WO2024104950A1

WO2024104950A1 - Method for separating an aqueous effluent containing algae-bacteria mixed granular biomass

Info

Publication number: WO2024104950A1
Application number: PCT/EP2023/081594
Authority: WO
Inventors: Mathieu Haddad; Troy Holst; Clément ROCHE; Thibaut SAUR
Original assignee: Suez International
Priority date: 2022-11-14
Filing date: 2023-11-13
Publication date: 2024-05-23

Abstract

The invention relates to a method for separating an aqueous effluent containing algae-bacteria mixed biomass, present in granular form and in suspension, and treated water, the method comprising: (a) a first step of physically separating at least a portion of the granular biomass from the treated water, carried out with no prior addition of a separation aid compound, producing a first thickened stream enriched in granular biomass, the granules of which have dimensions larger than a threshold, and a first aqueous stream depleted of granular biomass, the granules of which have dimensions smaller than the threshold, and enriched in biomass in suspension; (b) a first step of dehydrating the first thickened stream enriched in granular biomass, producing a first cake and a second aqueous stream depleted of granular biomass; (c) a second step of physically separating the first aqueous stream, producing a second thickened stream enriched in biomass in suspension and a third aqueous stream forming a treated water; and (d) a second step of dehydrating the second thickened stream enriched in biomass in suspension, producing a second cake and a fourth aqueous stream depleted of biomass in suspension.

Description

Process for separating an aqueous effluent containing mixed algae-bacteria granular biomass

Field of the invention

The invention relates to a process for separating an aqueous effluent containing mixed algae-bacteria granular biomass and treated water.

State of the art

Conventional wastewater treatment processes face increasing pressures due to their high energy consumption, significant greenhouse gas emissions and low resource recovery potential.

To meet these needs, processes using microalgae-bacterial aggregates (“MABAs” or “Microalgal-bacterial aggregates” in English) for the treatment of wastewater have been developed.

Microalgae-bacteria are symbiotic systems in which the algae provide dioxygen (O ₂ ) by photosynthesis which is then used by the bacteria for nitrification and carbon reduction, and synthesize polyphosphate which reduces the consumption of energy, and use carbon dioxide (CO ₂ ) produced by bacteria, thus reducing greenhouse gas emissions. Therefore, microalgae-bacteria systems have the potential to self-maintain their oxygen consumption, thereby reducing or even eliminating the aeration required for bacterial action and thus reducing associated energy costs. , while limiting the emission of greenhouse gases generated by bacteria.

However, due to their small size (3-30 µm) and negatively charged algal cells, it is difficult and expensive to separate microalgae and/or MABAs from their growth medium. In addition, free algae have a low settling capacity, which requires a long settling time or large sedimentation surfaces. It is estimated that approximately 20-30% of the operating costs of microalgae cultivation are related to the separation of algae, mainly through coagulation/flocculation, centrifugation, flotation, etc. Recently, there has been growing interest in using algal immobilization or fixation processes to address this separation problem. The immobilization of algae in several materials such as alginate and chitosan thus showed better settleability. However, their large-scale application for resource recovery is limited due to the high cost of the operation (mainly coming from the added materials) and the residual polymer materials added in the culture medium.

There are typically four strategies for harvesting and dehydrating MABAs which are direct centrifugation, gravity decantation followed by centrifugation, dissolved air flotation followed by centrifugation and membrane filtration followed by centrifugation . In addition, a thickening step upstream of dehydration makes it possible to reduce the size and energy consumption of the dehydration step, which is generally carried out by centrifugation.

Today, the first and second strategies are generally considered uneconomical when treating wastewater. Indeed, direct centrifugation induces high CAPEX and OPEX while the use of gravity decantation requires a large footprint due to the low Mohlman Index (“SVI” or “sludge volume index” in English) of the sludge. The use of a dissolved air flotation process requires the addition of polymers, which generates high costs and complicates the process. Finally, although membrane filtration as a thickening approach seems attractive in that it does not require the addition of chemical compounds such as polymers, it is limited by the cost and lifespan of the membrane (approximately 8 years), which compromises economic feasibility on an industrial scale. In addition, all the solutions mentioned above concentrate the granular microalgae-bacteria aggregates with all the sands and microsands and other contaminants (microplastics, possibly added chemical compounds, etc.) present in the wastewater. These contaminants have no value from an application point of view (agronomic, extraction of value-added products) and therefore reduce the quality of the final product.

Aerobic algal-bacterial granular sludge (“AGS” or “Algal-bacterial Granular Sludge” in English), also called “OPS” or “Oxygenic Photo Granules” in English, has attracted the attention of researchers to the extent that the bacteria in granular form could significantly improve the settling speed of microalgae. These sludges are obtained by introducing microalgae into an aerobic granular sludge (AGS) exploitation system leading to the establishment of granular algae-bacteria symbiosis systems. Compared to MABAs (free culture consortia), the use of aerobic pellets tends to be more attractive. Aerobic bacterial granular sludge has thus been proposed as an efficient and innovative technology for wastewater treatment, which possesses high settling speed, rich biomass retention, simultaneous removal capacity of organic matter and nutrients, and resistance to shock loads and toxic substances.

However, the separation and dehydration techniques for these granular sludges remain similar to the techniques presented above for MABAs with the same drawbacks.

There is therefore a need for an improved process for separating microalgae-bacteria granular biomass from an aqueous effluent, making it possible in particular to dehydrate the biomass without requiring the addition of a chemical compound and to produce a high quality solid residue which mainly contains algal bacterial biomass advantageously containing a very limited quantity of sand or microsand, microplastics and mineral materials or other contaminants which are not part of the biomass and which are present in wastewater. There is also a need for an improved biomass separation process making it possible to recover an aqueous effluent having a very low solids content.

