrogen treatment solution can be proposed for each possible BOD5/N-NH ₄ ratio of the digestate, the quantity of residual carbon necessary at the lagoon outlet does not is more constrained. Volume capacities can then be used optimally. The invention not only proposes rethinking these storage structures into carbon treatment and recovery structures, but in addition maximizes the potential for carbon treatment and recovery by allowing downstream biological treatment independent of the BOD5/N-NH ₄ ratio of the digestate.

Methanization processes in covered lagoons have the advantage of involving low energy consumption, because they are neither heated nor agitated. In addition, the residence time of the effluents in the covered lagoon is significant, making the process robust and capable of undergoing significant load variations (in terms of flow rate and BOD5). These processes are particularly effective and recommended in hot countries, where the ambient temperature allows high biological activity to be maintained. The simplicity of implementation and the hardiness to the exploitation of anaerobic lagoons also make them suitable technologies for application to the treatment of carbon-laden leachates from landfills in developing countries.

Furthermore, the coupling with a nitrogen treatment proposed by the invention makes it possible to maximize the potential of these lagoons, in particular a maximized production of biogas with low energy consumption. No restriction on the amount of carbon is necessary, unlike the conventional nitrification/denitrification process. Hydraulic residence times are therefore no longer limited by downstream needs and carbon reduction no longer needs to be restricted.

For example, treatment of nitrogen by nitrate shunt saves 25% of air requirements for nitrogen oxidation (and therefore the associated energy) compared to nitrification/denitrification. Furthermore, carbon demand is reduced by 40% and sludge production is also reduced by 40%. The deammonification process, on the other hand, allows the air requirements associated with the oxidation of nitrogen to be reduced by a further notch. This is an energy saving linked to aeration of 50% compared to nitrification/denitrification. It completely eliminates carbon requirements and achieves a reduction in sludge production estimated at 80%.

These two solutions (nitrate and NP/A shunt), requiring much less carbon than nitrification/denitrification, make it possible to maximize the potential for energy recovery by the first stage of methanization. This implies that it is as much carbon that does not need to be sent to these processes; this therefore results in a significant reduction in the volume of the works of this second stage.

Another object of the invention relates to an installation for the biological treatment of an effluent, the effluent to be treated containing nitrogen in the form of ammonium and having a concentration of carbonaceous matter measured by the chemical oxygen demand (COD) greater than or equal to 1g per liter of effluent, the installation comprising:

- a covered lagoon equipped with a biogas recovery device and a liquid digestate evacuation device, and optionally equipped with baffles and/or biological supports,

- optionally a liquid digestate storage capacity,

- at least one reactor for the biological treatment of the digestate.

Usually, the lagoon has at one of its ends an inlet for the effluent to be treated and at its other end an outlet for the digestate. Advantageously, it may include at least one other inlet for the effluent located between the first inlet and the outlet.

Description of figures

Other features and advantages of the invention will become apparent on reading the description given below of a particular embodiment of the invention, given by way of indication but not limitation, with reference to the appended drawings in which:

There schematically represents a processing installation according to one embodiment of the invention.

There represents the cumulative production of methane as a function of time of the test of example 1.

There represents a graph showing the nitrogen loads applied (kg N/m3/d) and the associated reduction rates (%) during phases 1 to 4 in the two bioreactors R1 and R2 of Example 2.

There represents a graph indicating the impact of temperature on the nitrogen loads applied (kg N/m3/d) and the associated reduction rates (%) during phases 5 and 6 in the two bioreactors R1 and R2 of the example 2.

There represents a graph indicating the impact of temperature on the nitrogen concentrations in the effluents during phases 4 to 6 in the two bioreactors R1 and R2 of example 2.

Detailed description of the invention

Abbreviations

Anammox (ANaerobic AMMonium Oxidation): Anaerobic oxidation of ammonium

BOD: Biological oxygen demand (quantity of oxygen necessary to oxidize all organic matter (biodegradable) by biological means)

BOD5: Biological oxygen demand measured after 5 days

COD or CODtot: Chemical oxygen demand (measurement of all oxidizable substances, whether biodegradable or not)

CODs: Measurement of the quantity of soluble oxidizable substances after filtration at 0.45µm

HRT (hydraulic retention time): Hydraulic retention time

N/DN: Nitrification / Denitrification

NLR (Nitrogen Loading Rate): Nitrogen load applied

NP: Partial nitration

NP/A: Partial nitritation/ Anammox

RR (Removal Rate): Reduction rate

SBR (Sequencing Batch Reactor): Sequential batch reactor

TKN (Total Kjeldahl Nitrogen): Total Kjeldahl Nitrogen

TN (Total Nitrogen): Total Nitrogen

TSS (Total Suspended Solid) or MES: Total Suspended Solids

DM: Dry Matter, includes both suspended solids and dissolved salts

VM: Volatile Matter

VMS: Volatile Matter in Suspended Matter

Nature of the treated effluent

The effluent treated by the method according to the invention has a COD of at least 1 g per liter of effluent.

“Effluent” means a single effluent or a mixture of effluents.

Advantageously, the COD of the effluent will be at least 4g/L, preferably at least 15g/L, or even at least 20g/L. COD can be measured according to standard NFT 90-101-February 2001 or ISO 6060-1989.

This effluent can be chosen from:

• leachate from a storage facility, preferably non-hazardous waste, also called landfill leachate,
• a liquid collected in the bottom of a collection truck, advantageously from this non-hazardous waste, (also called "truck juice"),
• a liquid agricultural effluent, in particular from agriculture or animal husbandry activities, for example slurry,
• a liquid effluent from the food industry, which includes industrial residues/co-products from the processing of animal and plant products (dairy, starch factory, brewery, slaughterhouse, tannery, etc.)
• an organic liquid effluent from other industries (textiles, paper, cosmetics, pharmaceuticals, etc.),
  - a mixture of two or more of these effluents.