A first subject of the invention relates to a process for separating an aqueous effluent containing mixed algae-bacteria biomass present in granular and suspended form, and optionally impurities, and treated water, the process comprising:
(a) A first step of physical separation of at least part of the granular biomass from the treated water carried out without prior addition of a separation aid compound, producing a first thickened flow enriched in granular biomass whose granules present dimensions greater than a threshold and a first aqueous flow depleted in granular biomass whose granules have dimensions lower than the threshold and enriched in suspended biomass;
(b) A first step of dehydration of the first thickened flow enriched in granular biomass producing a first cake and a second aqueous flow depleted in granular biomass,
(c) A second step of physical separation of the first aqueous flow producing a second thickened flow enriched in suspended biomass, and optionally in impurities, and a third aqueous flow forming treated water;
(d) A second step of dehydration of the second thickened stream enriched in suspended biomass, and optionally in impurities, producing a second cake and a fourth aqueous stream depleted in suspended biomass.

This sequence of steps, and in particular the use of two successive steps of separation of the granular biomass then of the suspended biomass, makes it possible to dehydrate the biomass without requiring the addition of a polymer-type separation aid compound and to produce a first cake which forms a high quality solid residue containing mainly mixed algae-bacteria granular biomass. This residue could in particular be reused in agriculture for example, particularly as a biostimulant. In addition, a second advantage is that the high value-added solid residue contains a very limited quantity of sand or microsand, microplastics and mineral materials or other contaminants which are not part of the biomass and which come from the treatment of wastewater having produces the aqueous effluent separated by the process according to the invention.

The granular biomass with high added value is recovered thanks to the first separation which corresponds to a coarse separation with a threshold determined to recover only algae-bacteria in granular form, generally without, or with a reduced quantity, of pollutants or other impurities . This step therefore does not require the addition of chemical compounds such as polymers or coagulants to promote the separation of the biomass.

The second separation step corresponds to a fine separation whose aim is to separate the remaining biomass in suspension, and optionally the impurities and pollutants, present in the first aqueous flow resulting from the first separation step to obtain water processed in compliance with rejection standards.

Advantageously, the threshold may be at least 50µm, optionally at least 70µm, 125µm, 0.250mm or 1mm. The threshold is set so that only the granular biomass mixed with algae-bacteria is retained, advantageously without pollutants and impurities, which makes it possible to obtain after dehydration a cake with high added value, because it is devoid of chemical compounds to aid in separation, with a significant fraction of biomass.

Preferably, the first physical separation step (a) can be a sieving separation step during which the aqueous effluent passes through a perforated support chosen from a grid, a sieve, a filter or a perforated plate. This step can for example be implemented using a device chosen from a draining table, a sieve, a vibrating sieve, a grid, a belt filter, a filter, a vibrating filter. Thus, the perforated support does not include, or is not, a membrane, which makes it possible to reduce the costs of implementing this step while ensuring effective separation of the granular biomass.

Preferably, the second separation step (c) can be chosen from membrane filtration and mesh filtration. These two types of separation thus allow fine and precise separation to retain the suspended biomass, and optionally the pollutants and impurities, present in the treated aqueous flow. In addition, the risk of clogging of the membrane or the fabric is reduced because the concentration of suspended matter (biomass) is lower since a major part has already been retained during the first physical separation.

In one embodiment, the first aqueous stream can be subjected to a biological treatment step before the second separation step (c). This can make it possible to further reduce nitrogen, carbon, phosphate and/or other pollution, and to improve the quality of the treated water leaving the second physical separation stage.

In one embodiment, a coagulant can be added to the first aqueous stream before the second separation step (c). For example, when the treated water must comply with strict phosphorus discharge standards, a coagulant, for example chosen from ferric chloride, alumina sulfate, polyaluminum chloride, WAC which is an aluminum polymer, or other, can be injected and thus make it possible to bind the residual phosphorus which could not be eliminated during the treatment of the aqueous effluent and to retain it during the second separation. The coagulation of the first aqueous flow before the second separation step (c) can make it possible to reduce the colloidal load to be filtered on the second separation step (c), which makes it possible to further reduce clogging in the case of a membrane or canvas filtration.

In one embodiment, a flocculant and/or a coagulant can be added to the second thickened stream enriched with suspended biomass before the second dehydration step (d). The flocculant will make it possible to improve the capture rate of free biomass, pollutants and impurities and to more easily dehydrate the second thickened flow enriched with suspended biomass and also to integrate a maximum of pollutants and impurities into the cake and not in the treated water.

Preferably, the second and/or fourth aqueous flow leaving steps (b) and (d) are sent upstream of the second separation step (c).

An advantage is to avoid reducing the hydraulic retention time (called "HRT" in English for "Hydraulic Retention Time") in a water treatment basin by a mixed granular biomass algae-bacteria since the aqueous flows leaving the Steps (b) and (d) do not need further treatment. Thus the dimensions of such a pond can be reduced while allowing a better proportion of biomass in the biological system because the suspended biomass will be less diluted. In addition, the flow rate in the separation step (a) can also be reduced, consequently the dimensions of the separation unit implementing step (a) can also be reduced.

The invention also relates to a wastewater treatment process in which:
(A) Water to be treated containing pollutants and impurities is provided, optionally water to be treated that has been previously sieved, degreased and/or desanded;
(B) The water to be treated is brought into contact with a mixed granular biomass of algae-bacteria, and an aqueous effluent is produced containing treated water depleted in pollutants, at least part of the biomass in granular form and at least part suspended biomass, and optionally impurities;
(C) The treated water, the biomass, and optionally the impurities, contained in the aqueous effluent are separated by subjecting the latter to the separation process described above.

Preferably, part of the first thickened flow enriched with granular biomass leaving the first separation step (a) is sent to the step (B) of bringing the water to be treated into contact with a granular biomass. This makes it possible to reduce the hydraulic residence time when implementing step (B) and thus to implement it in a smaller reactor. This approach makes it possible in particular to carry out "recovery thickening" without adding a chemical product, thus increasing the biomass concentration in the reactor and decoupling the solid residence time (called "SRT" in English for “Solid Retention Time”) from HRT.