“Non-hazardous waste” means waste that does not have any of the characteristics specific to hazardous waste, in particular as defined by the Basel Convention, and which is sometimes referred to as “ordinary waste”. These wastes usually include recyclable waste and organic waste. Recyclable waste includes glass, metals, cardboard, papers, wood, plastics, textiles. Organic waste includes biodegradable organic matter: it is mainly garden waste, leftover meals for households or collective catering, unsold items from the distribution of agro-food waste. Non-hazardous waste can also include waste of interest as a fuel: it can be residual household or industrial waste, rich in plastic materials in particular.

In the present invention, non-hazardous waste does not include waste from human economic activities, including agriculture, animal husbandry, food processing and other industries.

Advantageously, the effluent to be treated may have a nitrogen content TKN of 0.1 to 15 gN/L, preferably of 0.5 to 10 gN/L, preferably of 1 to 5 gN/L. This nitrogen content TKN can be measured according to standard NF EN 25663-January 1994 or ISO 5663:1984.

Advantageously, the effluent to be treated can have a conductivity of 0.5 to 45 mS/cm, preferably of 1 to 30 mS/cm, more preferably of 1.5 to 20 mS/cm. Conductivity can be measured according to standard NF EN 27888-January 1994 or ISO 7888:1985.

In a particularly advantageous embodiment, the method according to the invention can make it possible to treat a leachate, in particular a young leachate, a liquid collected in the bottom of collection trucks, in particular non-hazardous waste, or a mixture thereof. . In particular, the mixture of two or more of these effluents may present the characteristics of a young leachate.

Advantageously, the effluent to be treated may have one or more of the characteristics presented in Table 1 for leachates, preferably for a young or medium leachate, more preferably for a young leachate, optionally in combination with the different values of nitrogen TKN and/or the various conductivity values previously presented. In a preferred embodiment, the effluent to be treated may have the characteristics of a young or medium leachate, preferably of a young leachate, in combination with one or more of the preferential and/or preferred values of nitrogen content TKN and/or conductivity previously presented.

It should be noted that a leachate can be characterized by a BOD5/COD (gO ₂ /gO ₂ ) ratio of 0.1 to 1, as presented in table 1.

Generally, a leachate can be characterized by a COD/MES ratio of 5 or more, preferably strictly greater than 5. This ratio can be at most 100, preferably at most 80. Also, advantageously, in one embodiment, the effluent to be treated may have a COD/SS ratio in the value range of a leachate, in combination or not with the values of the characteristics of a leachate presented in table 1 and/or with the different values of nitrogen TKN and/or with the different values of conductivity previously presented.

In general, slurry has a COD/SS ratio strictly below 5.

SS can be measured using standard NF EN 872-June 2005.

Stage a) of anaerobic digestion

This anaerobic digestion step is implemented in a covered lagoon, also called an anaerobic lagoon, equipped with a biogas recovery device. Anaerobic lagoons are basins, partially or totally devoid of dissolved oxygen, generally unaerated, unheated and unmixed. Lagoons have the advantage of being very simple to build, of requiring low investment, operation and maintenance costs and of having good resistance to hydraulic and organic shocks.

Three main configurations of anaerobic lagoons can be considered:

• renovation of an existing pool with a floating cover, also called a negative pressure cover: the flexible cover is placed on top of the pool and held in place with external fixings. The system is surmounted by a grid of pipes recovering the biogas by overpressure. It is easy to design and deploy and therefore might be the most economical solution. However, as the top coating is in direct contact with the effluent, it can quickly deteriorate through corrosion. Anaerobic digestion could generate foam which could seep into the biogas line and threaten downstream processes. The evacuation of rainwater accumulated on the coating could also represent a significant complication.
• renovation of an existing lagoon with an inflated cover: the waterproof cover is fixed to the ground and permanently maintained at a constant positive pressure, for example by injection or by controlled extraction via a slight overpressure induced by the biological production of biogas. This avoids any contact between the effluent and the membrane and prevents clogging of the pipes by the foam. In order to improve wind resistance, a permanent structure can be installed. The limitation comes from the large quantity of biogas permanently stored and the health and safety measures to be taken. It also requires some energy (and the corresponding OPEX) for pressurization.
• "ready-to-use" reactor of the lagoon type, such as cushion basins or flexible tanks: this type of digester consists of an autonomous geomembrane bag, with or without a rigid retaining structure, which can be placed on flat ground or in an existing empty basin or specially dug landing platform. This type of system offers the best air and water tightness. They can be designed with a separate area dedicated to liquids and gases, which increases the life of the overall system. This "plug-and-play" system is easy to deploy, modular and adaptable to many environments. As for inflated blankets, specific health and safety measures must be taken.

The conditions for implementing this step, namely the temperature and the residence time, can advantageously be chosen in order to maximize the production of biogas, in particular according to the carbonaceous matter (COD) content of the effluent to be treated. and also the flow of effluent entering the covered lagoon and its volume capacity, in other words the induced residence time. Thus, the implementation conditions will not depend on the BOD5/N-NH4 ratio of the first effluent leaving the anaerobic digestion stage.

Those skilled in the art are able to determine appropriate implementation conditions depending on the bacteria present in the covered lagoon.