In one embodiment, during step (B) the aqueous effluent produced can be subjected to a pre-separation step producing a first so-called muddy effluent comprising at least 90% by mass of the granular biomass, and a second so-called aqueous effluent containing the rest of the biomass, and, during step (C), the first muddy effluent is sent to step (a) of the separation process. The second aqueous effluent is sent to step (c) of the separation process, optionally after a biological treatment step.

The invention also relates to an installation for separating an aqueous effluent containing mixed algae-bacteria biomass present in granular and suspended form, optionally impurities, and treated water, in particular suitable for implementing the separation process. according to the invention. The installation includes:
(a) A first physical separation unit, in particular adapted to carry out step (a) of the separation process, comprising an aqueous effluent supply pipe, a first evacuation pipe for a first thickened flow enriched in biomass granular whose granules have dimensions greater than a threshold and a second evacuation pipe for a first aqueous flow depleted in granular biomass whose granules have dimensions less than the threshold and enriched in suspended biomass;
(b) A first dehydration unit, in particular adapted to carry out step (b) of the separation process, comprising a supply pipe connected to the first discharge pipe of the first separation unit and a discharge pipe a second aqueous flow depleted in granular biomass;
(c) A second physical separation unit, in particular adapted to carry out step (c) of the separation process, comprising a supply pipe connected to the second discharge pipe of the first separation unit, a third pipe d the evacuation of a second thickened flow enriched in suspended biomass, and optionally in impurities and a fourth pipe for evacuating a third aqueous flow forming treated water;
(d) A second dehydration unit, in particular adapted to carry out step (d) of the separation process, comprising a supply pipe connected to the third discharge pipe of the second separation unit and a discharge pipe a fourth aqueous flow depleted in suspended biomass, and optionally in impurities.

Preferably, the first separation unit may be a sieving separation unit in which the aqueous effluent passes through a perforated support chosen from a grid, a sieve or a perforated plate.

Preferably, the second separation unit can be chosen from a membrane filtration unit and a fabric filtration unit.

In one embodiment, a coagulant injection line can be connected to the supply line of the second separation unit.

In another embodiment, at least one injection line for a flocculant and/or a coagulant can be connected to the supply line of the second dehydration unit.

In one embodiment, the evacuation pipe for a second aqueous flow depleted in granular biomass and/or the evacuation pipe for a fourth aqueous flow depleted in suspended biomass, and optionally in impurities, can be connected to the supply line of the second physical separation unit.

In one embodiment, the separation installation may further comprise a biological treatment unit connected to the second discharge pipe of the first separation unit and to the supply pipe of the second physical separation unit.

The invention also relates to a wastewater treatment installation comprising:
(A) A pipe supplying water to be treated containing pollutants, and optionally impurities;
(B) A unit for bringing the water to be treated into contact with a mixed algae-bacteria granular biomass, comprising a supply pipe connected to the water supply pipe to be treated and a water evacuation pipe an aqueous effluent containing treated water depleted in pollutants, at least part of the biomass in granular form and at least part of the suspended biomass, and optionally impurities;
(C) A separation installation according to the invention, said aqueous effluent discharge pipe being connected to the aqueous effluent supply pipe of the first physical separation unit of the separation installation according to the invention .

Preferably, the first discharge pipe of the first thickened flow enriched with granular biomass is connected to a recirculation pipe connected to the unit for bringing the water to be treated into contact with granular biomass.

In one embodiment, the contacting unit comprises pre-separation equipment having a first outlet for discharging a first muddy effluent and a second outlet for discharging the rest of the effluent forming a second effluent aqueous, the first outlet being connected to said discharge pipe of the contacting unit and the second outlet being connected to the supply pipe of the second physical separation unit of the separation installation, optionally via a biological treatment unit.

DéfinitionsDefinitions /abréviations/abbreviations

Mixed algae-bacteria biomass is a symbiotic system between microalgae and bacteria. Bacteria are strict aerobic, micro-aerophilic, facultative aero-anaerobic or aerotolerant anaerobic microorganisms capable of eliminating carbon, nitrogen, phosphorus or even organic matter present in wastewater. Microalgae are single-celled photosynthetic microorganisms less than 400 micrometers in size and normally between 1 and 30 micrometers in diameter. The mixed algae-bacteria biomass can include a majority of eukaryotic microalgae or prokaryotic microalgae. The latter generally contain prokaryotic filamentous cyanobacteria as the main microalgae.

The mixed algae-bacteria biomass used is in granular form. Typically, the dimensions of the granules are of the order of 50 to 500 micrometers. As is known, biomass develops in granular form with the help of extracellular polymeric substances present in the system which are natural polymers secreted by microorganisms.

The size of the mixed algae-bacteria biomass granules can be measured, for example, by sieving, by laser diffraction according to ISO 13320:2020, by particle size analysis by a static image analysis method according to ISO 13322 -1:2014, for example by optical microscopy, or by a dynamic image analysis method according to standard ISI 13322-2-2021, or by dynamic light diffusion.

The most commonly used method for measuring granule size is sieving, which consists of pouring the product to be analyzed through a series of standardized sieves (e.g. according to ASTM E11-22) stacked in descending order of mesh openings, the last sieve having the smallest meshes. This method generally applies to regularly shaped particles with a size between 40µm and 8mm.

Suspended biomass is biomass in free culture. In general, a mixed algae-bacteria biomass in suspension forms aggregates of 3 to 30µm.

By “wastewater”, we mean urban wastewater whose origin is essentially domestic but a part of which may be of industrial origin, or even industrial wastewater, in particular that resulting from the agri-food industry or any other industry producing effluents loaded with carbonaceous matter and nitrogen for example.

Dryness is a unit of measurement representing the mass percentage of dry matter in a sludge.

The acronym “TSS” stands for Total Suspended Solid and refers to the dry mass of suspended solids present in an aqueous effluent. It can be determined by standard NF EN 872 (June 2015) or ISO 11923:1997.

Description of figures

Other particularities and advantages of the invention will emerge on reading the description given below of several particular embodiments of the invention, given for information but not limitation, with reference to the appended drawings in which:

There is a schematic representation of the separation installation according to one embodiment of the invention.