In general, it is unlikely to regulate the temperature of a covered lagoon, in particular a lagoon with a large volume capacity. However, one could consider regulating the temperature of the covered lagoon by providing a heat flow, for example by heating the effluent to be treated.

In a preferred embodiment, the temperature of a covered lagoon is not regulated, the production of biogas can then be maximized by an appropriate choice of the residence time of the effluent to be treated. This residence time may be determined by those skilled in the art depending on the temperature of the covered lagoon, the content of the charge to be treated in carbonaceous material, for example depending on the COD, the volume of the covered lagoon and the volume of effluent to be treated.

In one embodiment, the optimization of the production of biogas, corresponding to a maximization of the degradation of the COD, is preferably achieved by minimizing the residence time in the covered lagoon. This makes it possible to minimize the volume of the lagoon covered in order to control costs (CAPEX and OPEX).

In general, the anaerobic digestion can be carried out at a temperature of 5 to 55°C, preferably of 15 to 50°C, and more preferably between 20 to 45°C, or even 25 to 40°C, or any other temperature range defined by these limits.

Preferably, the temperature of the covered lagoon is not regulated: the temperature of the digestion step will therefore depend on the outside temperature.

In general, the residence time of the effluent in the anaerobic lagoon can be at least 1 day, preferably at least 5 days. Very long residence times (365 days or more) are also possible. More often, the residence time may be 1 to 150 days, preferably 5 to 100 days, more preferably 15 to 100 days, more preferably 20 to 70 days, or any other interval defined by these limits. . The residence time may also be adjusted according to the volume of effluent to be treated and its COD content, these being likely to vary suddenly, in particular due to weather conditions.

The covered lagoon can be supplied with effluent to be treated continuously or discontinuously. In the latter case (so-called “batch” operation), a volume can be left at the bottom of the covered lagoon containing the biomass. A continuous supply may result from a continuous production of the effluent to be treated, which can then directly supply the covered lagoon directly, such as landfill leachate, or possibly via an effluent recovery tank upstream of the lagoon. covered. We prefer a continuous supply of the lagoon in order to preserve the balance of the biomass.

In one embodiment, it is also possible to provide a stepped supply of the covered lagoon: one or more effluents can then be introduced at different points of the lagoon. This stepped supply can be a function of the COD of the effluents to be treated: a first effluent with a COD higher than a second effluent being introduced into the lagoon upstream of this second effluent. The points of introduction of the different effluents can then be chosen according to the residence time necessary to treat the effluent. The stepped supply could also be used to introduce the same effluent or different effluents at different temperatures: one or more heated effluents can be introduced at different points in the lagoon.

In one embodiment, the covered lagoon can be equipped with biological supports and/or baffles, the baffles being formed for example of horizontal or vertical walls. These baffles and biological supports make it possible to facilitate the fixation and development of the biomass (acceleration of the degradation kinetics) and to facilitate the sedimentation of the particles contained in the effluent (acceleration of the separation of the particulate COD and the degradation of that -this).

The anaerobic digestion step produces a digestate with a reduced carbon content. Advantageously, the COD of this first liquid effluent can be reduced by 60 to 90%, by 70 to 90%, or even by 80 to 95%.

Step b) of reducing the nitrogen content

The digestate from the anaerobic digestion is subjected to a biological treatment step producing a second liquid effluent with reduced nitrogen content.

This step b) can comprise one of the treatments b1) to b5) described below.

The choice of treatment may in particular depend on the carbonaceous matter and nitrogen content of the digestate and on the target carbonaceous matter and/or nitrogen contents desired for the second liquid effluent. In particular, the treatment can be chosen according to the BOD5/N-NH ₄ + ratio between the BOD5 and the concentration of ammonium ions NH4+ noted N-NH4+ (mass of the nitrogen element) of the outgoing digestate.

The BOD5 can be determined using the standards NFT 90-103-December 1975, EN 1899-1-May 1998, ISO 5815-1:2019. The N-NH4+ concentration can be determined using standards NFT 90-015 1 & 2 (January 2000) and ISO 7150-1:1984.

The treatment b1) comprises, or consists of, a biological nitrification/denitrification treatment, comprising in particular a biological nitrification treatment, in particular under aerobic conditions, and a biological denitrification treatment, in particular under anoxic conditions.

In a known manner, the biological nitrification treatment is carried out under aerobic conditions by autotrophic microorganisms capable of oxidizing the ammonium ions (NH4+) into nitrite ions (NO2-) then into nitrate ions (NO3-). This step is usually carried out in two sub-steps of nitritation and nitration. The biological denitrification treatment is carried out under anoxic conditions, by heterotrophic micro-organisms able to reduce the nitrate ions produced during the first treatment into nitrite ions, then the nitrite ions into gaseous nitrogen (N2).

Treatment b1) (N/DN treatment) is more particularly suited to the treatment of effluents having a BOD5/N-NH ₄ + ratio greater than or equal to 3 gO ₂ /gN.

The treatment b2) comprises, or consists of, a biological nitritation/denitritation treatment, comprising in particular a biological nitritation treatment, in particular under aerobic conditions, and a biological denitritation treatment, in particular under anoxic conditions. This treatment is also known as a “nitrate shunt”.

In known manner, the nitritation treatment is carried out under aerobic conditions by autotrophic microorganisms capable of oxidizing ammonium ions (NH4+) to nitrite ions (NO2-). The denitritation treatment is carried out under anoxic conditions by heterotrophic micro-organisms capable of reducing nitrite ions (NO2-) to dinitrogen (N2).

Treatment b2) is more particularly suited to the treatment of effluents having a BOD5/N-NH ₄ + ratio of 1.7 to 3 gO ₂ /gN.

The treatment b3) comprises, or consists of, a biological deammonification treatment, comprising in particular a biological treatment of partial nitritation, in particular under aerobic conditions, and a biological treatment of anaerobic oxidation of the ammonium ions in the presence of nitrite ions.

In known manner, the partial nitritation treatment is carried out under aerobic conditions by autotrophic microorganisms capable of oxidizing ammonium ions (NH4+) to nitrite ions (NO2-). The anaerobic oxidation treatment is carried out under anaerobic conditions by autotrophic micro-organisms capable of oxidizing ammonium ions (NH4+) to dinitrogen (N2) in the presence of nitrite ions (NO2-) (Anammox bacteria).

Treatment b3) (NP/A treatment) is more particularly suited to the treatment of effluents with a BOD5/N-NH ₄ + ratio of less than 1.7 gO ₂ /gN, for which it is not desired to further reduce the carbon content.

Provision may also be made to combine the treatments b2) and b3): thus in a first configuration, the treatment b4) comprises, or consists of, the successive performance of the treatments b2) and b3), and optionally a recirculation of a part of the effluent from the biological deammonification treatment to the biological treatment b2). In this first configuration, the ammonia pollution (NH ₄ ⁺ ) will be treated as much as the carbon allows in the nitrate shunt (treatment b2). The nitrogenous composition of the outlet effluent from the nitrate shunt will be predominantly ammoniacal, like the inlet effluent. The process will therefore allow a reduction in the ammonia flow but without a major change in the distribution of the nitrogenous species between the inlet and the outlet of the treatment b2). Treatment b3) (partial nitritation followed by ammonium oxidation) downstream of the nitrate shunt allows treatment of the residual ammoniacal pollution.

In a second configuration, the treatment b5) comprises, or consists of, a biological treatment of partial nitritation, a biological treatment of denitritation and a biological treatment of anaerobic oxidation of ammonium ions in the presence of nitrite ions. Partial nitritation and denitritation biological treatments are implemented with the same bacteria as a nitrate shunt treatment. These two treatments thus correspond to a modified nitrate shunt treatment b2) in which the nitritation treatment has been modified in order to include the partial nitritation carried out in b3).

The biological treatment of anaerobic oxidation of ammonium ions in the presence of nitrite ions is carried out under anaerobic conditions by autotrophic micro-organisms capable of oxidizing ammonium ions (NH ₄ ⁺ ) to dinitrogen (N ₂ ) in the presence of nitrite ions (NO ₂ ^- ) (Anammox bacteria). This treatment therefore corresponds to only part of the deammonification treatment described above, ie to a modified treatment b3).

The treatment b2) thus modified makes it possible to treat the ammoniacal pollution (NH ₄ ⁺ ) as much as the carbon allows while supporting the partial nitritation of a deammonification treatment (step prior to the action of the Anammox bacteria). There will therefore not only be a reduction in nitrogen in this stage, but also a transformation in the distribution of nitrogenous species. This treatment will preferably be carried out under conditions making it possible to obtain at the outlet an effluent having a ratio of nitrite/ammonium concentrations allowing it to be treated by the Anammox bacteria. This ratio is for example from 0.8 to 1.8, preferably from 1.1 to 1.5 gN/gN. The effluent obtained at the outlet of the modified treatment b2) is then treated with Anammox bacteria in order to reduce the residual nitrogen.

Partial nitritation can be obtained in the usual way by controlling parameters such as the concentrations of nitrogenous species, the oxygen supply, the hydraulic residence time and the age of the sludge.

The first and second configurations b4), b5) are particularly suitable for the treatment of effluents having a BOD5/N-NH ₄ + ratio of less than 1.7 gO ₂ /gN, the content of remaining carbonaceous matter of which it is desired to further reduce.

In a particular embodiment of the treatments b4) and b5), it is possible to provide for a recirculation of part of the effluent rich in nitrates resulting from the biological treatment of anaerobic oxidation by the Anammox bacteria, towards the biological treatment b2), in particular towards at least one of the biological treatments of nitritation and denitritation of b2), or towards the treatment b2) modified, in particular towards at least one of the biological treatments of partial nitritation and denitritation of b2) modified. This should allow degradation of these nitrates during the denitritation carried out in b2) or b2) modified. Indeed, when the nitrogen content is too high at the input of the treatment by the Anammox bacteria, these producing nitrate up to 11% of the load applied in ammoniacal and nitrous nitrogen, it is possible that too large a quantity of nitrates is still present in the effluent. One solution is to recirculate a fraction of this effluent to b2) or b2) modified where it can be treated by nitritation and denitritation.

In one embodiment, the treatment b3) may be followed by a post denitrification treatment in order to eliminate any nitrogen still present. This post-denitrification treatment can be a biofiltration treatment, combining biological purification by fixed cultures and retention of suspended solids. For example, the Biofor® process can be used for this purpose. An addition of exogenous carbon (methanol or equivalent) may then be necessary. Other treatments such as biological discs and bacterial beds (fixed bacterial culture), activated sludge (free bacterial culture), reed beds can also be considered as post denitrification treatment.

The various treatments b1) to b5) implement well-known biological treatments. Those skilled in the art will be able to choose the conditions for each of these treatments, in particular temperature and residence time, to obtain the desired elimination of nitrogen, or even carbon, depending on the bacteria present, the nitrogen content of the digestate , or even carbon, and the volume of digestate to be treated.