There is a schematic representation of the separation installation according to a first embodiment of the invention.

There is a schematic representation of the separation installation according to a second embodiment of the invention.

There is a schematic representation of the installation according to a third embodiment of the invention.

There is a schematic representation of the installation according to a fourth embodiment of the invention.

Detailed description of the invention

The alternative configurations of the different stages of the process presented below can be combined depending on the separation objective decided.

Process of treatment ' water x worn s

The wastewater treatment process firstly comprises a step (A) of supplying water to be treated typically containing pollutants (such as organic matter, in particular compounds containing carbon, nitrogen and /or phosphorus) and impurities (such as sand, microplastics, mineral matter, etc.). The water to be treated may be municipal and/or industrial wastewater that has not undergone prior treatment or water to be treated that has already undergone one or more treatment stages beforehand. The pretreatment step(s) may for example be primary treatment steps which generally make it possible to reduce the solids and/or organic matter content of the water to be treated. This may in particular involve sieving, desanding and/or degreasing. Advantageously, the pretreatment step is only sieving (also called “screening”), desanding and/or degreasing, preferably without prior treatment by decantation. These treatments are typically carried out by passing through successive meshes, comprising meshes of several centimeters (between 10 cm and 5 cm), then 10 mm, 6 mm, and, more rarely, 3 mm, 2 mm or 1 mm. . These preliminary treatments are well known and will not be detailed further.

The treatment process then comprises a step (B) of bringing the water to be treated into contact with a mixed granular biomass of algae-bacteria. Mixed algae-bacteria granular biomass is particularly capable of eliminating pollutants contained in the water to be treated. During the biological treatment of water by contact with the mixed biomass, part of the biomass develops in free culture and not in granular form so that the aqueous effluent leaving the wastewater treatment process comprises at the both biomass in granular form (or granular biomass) and biomass in suspension (in free culture).

The contacting step thus produces an aqueous effluent containing treated water depleted in pollutants, mixed algae-bacteria biomass in granular and suspended form, and optionally impurities. The mass proportion of granular biomass in the effluent can be from 25%m to 80%m.

In one embodiment, during step (B), the aqueous effluent resulting from the bringing into contact of water with the biomass can be subjected to a pre-separation step producing a first so-called muddy effluent comprising at least least 90% by mass of the granular biomass, and possibly part of the suspended biomass, and a second so-called aqueous effluent containing the rest of the biomass. The first muddy effluent may in particular contain from 60 to 100% by mass, advantageously from 70 to 100% by mass or from 80 to 100% by mass, and more advantageously from 90 to 100% by mass, of granular biomass, and optionally part of the suspended biomass. The second aqueous effluent then contains the rest of the biomass, namely mainly (generally more than 90% by mass) the suspended biomass, and, where appropriate, the rest of the granular biomass. This second aqueous effluent may include a quantity of TSS solids from 10 mg/L to 2g/L, from 100mg/L to 1 g/L or in any range defined by two of these limits. These two effluents are then treated separately during step (C), as described below.

This pre-separation step is typically a decantation or clarification step. The quantities of biomass present in each flow can be modulated as a function of the settling/clarification time in a manner known to those skilled in the art. This pre-separation step can be implemented by means of pre-separation equipment which may include a decanter or clarifier integrated or not into the contacting unit in which step (B) is implemented. ). By way of example, the contacting unit may comprise at least one sequenced biological reactor (also designated by the acronym “SBR” from English “Sequenced Batch Reactor”) operating sequentially in mixing and rest modes. to separate the first muddy effluent from the second aqueous effluent. Alternatively, the contacting unit may comprise a contacting reactor connected to a clarifier or decanter to separate the first muddy effluent from the second aqueous effluent.

The treated water present in the aqueous effluent is then separated from the biomass, and optionally from the impurities, during a step (C) corresponding to the separation process according to the invention described below.

When step (B) comprises a pre-separation step, during step (C), the first muddy effluent is sent to step (a) of the separation process according to the invention, the second effluent aqueous being sent to step (c) of the separation process. This makes it possible on the one hand to reduce the volume of effluent sent in step (a) of the separation process and thus to reduce the volume and cost of the equipment used, and on the other hand to improve the separation carried out during this step (a).

The second aqueous effluent, containing mainly the suspended biomass, can be treated during the second separation step (c) described below, optionally after a biological treatment step. The latter makes it possible to improve the quality of the treated water leaving the separation process according to the invention by making it possible to further eliminate various pollution that may be present. This biological treatment can be an aerobic or anoxic biological process for eliminating nitrogen and/or carbon and/or phosphate pollution using appropriate fixed, free or mixed bacterial cultures. This treatment can be implemented in a biological treatment unit such as for example an activated sludge biological reactor, a sequenced biological reactor (SBR), a fluidized fixed culture biological reactor (also designated by the acronym "MBBR" from the English “Moving Bed Biofilm Reactor”), a reactor with free cultures and fixed cultures in suspension (e.g. implemented by IFAS technology – “Integrated fixed-film activated sludge”/fixed film activated sludge), a bacterial bed reactor, a biofiltration reactor, a biological disk reactor also called “Rotating Biological Contactor” in English. Since the biological treatment unit is produced on a smaller volume compared to the volume initially treated during the contacting stage, smaller equipment can be used. When the biological treatment unit uses free cultures, it produces a clarified effluent which is sent to the second separation stage (c) and sludge which can be sent to the second dehydration stage (d).

Process for separating an aqueous effluent

First stage (a) of physical separation

The aqueous effluent produced during step (B) of the wastewater treatment process, or the first pre-separated muddy effluent, is then sent to a first step (a) of physical separation without prior addition of compound separation aid, for example of the coagulant and/or flocculant type. The first step (a) of physical separation produces a first thickened flow enriched in granular biomass whose granules have dimensions greater than a threshold and a first aqueous flow depleted in granular biomass whose granules have dimensions less than the threshold and enriched in biomass in suspension.

The objective of this step (a) is to separate the granular biomass from the treated water, and optionally also from pollutants and impurities, to obtain a flow enriched with mixed algae-bacteria granular biomass, advantageously comprising few pollutants and impurities. . The threshold is chosen in order to separate the majority of the granular biomass, in particular from 80% m to 100% m, preferably from 90% m to 99% m of the granular biomass. This threshold can be determined based on the dimensions of the biomass granules. In general, a threshold of at least 50 micrometers and optionally at least 1 millimeter allows the majority of the granular biomass to be recovered. The threshold can also be at least 70µm, in particular to separate biomass from silt and clay, or at least 125µm or at least 250 micrometers, in particular to separate biomass from very fine or fine sand.

Step (a) is typically a sieving separation step during which the aqueous effluent passes through a perforated support chosen from a grid, a sieve, a filter or a perforated plate. This step therefore does not use membranes likely to clog and does not use chemical compounds to aid separation.

Step (a) can be implemented by means of a device chosen from a draining table, a sieve, a vibrating sieve, a grid, a belt filter, a filter, a vibrating filter, a filter press , a piston press, a screw press. The vibrating screen, belt filter or vibrating filter is a screen or filter having openings through which the aqueous effluent circulates. The vibrating screen or filter will vibrate to filter and separate the biomass. For this separation, the threshold can be reached by choosing a sieve whose openings have a diameter corresponding to the threshold.

Part of the first thickened flow enriched with granular biomass leaving step (a) can optionally be returned to step (B) of the wastewater treatment process. The recirculation of the biomass makes it possible to return a fraction rich in biomass to the reactor of step (B) and thus increase the efficiency of wastewater treatment by improving the performance of biological treatment.

In one embodiment, the first aqueous stream depleted of granular biomass produced by step (a) is subjected to an optional biological treatment step before being sent to step (c) of physical separation. This biological treatment can be as previously described. When step (B) includes a pre-separation step and produces a second aqueous effluent which is subjected to a biological treatment step before being sent to step (c), the same treatment unit can be used. biological or separate biological treatment units for treating this second aqueous effluent and the first aqueous flow depleted in granular biomass produced by step (a).

First stage (b) dehydration

The first thickened flow enriched with mixed algae-bacteria granular biomass leaving step (a), or optionally a part of this first thickened flow enriched with granular biomass, is then dehydrated. During step (b), a first cake with high added value, without added chemical compounds, is produced as well as a second aqueous flow depleted in granular biomass.

This first step (b) of dehydration can be preceded by a thickening step to reduce the volume of the sludge. The thickening step can be a thickening step by gravity settling with recovery of the granules at the bottom of the thickener while the liquid is evacuated by an overflow or a dynamic thickening step. Dynamic thickening can be carried out by microbubble flotation (also called dissolved air flotation), by dewatering (e.g. using a drip or thickening grid, a dewatering table, or a dripping drum or thickener drum) or by centrifugation. Microbubble flotation is based on the injection of gas into the liquid containing the granules, which separates the liquid and solid phases by difference in density. The thickening step can also include thickening by gravity settling and dynamic thickening, advantageously thickening by gravity settling followed by dynamic thickening.

Dehydration makes it possible to reduce the water content of the biomass to reach a dryness of approximately 10% to 20%. Dehydration can be carried out by filtration (a filter press, a piston press, a screw press) and/or by centrifugation in the usual manner.

The richness of the cake in mixed microalgae-bacteria biomass thus produced allows its subsequent use as biofertilizer in agriculture, for spreading fields for example. Compared to existing processes, this cake has the advantage of having a very low content of pollutants, and optionally of impurities, and is free of chemical compounds to aid separation of the flocculant and/or coagulant type usually used to improve the recovery of mixed biomasses. Due to its low content of pollutants, and optionally impurities, this cake is therefore particularly suitable for use in agriculture, particularly in organic farming.

Preferably, the second aqueous flow depleted in granular biomass produced by step (b) is sent entirely or in part to the second separation step (c), and optionally to step (B) of wastewater treatment described previously.

Second step (vs) physical separation

During the first step (a) of physical separation, the granular biomass is mainly separated from the rest of the aqueous effluent: the first aqueous flow which results is thus (i) depleted in mixed granular biomass algae-bacteria whose granules present dimensions below the threshold and (ii) enriched in suspended biomass.

The first aqueous stream produced during separation step (a) is thus sent, directly or after the optional biological treatment step, to a second physical separation step (c) to produce a second thickened stream enriched in biomass. in suspension and a third aqueous flow depleted in mixed algae-bacteria biomass, generally also depleted in pollutants and impurities. This third flow forms treated water.

The objective of this step (c) is to carry out further separation of the remaining biomass, namely essentially suspended biomass, and advantageously pollutants and impurities, from the water to obtain treated water. The conditions for implementing this second step (c) can thus be chosen based on target levels of pollutants and/or impurities in the treated water, these target levels being able to be set based on water discharge standards. processed. The second thickened flow enriched in suspended biomass thus generally includes the pollutants and impurities initially present in the wastewater (sand, microplastics, mineral matter, etc.).

The second physical separation step (c) is typically chosen from membrane filtration (c1) and cloth filtration (c2).

Membrane filtration (c1) corresponds to membrane separation which consists of separating several compounds from a liquid using one or more membranes. Depending on the difference in permeability of the membrane with respect to the compounds present in the liquid and the size of the pores of the membrane, it is possible to separate biomass, pollutants and impurities from an aqueous effluent. Such membrane filtration (c1) will be favored when the treated water must respect a strict target solids content. For example, microfiltration and/or nanofiltration techniques could be used to carry out this step.

Cloth filtration (c2) consists of circulating the first flow enriched in biomass through a filter cloth which will help retain particles too large to pass through the mesh of the cloth. This type of filtration has the advantage of being inexpensive and will be preferred when the treated water must have a target solids content that is not restrictive.

Optionally, before the second physical separation step (c), a coagulant can be added to the first aqueous flow enriched with granular biomass. The addition of coagulant can in particular make it possible to bind the residual phosphorus which could not be eliminated during the treatment of waste water and thus to be able to retain it during filtration. The coagulant used can be, for example, ferric chloride, alumina sulfate, or polyaluminium chloride (PAC).