Treatments b1) to b5) each include steps under aerobic and anoxic conditions. These steps can be carried out by implementing aerated/non-aerated cycles in a single reactor, for example a sequential reactor also called SBR reactor (Sequential Batch Reactor), a reactor with mixed cultures and continuous feeding (Meteor ^TM -IFAS), or in separate reactors, in particular with recirculation between them. The use of separate reactors allows continuous treatment of the effluents. In any case, the invention is not limited by the number of reactors used, in particular several reactors each operating according to aerated/non-aerated cycles can be provided, or several continuous feed reactors or successive reactors comprising SBRs and continuous feed reactors.

By way of example, the treatments described above could for example be implemented as described in the documents WO2007014994A1 (nitrate shunt), WO2014111878A1 (NP/A), WO2015004583 (N/DN), WO2016020805A1 (NP/ A), WO2019069031A1 (NP/A).

Description of the installation

With reference to the , an installation 1 for the biological treatment of an effluent according to one embodiment of the invention is described. In the figure, the arrows indicate the directions of fluid circulation.

This installation includes a covered lagoon 10 equipped with a biogas recovery device 12 and a liquid digestate evacuation device 14.

The lagoon may consist of a dug basin, the bottom of which will be covered with an impermeable material (concrete, geomembrane, etc.) in order to prevent infiltration into the surrounding environment. This structure will be surmounted by a cover which may be floating or with arches, which will ensure the tightness of the system, in order to ensure complete capture of the gas produced. The system may also be fitted with safety equipment to manage the gas pressure in the containment.

The structure may also consist of a “pillow” or “pillow tank” type system, which consists of an all-in-one fermentation ladle, one part of which is dedicated to liquid storage and the other to biogas storage.

This covered lagoon 10 is connected to an effluent source 2, which may be a recovery tank, as shown in the , or a discharge drain, or any other suitable device.

Downstream of the covered lagoon 10, a storage capacity 16 for the liquid digestate can be provided, preferably an open lagoon, although a tank or similar can be envisaged.

The digestate is then treated in at least one biological treatment reactor 20. This may be an SBR reactor operating in alternating aerobic/anoxic cycles, or two reactors, one for the aerobic phases, the other for the anoxic phases.

Examples of implementation of the treatment of the digestate by the treatments b1)-b5) are described below. However, the invention is not limited by these embodiments.

The treatment b1) of the digestate by N/DN can be implemented using two reactors, one for the aerobic stages, the other for the anoxic stages, with fluid recirculation between the two reactors.

The treatment b2) of the digestate by nitrate shunt can be implemented in a single reactor, preferably an SBR reactor.

The treatment b3) of the digestate by NP/A can be implemented in a single reactor, for example a constant level SBR reactor.

The treatment b4) of the digestate by nitrate shunt then NP/A can be implemented by means of a reactor dedicated to the nitrate shunt treatment and another reactor dedicated to the NP/A treatment, with optional recirculation between the two reactors.

The treatment b5) of the digestate can be implemented by means of a reactor dedicated to the treatment by shunt of modified nitrates (for example by extending the aeration of the reactor to carry out partial nitritation) and another reactor for the treatment by Anammox bacteria.

Examples

Example 1: methanogenic potential test

The methanogenic potential (denoted BMP) corresponds to the maximum quantity of methane produced by a compound during its degradation. Methanogenic potential tests (BMP tests) were carried out in order to determine the achievable COD reduction and the associated methane production.

Four effluents (substrates) were studied: 3 truck juices MJC1, MJC2 and MJC4 and 1 leachate ML1. Their biochemical composition is reported in Table 3.

pH DM (g/L) MV (%MS) CODtot (gO ₂ /L) TSS (mg/L) MVS (%MES) Method of analysis NF EN 12880 - Nov 2000 NF EN 15169-May 2007 ISO 6060-1989, DIN 38409-H41-H44 NF EN 872- June 2005 NF EN 872- June 2005 MJC 1 4.32 65.72 74 78.90 5534.6 88.4 MJC 2 7.12 64.78 52 57.57 6088.3 37.4 ML1 7.56 61.31 48 54.07 12112 29.2 MJC 4 5.21 59.08 79 70.38 -

The juices studied, particularly MJC 1 and MJC 4, have characteristics of organic liquors: acid pH and high organic matter content. ML1 leachate and MJC 2 juice are less organic: they contain 29.4 and 33.7g MV/L respectively. Their pH is greater than 7. It should be noted that the amount of suspended matter in the leachate tested is high: 12g/L; and not very organic (29%). This can be partly explained by the fact that the leachates studied are drained on leaving the landfill directly on clay soil, thereby entraining large quantities of clay.

The BMP tests were performed as follows. The flasks are prepared in triplicate, by mixing each substrate tested with the inoculum according to the Substrate/Inoculum ratio of 0.8 g of COD substrate/g of MV inoculum, for a total volume of 400 mL (substrate volume + inoculum). These flasks are then incubated in anaerobiosis at a temperature of 37°C. Methane production is automatically measured continuously, using a volumetric flowmeter. Flasks containing sodium hydroxide make it possible to absorb the CO ₂ produced by the biomass before the gas reaches the metering flowmeter.

For each series of tests, control vials (or cold references) are prepared with only inoculum. They make it possible to monitor the production of endogenous methane, i.e. resulting from the degradation of the residual organic matter contained in the inoculum. This endogenous production is deduced from the production of the test bottles, in order to determine the production resulting from the degradation of the substrate itself. The following solutions are added to each vial, in order to prevent any deficiency in the medium which could falsify the test:

• 0.4 mL of micronutrient solution (Fe, B, Zn, Cu, Mn, Mo, Al, Co, Ni, Se),
• 4mL of macronutrient solution (NH4+, Na, Mg, Ca)
• 8mL of phosphate buffer

Total COD was determined using Hach Lange LCK 514 kits according to ISO 6060-1989, DIN 38409-H41-H44 methods. The N-NH4+ nitrogen concentration was determined using Hach Lange LCK 303 kits, according to ISO 7150-1:1984, DIN 38406 E5-1, UNI 11669:2017 methods.