Optionally, the treated water leaving the second physical separation stage can, before being rejected, reinjected as a tablecloth, or reused as bathing water, irrigation water, process water, or to be the source water for subsequent direct or indirect treatment aimed at the production of drinking water, undergo a treatment step, for example a biological treatment step of the type previously described, and/or a treatment step for eliminating micropollutants or microorganisms. This elimination step can be disinfection, for example using oxidants (e.g. chlorine, peracids), ultraviolet rays, or ozonation.

Second stage (d) of dehydration

The second thickened stream enriched with suspended biomass leaving step (c) is then dehydrated. During step (d), a second cake with low added value is produced as well as a fourth aqueous stream depleted of suspended biomass.

This second step (d) of dehydration can be preceded by a thickening step making it possible to reduce the volume of the sludge. This thickening step can be carried out by gravity settling and/or by dynamic thickening as described previously. In one embodiment, the thickened stream leaving this thickening stage can be subjected to an anaerobic digestion stage producing biogas and digested sludge, which are then sent to the second dehydration stage (d). This makes it possible to convert the second thickened stream enriched with suspended biomass leaving step (c) into biogas and to reduce the volume of dehydrated effluent during step (d). This anaerobic digestion step can be implemented in a free or mixed culture methanization reactor.

Dehydration makes it possible to reduce the water content of the biomass to reach a dryness of approximately 10% to 30%. Dehydration can be carried out by filtration and/or centrifugation, in the usual manner.

The cake thus produced cannot be used in agriculture since the second cake generally includes, in addition to the biomass, impurities from the aqueous effluent entering the separation process such as sand and pollutants, or even chemical compounds which had to be added to facilitate separation. It will therefore be disposed of in a landfill for example.

Preferably, the fourth aqueous stream depleted in biomass produced by step (d) is sent partially or entirely to the second separation step (c), and optionally to the wastewater treatment step (B) described previously.

Optionally, a flocculant and/or a coagulant can be added to the second thickened stream enriched with biomass before the second dehydration step (d) to flocculate the maximum amount of free biomass, pollutants and impurities in order to more easily dehydrate this stream and also to integrate a maximum of pollutants and impurities into the cake and not into the fourth aqueous flow depleted in biomass.

As part of the water and biomass initially contained in the aqueous effluent treated by the separation process according to the invention has been reduced by the implementation of the first separation step, the flow of biomass and contaminants to be treated in this second physical separation step is much lower, so that it is possible to consider the use of less expensive and less restrictive techniques in terms of maintenance than if the aqueous effluent leaving the biological reactor was directly addressed in steps (c) and (d). It is also possible to use, during this second separation stage, chemical compounds to aid separation such as flocculants and/or coagulants in order to improve the quality of the recovered treated water. These chemical compounds will then be found in a second cake obtained after dehydration, as described above. This cake with low added value due to the presence of these chemical reagents and pollutants and/impurities initially contained in the wastewater, although not recoverable, is in small quantity so that, overall, the separation process according to the invention makes it possible on the one hand to optimize the valorization of biomass, in particular in granular form, by obtaining the first cake, and on the other hand to optimize the treatment of water, in particular by reducing the CAPEX and OPEX of the second stages of separation and dehydration.

Description of the installation

In reference to the , the installation 100 for separating an aqueous effluent containing mixed algae-bacteria biomass in granular and suspended form, pollutants, and optionally impurities, and treated water comprises a first physical separation unit 110 adapted to put implementing step (a) of the process, a first dehydration unit 120 adapted to implement step (b) of the process, a second physical separation unit 130 adapted to implement step (c) of the process process and a second dehydration unit 140 adapted to implement step (d) of the process.

The first separation unit 110 is a unit configured to separate granules of mixed algae-bacteria granular biomass of dimensions greater than a threshold. The first separation unit 110 is for example a sieving separation unit. For example, we could use a draining table, a sieve, a vibrating sieve, a grid, a belt filter, a filter, a vibrating filter. The first unit 110 comprises a supply pipe 1 of an aqueous effluent, a first evacuation pipe 2 of a thickened flow enriched with mixed granular biomass algae-bacteria whose granules have dimensions greater than a threshold and a second evacuation pipe 3 of a first aqueous flow depleted in granular biomass mixed algae-bacteria whose granules have dimensions less than the threshold.

The first dehydration unit 120 is a unit chosen from a centrifugation unit, a filtration unit and a combination of these units, making it possible to produce a mixed algae-bacteria granular biomass cake with high added value. The first unit 120 comprises a supply pipe 4 connected to the first discharge pipe 2 of the first separation unit 110 and an discharge pipe 5 of a second aqueous flow depleted in mixed granular biomass algae-bacteria. The first unit 120 also includes a dehydrated biomass evacuation pipe not shown in the figures.

The second physical separation unit 130 is for example chosen from a membrane filtration unit and a fabric filtration unit. The second separation unit 130 comprises a supply pipe 6 connected to the discharge pipe 3 of the first separation unit 110, a third discharge pipe 7 of a second thickened flow enriched with mixed algae-bacteria biomass in suspension and a fourth evacuation pipe 8 of a third aqueous flow depleted in mixed algae-bacteria biomass in suspension and forming treated water.

The second dehydration unit 140 is a unit chosen from a centrifugation unit, a filtration unit and a combination of these units, making it possible to produce a mixed algae-bacteria biomass cake with low added value also containing pollutants and impurities. The second unit 140 comprises a supply pipe 9 connected to the third discharge pipe 7 of the first separation unit 130 and an discharge pipe 10 of a fourth aqueous flow depleted in mixed algae-bacteria biomass in suspension. The second unit 140 also includes a dehydrated biomass evacuation pipe not shown in the figures.

There presents an embodiment of a wastewater treatment installation 250 comprising a separation installation 200. The units 210, 220, 230 and 240 are identical to the units 110, 120, 130 and 140 described with reference to the , the pipe numbering remains the same for these units.

In reference to the , the wastewater treatment installation 250 comprises a supply pipe 11 of water to be treated containing pollutants, and optionally impurities, capable of carrying out step (A) of the treatment process, a unit 251 of bringing into contact the water to be treated capable of carrying out step (B) of the treatment process and an evacuation pipe 12 of an aqueous effluent capable of carrying out step (C) of the treatment process.

Unit 251 is a unit for bringing water to be treated into contact with a mixed granular biomass of algae-bacteria capable of eliminating pollutants. It comprises a supply pipe 13 connected to the pipe 11 for supplying the water to be treated and an evacuation pipe 12 of an aqueous effluent connected to the supply pipe 1 of the unit 210 of the installation 200 separation. Unit 251 can be a photobioreactor, in particular an open culture tank in which the water to be treated circulates with a certain residence time, a bubble column or a closed tubular reactor.

Optionally, a recirculation pipe 14 connects the evacuation pipe 2 of the separation unit 210 of the installation 200 and the unit 251 of the treatment installation 250 so that part of the first flow is enriched with granular biomass mixed algae-bacteria leaving step (a) is returned to step (B) of the wastewater treatment process.

Preferably, the evacuation pipes 5, 10 of the units 220 and 240 of the separation installation 200 are connected to the supply pipe 6 of the second separation unit 230 by a pipe 20. Optionally, the evacuation pipes 5, 10 can also be connected to pipe 11 of installation 250 via pipe 15.

There presents another embodiment of the wastewater treatment installation 350 comprising a separation installation 300. The units 310, 320, 330, 340 and 351 are identical to the units 210, 220, 230, 240 and 251 described in reference to the , the pipe numbering remains the same for these units.

In reference to the , the separation installation 300 comprises a coagulant injection line 16 connected to the supply line 6 of the second separation unit 330.

The separation installation 300 also includes a flocculant and/or coagulant injection line 17 connected to the supply line 9 of the second dehydration unit 340.

The installation 300 finally comprises, downstream of the second separation unit 330, a unit 360 for disinfection of the treated water comprising a supply pipe 18 connected to the evacuation pipe 8 of the unit 330 and a pipe evacuation 19 of disinfected water. This unit 360 can also be a biological treatment unit or include both a disinfection unit and a biological treatment unit.

There presents another embodiment of the wastewater treatment installation 450 comprising a separation installation 400. The units 410, 420, 430, 440 and 451 are identical to the units 310, 320, 330, 340 and 351 described in reference to the , the pipe numbering remains the same for these units.

In the embodiment of the , the separation installation 400 comprises a biological treatment unit 470 connected to the second discharge pipe 3 of the first separation unit 410 and to the supply pipe 6 of the second separation unit 430. This treatment unit optional biological 470 can also be provided in the embodiments shown in Figures 1-3. The separation installation 400 also includes an anaerobic digestion unit 480 supplied with effluent by the evacuation pipe 7 of the second separation unit 430 and supplying digested sludge to the supply pipe 9 of the second dehydration unit 440. The anaerobic digestion unit 480 also produces biogas recovered via a pipe 21. This optional anaerobic digestion unit 480 can also be provided in the embodiments shown in Figures 1-3.

There presents another embodiment of the wastewater treatment installation 550 comprising a separation installation 500. The units 510, 520, 530, 540, 570, 580 are identical to the units 410, 420, 430, 440, 460, 470, 480 described with reference to the , the pipe numbering remains the same for these units.

In the embodiment of the , the contacting unit 551 is equipped with pre-separation equipment 552, whether or not integrated into a reactor of the contacting unit, as previously described. This pre-separation equipment 552 comprises a first outlet 552a for evacuating a first muddy effluent and a second outlet 552b for evacuating the rest of the effluent forming a second aqueous effluent. The first outlet 552a is connected to the discharge pipe 12 of the unit 551 to send the first muddy effluent into the first separation unit 510. The second outlet 552b is connected either directly to the supply pipe 6 of the second separation unit 530, or at the inlet of the optional biological treatment unit 570 via a pipe 12b. Note that two biological treatment units 570 could be considered instead of just one, each unit then treating a flow coming from one of the units 551 and 510. Note also that the first aqueous flow leaving the first unit of separation 510 via the evacuation pipe 3 could be sent directly to the second separation unit 530 without going through the biological treatment unit 570.

Such pre-separation equipment 552 can also be provided in the contacting units of the embodiments previously described.

Generally speaking, screening and/or degreasing and/or grit removal units could be provided upstream of units 251, 351, 451, 551 to treat incoming wastewater. Preferably these units are not preceded by a decantation unit.

We can also provide between each physical separation unit 110, 130; 210, 230; 310, 330; 410, 430; 510, 530) and the associated dehydration unit (120, 140; 220, 240; 320, 340; 420, 440; 520, 540), a thickening unit (not shown in the figures) such as a basin decantation, a flotation reactor, a centrifuge, a dripping drum or even a dripping table or grid. When present, the anaerobic digestion unit 480, 580 may be located downstream of an optional thickening unit.

The different embodiments presented with reference to Figures 1 to 5 can be combined depending on the treatment and separation objective decided.