The results are represented on the . The cumulative methane production curves are homogeneous for all the substrates tested and show typical patterns of rapidly degrading substrates. No latency time or inhibition phenomenon was observed at this stage. The slowest kinetics are observed for the “MJC2 and ML1” substrates, which have been identified as being substrates with a lower organic content. The methanogenic potentials of the substrates are homogeneous and between 250 and 300 NmL CH4 / g incoming COD. The methanogenic potentials per gram of COD in the effluent and the organic matter degradation rates for the studied substrates are collated in Table 4. The degradation rates were calculated by considering that the theoretical methanogenic potential of one gram of Degraded COD is 350mL of CH4.

Substrate (effluent) Methanogenic potential (NmL CH ₄ / g COD effluent) Degradation rate (%COD) Methanogenic potential/ raw material (Nm ³ CH ₄ / m ³ effluent) MJC1 278 79.4 21.9 MJC 2 268 76.5 15.4 MJC 4 281 89.9 20.1 ML1 256 72 13.6

The organic matter degradation rates are very interesting since they vary between 70 and 90% of the incoming COD. These substrates are therefore largely biodegradable by methanation. Their methanogenic potential relative to the raw material is between 14 and 22 Nm ³ CH ₄ /m ³ of incoming substrate.

Example 2: laboratory tests, elimination of nitrogen by the Anammox route

Throughout the duration of the trial, leachate was sampled from a storage facility in France operated by Suez every two to four weeks. Table 5 gives the mean values of the characteristics of the leachates obtained for the entire test.

pH Conductivity mS/cm N-NH ₄ mgN/L CODtot
mg/L CODs
mg/L Alkalinity mgCaCO ₃ /L PPO ₄ mgP/L N-NO ₂ mgN/L N-NO ₃ mgN/L Mean 8.3 5.9 414 705 662 2531 1.7 0.5 3.7 standard error 0.1 1.2 135 484 480 917 0.9 0.4 0.7

The leachate used thus has a CODtot/N-NH4 ratio of 1.7 gO ₂ /gN.

For the entire test, the leachate was used to feed the reactors after a prior dilution of 60% with dechlorinated water. After dilution, we therefore have on average: CODtot = 282 mg O ₂ /L and N-NH4 = 165 mgN/,/L, with an unchanged CODtot/N-NH4 ratio.

The dilute leachate was then spiked with sodium nitrite to simulate prior partial nitritation. The doping was carried out so as to obtain a nitrite to ammonium ratio of 1.32 gN/gN, in accordance with the stoichiometric ratio of the Anammox reaction. This dilution followed by doping makes it possible to simulate a leachate which contained, before NP, a concentration of N-NH4+ equal to the sum of the ammonium and nitrite ions. Throughout the test, this value varied between 300 and 800 mgN/L. Taking into account the standard deviations (see table 5), a range of CODtot/N-NH4 ratio ranging from 1.1 to 0.25 gO ₂ /gN was studied. Such a range of ratios is consistent with effluents coming out of anaerobic digestion and whose carbon has already been at least partially treated.

Two bioreactors, R1 and R2, fully equipped and automated of the BioFlo 310 type from New Brunswick Scientific - from Eppendorf, were set up. Both reactors consist of a 7.25 liter glass tank with a heat exchanger placed under the reaction vessel. A pump is used to feed the solution to the bottom of the reactor. The evacuation of the reactor is ensured by the overflow of the effluent when feeding the reactor. The granules contained in the effluent are recovered and returned to the reactor. A three-blade stirring system homogenizes the biomass. The speed was set at 80 rpm to ensure good mixing without compromising the integrity of the Anammox granules.

The two reactors were operated using the SBR (Sequencing Batch Reactor) process, with the following cycle:

- Simultaneous supply and emptying: supply of the water to be treated at the bottom of the reactor, the treated water flowing by overflow from the reactor at the same time.

- Reaction: The reaction vessel was mixed and maintained under anoxic conditions for the Anammox reaction to occur.

- Decantation: the Anammox biomass is decanted for 15 minutes.

The frequency and duration of the cycles varied during the test depending on the targeted applied load, performance, nitrogen concentration of the inlet effluent and the desired hydraulic retention time.

Different phases were considered during the test, summarized in Table 6. First of all, around three months were devoted to starting up the reactor and adapting the Anammox biomass to a leachate matrix ( stages 1 and 2). Next, two months were spent monitoring performance while a gradual increase in NLR was applied. During these periods, a leachate with a TN nitrogen content of 300 mgN/L was used (phase 3). Then, the TN nitrogen content and feed conductivity were gradually increased up to 800 mgN/L and 8 mS/cm respectively (phase 4). Here too, performance was monitored for a month and a half. Subsequently, a temperature test was carried out for three and a half months (phases 5 and 6). All the previous phases were carried out at 26°C. The temperature was gradually lowered to 14°C. This period unfortunately suffered from technical problems on the equipment which compromised the proper functioning of the reactors. During these first five phases, R1 and R2 functioned as replicas of each other. In phase 6, the temperature was returned to 20°C only in R1, R2 remaining at 14°C, to test the resumption of bacterial activity.