Claims

Process for separating an aqueous effluent containing mixed algae-bacteria biomass present in granular and suspended form and treated water, the process comprising:
(a) A first step of physical separation of at least part of the granular biomass from the treated water carried out without prior addition of separation aid compounds, producing a first thickened flow enriched in granular biomass whose granules present dimensions greater than a threshold and a first aqueous flow depleted in granular biomass whose granules have dimensions lower than the threshold and enriched in suspended biomass;
(b) A first step of dehydration of the first thickened flow enriched in granular biomass producing a first cake and a second aqueous flow depleted in granular biomass,
(c) A second step of physical separation of the first aqueous stream producing a second thickened stream enriched with suspended biomass and a third aqueous stream forming treated water;
(d) A second step of dehydration of the second thickened stream enriched in suspended biomass producing a second cake and a fourth aqueous stream depleted in suspended biomass.
Separation method according to claim 1, characterized in that the threshold is at least 50µm, optionally at least 70µm, 125µm, 0.250mm or 1mm.
Separation process according to claim 1 or 2, characterized in that it comprises at least one of the following characteristics:
- the first physical separation step (a) is a sieving separation step during which the aqueous effluent passes through a perforated support chosen from a grid, a sieve, a filter or a perforated plate, optionally placed implemented by means of a device chosen from a draining table, a sieve vibrating or not, a grid, a belt filter, a filter vibrating or not;
- the second separation step (c) is chosen from membrane filtration and cloth filtration.
Separation process according to any one of claims 1 to 3, characterized in that the first aqueous flow is subjected to a biological treatment step before the second separation step (c).
Separation process according to any one of claims 1 to 4, characterized in that it comprises at least one of the following characteristics:
- a coagulant is added to the first aqueous flow before the second separation step (c);
- a flocculant and/or a coagulant is added to the second thickened flow enriched with suspended biomass before the second dehydration step (d).
Separation process according to any one of claims 1 to 5, characterized in that the second and/or fourth aqueous flow leaving steps (b) and (d) are sent upstream of the second separation step (c).
Wastewater treatment process in which:
(A) Water to be treated containing pollutants is provided, optionally water to be treated that has been previously sieved, degreased and/or desanded;
(B) The water to be treated is brought into contact with a mixed granular biomass of algae-bacteria, and an aqueous effluent is produced containing treated water depleted in pollutants, at least part of the biomass in granular form and at least part suspended biomass;
(C) The treated water and the biomass contained in the aqueous effluent are separated by subjecting the latter to the separation process according to any one of the preceding claims.
Treatment method according to claim 7, characterized in that part of the first thickened flow enriched in granular biomass leaving the first separation step (a) is sent to the step (B) of bringing the water into contact with treat with granular biomass.
Treatment method according to claim 7 or 8, characterized in that:
- during step (B) the aqueous effluent produced is subjected to a pre-separation step producing a first so-called muddy effluent comprising at least 90% by mass of the granular biomass, and a second so-called aqueous effluent containing the remainder of the biomass, and
- during step (C), the first muddy effluent is sent to step (a) of the separation process,
- the second aqueous effluent is sent to step (c) of the separation process, optionally after a biological treatment step.
Installation (100, 200, 300, 400, 500) for separating an aqueous effluent containing mixed algae-bacteria biomass present in granular and suspended form and treated water, characterized in that it comprises:

A first unit (110, 210, 310, 410, 510) for physical separation, comprising a supply pipe (1) for aqueous effluent, a first discharge pipe (2) for a first thickened flow enriched with granular biomass whose granules have dimensions greater than a threshold and a second evacuation pipe (3) of a first aqueous flow depleted in granular biomass whose granules have dimensions less than the threshold and enriched in suspended biomass;

A first dehydration unit (120, 220, 320, 420, 520), comprising a supply line (4) connected to the first discharge line (2) of the first unit (110, 210, 310, 410, 510) for separation and a discharge pipe (5) for a second aqueous flow depleted in granular biomass;

A second physical separation unit (130, 230, 330, 430, 530), comprising a supply line (6) connected to the second discharge line of the first unit (110, 210, 310, 410, 510) separation, a third discharge pipe (7) of a second thickened flow enriched with suspended biomass and a fourth discharge pipe (8) of a third aqueous flow forming treated water;

A second dehydration unit (140, 240, 340, 440, 540), comprising a supply line (9) connected to the third discharge line of the second separation unit (130, 230, 330, 430, 530 ) and a discharge pipe (10) for a fourth aqueous flow depleted of suspended biomass.
Separation installation (300, 400, 500) according to claim 10, characterized in that it comprises at least one of the following characteristics:
- a coagulant injection pipe (16) connected to the supply pipe (6) of the second separation unit (330)
- at least one line (17) for injecting a flocculant and/or a coagulant connected to the supply line (9) of the second dehydration unit (340).
Separation installation (300) according to any one of claims 10 or 11, characterized in that at least one pipe chosen from the evacuation pipe (5) of the second aqueous flow depleted in granular biomass and the evacuation pipe (10) of the fourth aqueous flow depleted of suspended biomass is connected to the supply line (6) of the second physical separation unit (330).
Separation installation (400, 500) according to any one of claims 10 or 11, characterized in that it comprises a biological treatment unit (470, 570) connected to the second evacuation pipe (3) of the first separation unit (410, 510) and the supply line (6) of the second physical separation unit (430, 530).
Installation (250, 350, 450, 550) for treating wastewater, characterized in that it comprises:

A pipe (11) for supplying water to be treated containing pollutants;

A unit (251, 351, 451, 551) for bringing the water to be treated into contact with a mixed granular biomass of algae-bacteria, comprising a supply pipe (13) connected to the pipe (11) for supplying water water to be treated and a discharge pipe (12) for an aqueous effluent containing treated water depleted of pollutants, at least part of the biomass in granular form and at least part of the suspended biomass;

A separation installation (100, 200, 300, 400, 500) according to any one of claims 10 to 13, said evacuation pipe (12) of an aqueous effluent being connected to the supply pipe (1) in aqueous effluent from the first physical separation unit of the separation installation (100, 200, 300, 400, 500).
Treatment installation (250, 350, 450, 550) according to claim 14, characterized in that it comprises at least one of the following characteristics:
- the first evacuation pipe (2) of the first thickened flow enriched with granular biomass is connected to a recirculation pipe (14) connected to the unit (251, 351, 451, 551) for bringing the water into contact to be treated with granular biomass,
- the contacting unit (551) comprises equipment for pre-separation (552) of the aqueous effluent having a first discharge outlet (552a) of a first muddy effluent and a second discharge outlet ( 552b) of the rest of the effluent forming a second aqueous effluent, the first outlet (552a) being connected to said evacuation pipe (12) of the contacting unit and the second outlet (552b) being connected to the supply line (6) of the second physical separation unit (530) of the separation installation, optionally via a biological treatment unit (570).