Analyzes were carried out to monitor the performance of the processes on the two reactors. They relate to NH4+, NO2-, NO3-, pH, conductivity. They were carried out daily for the samples taken from the tank at the beginning and at the end of the reaction batch and every week for the feed solution. Hach microtube kits were used.

Results and discussion

1.1 Starting and increasing the load

Throughout this period, the temperature in the two reactors was constant and equal to 26°C. There illustrates the NLR applied and the nitrogen reductions (RR) observed in each reactor for the first four phases. No RR was measured for the first six days because the bioreactors were operating in batch mode. The load gradually increased (phase 1), then stabilized at approximately 0.13 kgN/m3/d for the biomass to adapt to a leachate matrix (phase 2). During this period, the abatement rates slowly increased from 70 to 85%, which is very close to the maximum stoichiometric RR of 89%.

Subsequently, from day 82, the daily input rate was increased (phase 3) by increasing the number of cycles per day and the volume at each feed. Thus, in 53 days, the load was multiplied by 3. Despite this very significant increase in the NLR associated with a 2.5 times reduction in the hydraulic retention time (HRT), the RRs were always maintained between 80 and 90 %.

From phase 4, the concentration of TN in the diet was gradually increased to 800 mgN/L (345 mgN-NH ₄ /L and 455 mgN-NO ₂ /L). Consistent with increasing nitrogen concentration, inlet flow was decreased and HRT increased to keep NLR constant. The measured RR, stable between 80 and 90% with an average value of 88% for the two reactors, reflected the very good performance despite the increase in the TN concentration. It should also be noted that during phase 4, the conductivity of the feed solution increased from about 4 to 8 mS.cm ^-1 .

The outlet TN concentration, corresponding to the sum of three nitrogen species (ammonium, nitrites, nitrates) was always kept below 100 mgN/L. This phase therefore highlights the suitability of the Anammox-based treatment for a wide range of nitrogen concentrations in the leachates (up to 800 mg/L) and conductivity (up to 8 mS.cm ^-1 ).

The Anammox activities measured in the two reactors also confirmed the good acclimatization and the growth of the bacterial population (not shown). Activities of more than 300 mgN/L/d were obtained at the end of the third phase. During the fourth phase, the change in power quality in terms of TN concentration and conductivity did not affect activities. An increase was even observed up to 390 mgN/L/d for R1 and more than 700 mgN/L/d for R2.

1.2 Temperature test

From the 176th day, the temperature was gradually lowered from 26°C to 14°C in the two reactors R1 and R2 (phase 5). In phase 6, the recovery of R1 after the restoration of a temperature of 20°C was studied, while R2 remained at 14°C. For this entire temperature test, the same NLR as in phase 4 was targeted (0.4 kgN/m ³ /d) and the same feed solution was used (800 mgN/L). In addition, certain technical problems had a critical impact on the operation of the reactors from day 208 to day 235 and from day 261 until the end. These periods are represented by a red line on the x-axis in Figures 4 and 5.

There shows the NLR and the RR (top) as well as the evolution of the temperature in the reactors (bottom). No RR was calculated for the first malfunction period. The temperature started to drop from day 176 and gradually reached 18°C on day 189, a loss of about one degree every two days. The temperature was maintained at 18°C until the 196th day. During this first period, the RR was still very high, with average values of 87% for R1 and R2. Thus, for this temperature range, 18°C-26°C, the performances remained stable.

When the temperature was set at 16°C (day 196), the RR, especially for R1, decreased slightly. The average RR between day 196 and day 206 (before technical issues) was indeed 81 and 83% for R1 and R2, respectively. This loss of performance was amplified later, but not only due to the malfunctioning of the equipment. Different reasons can explain this behavior and the total concentration of nitrogen in the output presented in the helps to understand the evolution of performance described above.

First, the total nitrogen concentration at the start of phase 5 (until day 196) stabilized around 100 mgN/L. From the 196th day, the TN concentration increased to values above 150 mg/L. Moreover, while the percentage of N-NO ₃ in the TN concentration at discharge between day 176 and day 196 was 87 and 85%, this ratio fell to 48 and 66% in R1 and R2, respectively, the next days. This proves that more substrates have accumulated in the reactor rather than being transformed into N ₂ and N-NO ₃ . It highlights a decrease in the activity of Anammox at 16°C. Consequently, high concentrations of nitrites, potentially inhibitors of the Anammox reaction, were measured (greater than 100 mgN-NO ₂ /L). Coupled with the drop in temperature, this resulted in even more pronounced accumulation and a detrimental loop ending in the complete loss of Anammox activity.

From day 236, both reactors were operational again. An NLR of 0.07 kgN/m ³ /d was applied in order to gradually recover the Anammox activity. The RRs were very high, but that was a direct result of the manual reactor washdowns. For the two reactors, activities of between 50 and 150 mgN/L/d were observed. No stable activity or performance was quickly regained, as demonstrated by the R2 who continued the test at 14°C until day 290. Maximum activities of 150 mgN/L/d were obtained at 14°C with an RR of 80% for an NLR of between 0.12 and 0.15 kgN/m ³ /d.

On day 253, R1 was returned to 20°C to test the ability to recover Anammox biomass. Activities of 225 mgN/L/d were quickly obtained. Nevertheless, the performances have not been completely recovered compared to the beginning of phase 5. RRs between 70 and 85% have been measured for NLR in a range of 0.15 to 0.3 kgN/m ³ /d.

Findings

These tests confirmed that the Anammox biomass is suitable for nitrogen removal in the leachate. Mature leachates are the most relevant matrix given their low BOD5 content. Younger leachates have a high concentration of BOD5 and a reduced BOD5/nitrogen ratio which is more difficult to obtain. Prior treatment of young leachate by anaerobic digestion should therefore allow treatment with Anammox biomass, after partial nitritation.

The tests carried out within the framework of this particular embodiment on the mature leachates and confirmed in the first place that this matrix was not detrimental to the Anammox activity.

Claims (16)

Process for the biological treatment of an effluent, the effluent to be treated containing nitrogen in the form of ammonium and having a concentration of carbonaceous matter measured by the chemical oxygen demand greater than or equal to 1 g per liter of effluent, the process comprising:
a) a step of anaerobic digestion of the effluent to be treated to produce biogas and a digestate constituting a first liquid effluent having a reduced carbon content, this step being implemented in a covered lagoon equipped with a recovery device biogas,
b) a stage of biological treatment of the digestate obtained in stage (a) to produce a second liquid effluent with a reduced nitrogen content. Biological treatment process according to claim 1, in which step a) of anaerobic digestion is carried out under temperature and residence time conditions maximizing the production of biogas as a function of the carbonaceous matter content (COD) of the effluent to be treated, the flow rate of effluent entering the covered lagoon and the volume capacity thereof and the bacteria present in the covered lagoon, and optionally minimizing the residence time of the effluent to be treated in the covered lagoon. Biological treatment process according to any one of claims 1 or 2, in which, during step a), the covered lagoon is continuously fed and the digestate is continuously extracted. Biological treatment method according to any one of Claims 1 to 3, in which the digestate is stored in at least one storage capacity before its treatment in step b). Biological treatment method according to any one of Claims 1 to 4, in which step b) of biological treatment comprises:
b1) a biological nitrification/denitrification treatment. Biological treatment method according to any one of Claims 1 to 4, in which step b) of biological treatment comprises a treatment chosen from:
b2) a biological nitritation/denitritation treatment,
b3) a biological deammonification treatment,
b4) the successive performance of treatments b2) and b3), and optionally a recirculation of part of the effluent from the biological deammonification treatment towards the biological treatment b2),
b5) a biological treatment comprising a biological treatment of partial nitritation, a biological treatment of denitritation and a biological treatment of anaerobic oxidation of ammonium ions in the presence of nitrite ions, and optionally a recirculation of part of the effluent from the biological anaerobic oxidation treatment to at least one biological treatment chosen from the biological partial nitritation treatment and the biological denitritation treatment. Biological treatment process according to claim 6, in which treatment b3) is followed by a post-denitrification treatment in order to eliminate any nitrogen still present in the second liquid effluent. Biological treatment process according to any one of Claims 5 to 7, in which the treatment of step b) is chosen as a function of the BOD5/N-NH ₄ ratio of the digestate and target values of the residual nitrogen and matter contents. carbon content of the second liquid effluent, and in which:
- treatment b1) is suitable for treating effluents with a BOD5/N-NH ₄ + ratio greater than or equal to 3 gO ₂ /gN,
- treatment b2) is suitable for treating effluents with a BOD5/N-NH ₄ + ratio of 1.7 to 3 gO ₂ /gN,
- treatments b3), b4) and b5) are suitable for the treatment of effluents having a BOD5/N-NH ₄ + ratio of less than 1.7 gO ₂ /gN. Biological treatment process according to any one of Claims 1 to 8, in which the effluent to be treated is chosen from:
- leachate from a storage facility, preferably non-hazardous waste,
- a liquid collected in the bottom of a collection truck, advantageously non-hazardous waste,
- a liquid agricultural effluent, for example a slurry,
- a liquid effluent from the food industry, which includes industrial residues/co-products from the processing of animal and plant products,
- an organic liquid effluent from the textile, paper, cosmetics and pharmaceutical industries,
- a mixture of two or more of these effluents. Biological treatment process according to any one of Claims 1 to 9, in which the effluent to be treated has a nitrogen content TKN of 0.1 to 15 gN/L. Biological treatment process according to any one of Claims 1 to 10, in which the effluent to be treated has a conductivity of 0.5 to 45 mS/cm. Biological treatment process according to any one of Claims 1 to 11, in which the effluent to be treated has a BOD5/COD (gO ₂ /gO ₂ ) ratio of 0.1 to 1. Biological treatment process according to any one of Claims 1 to 12, in which the effluent to be treated has a COD/SS ratio of 5 or more, preferably strictly greater than 5. Biological treatment process according to any one of claims 1 to 13, in which the covered lagoon has at one of its ends a first inlet for the effluent to be treated and at its other end an outlet for the digestate, and the effluent to be treated is introduced into the lagoon covered in part or in whole by the first inlet and at least one other effluent to be treated, or another part of the first effluent to be treated, is introduced into the lagoon covered by at least one other inlet of the covered lagoon located between the first entrance and the exit. Installation (1) for the biological treatment of an effluent, the effluent to be treated containing nitrogen in the form of ammonium and having a concentration of carbonaceous matter measured by the chemical oxygen demand greater than or equal to 1 g per liter of effluent, the installation comprising:
- a covered lagoon (10) equipped with a biogas recovery device (12) and a liquid digestate evacuation device (14), and optionally equipped with baffles and/or biological supports,
- optionally a capacity (16) for storing the liquid digestate,
- at least one reactor (20) for the biological treatment of the digestate. Installation (1) for biological treatment according to claim 15, in which the covered lagoon comprises at one of its ends a first inlet for the effluent to be treated and at its other end an outlet for the digestate and in that it comprises at at least one other inlet for the effluent to be treated or another effluent to be treated located between the first inlet and the outlet.