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

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 granular algae-bacteria biomass Field of invention

[0001] The invention relates to a process for separating an aqueous effluent containing mixed granular algae-bacteria biomass and treated water. State of the art

[0002] Conventional wastewater treatment processes are facing increasing pressures due to their high energy consumption, significant greenhouse gas emissions and low resource recovery potential.

[0003] To meet these needs, processes using microalgal-bacterial aggregates (MABAs) for wastewater treatment have been developed.

[0004] Microalgae-bacteria are symbiotic systems in which algae provide oxygen (O2) through photosynthesis, which is then used by bacteria for nitrification and carbon removal, synthesize polyphosphate, which reduces energy consumption, and utilize carbon dioxide (CO2) produced by bacteria, thus reducing greenhouse gas emissions. Therefore, microalgae-bacteria systems have the potential to self-sustain their oxygen consumption, thereby reducing or even eliminating the aeration required for bacterial activity and thus reducing associated energy costs, while limiting the emission of greenhouse gases generated by bacteria.

[0005] 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-living algae have low settling capacity, requiring long settling time or large settling surfaces. It is estimated that approximately 20-30% of the operating costs of microalgae cultivation are related to algae separation, mainly by coagulation / flocculation, centrifugation, flotation, etc. Recently, there has been growing interest in the use of algae immobilization or fixation processes to solve this separation problem. Immobilization of algae in several materials such as alginate and chitosan has thus shown better settling ability.However, their large-scale application for resource recovery is limited due to the high cost of operation (mainly from added materials) and residual polymeric materials added to the culture medium.

[0006] There are typically four strategies for harvesting and dehydrating MABAs, which are direct centrifugation, gravity settling followed by centrifugation, dissolved air flotation followed by centrifugation, and membrane filtration followed by centrifugation. In addition, a thickening step upstream of dehydration reduces the size and energy consumption of the dehydration step, which is generally performed by centrifugation.

[0007] Today, the first and second strategies are generally considered uneconomical in wastewater treatment. Indeed, direct centrifugation induces high CAPEX and OPEX while the use of gravity settling requires a large footprint due to the low Mohlman Index (SVI) 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, even if membrane filtration as a thickening approach seems interesting since it does not require the addition of chemical compounds such as polymers, it is limited by the cost and the lifetime of the membrane (approximately 8 years), which compromises the economic feasibility at industrial scale.Furthermore, all the above-mentioned solutions concentrate the microalgae-bacteria granular aggregates with all the sand and microsand and other contaminants (microplastics, possibly added chemical compounds, etc.) present in the wastewater. These contaminants have no value from the application point of view (agronomic, extraction of value-added products) and therefore reduce the quality of the final product.

[0008] Algal-bacterial granular sludge (AGS), also known as OPS or Oxygenic Photo Granules, has attracted the attention of researchers as granular bacteria can significantly improve the settling rate of microalgae. This sludge is obtained by introducing microalgae into an aerobic granular sludge (AGS) system, leading to the establishment of symbiotic algae-granular bacteria systems. Compared to MABAs (free-living consortia), the use of aerobic granules 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 velocity, rich biomass retention, simultaneous removal capacity of organic matter and nutrients, and resistance to shock loads and toxic substances.

[0009] However, the techniques for separating and dehydrating these granular sludges remain similar to the techniques presented above for MABAs with the same drawbacks.

[0010] There is therefore a need for an improved process for separating microalgal-bacterial granular biomass from an aqueous effluent, in particular making it possible 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 matter or other contaminants which are not part of the biomass and which are present in the wastewater. There is also a need for an improved process for separating the biomass making it possible to recover an aqueous effluent having a very low solids content.

[0011] A first subject of the invention relates to a method for separating an aqueous effluent containing mixed algae-bacteria biomass present in granular and suspended form, and optionally impurities, and treated water, the method comprising: (a) A first step of physical separation of at least a portion of the granular biomass from the treated water carried out without prior addition of a separation aid compound, producing a first thickened stream enriched in granular biomass whose granules have dimensions greater than a threshold and a first aqueous stream depleted in granular biomass whose granules have dimensions less than the threshold and enriched in suspended biomass;(b) A first step of dehydration of the first thickened stream enriched in granular biomass producing a first cake and a second aqueous stream depleted in granular biomass,(c) A second step of physical separation of the first aqueous stream producing a second thickened stream enriched in suspended biomass, and optionally in impurities, and a third aqueous stream 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.;

[0012] 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 can in particular be reused in agriculture for example, in particular 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 matter or other contaminants which are not part of the biomass and which come from the treatment of the wastewater having produced the aqueous effluent separated by the process according to the invention.

[0013] High value-added granular biomass is recovered through the first separation, which corresponds to a rough 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.

[0014] The second separation stage corresponds to a fine separation whose aim is to separate the remaining suspended biomass, and optionally the impurities and pollutants, present in the first aqueous flow from the first separation stage to obtain treated water that meets discharge standards.

[0015] 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 mixed granular algae-bacteria biomass is retained, advantageously without pollutants and impurities, which makes it possible to obtain, after dehydration, a cake with high added value, because it is free of chemical compounds to aid separation, with a significant fraction of biomass.

[0016] Preferably, the first physical separation step (a) may be a screening 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 may for example 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. Thus, the perforated support does not comprise, or is not, a membrane, which makes it possible to reduce the costs of implementing this step while ensuring efficient separation of the granular biomass.

[0017] Preferably, the second separation step (c) can be chosen from membrane filtration and fabric filtration. These two types of separation thus allow a 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 fouling of the membrane or 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.

[0018] In one embodiment, the first aqueous stream may be subjected to a biological treatment step before the second separation step (c). This may further reduce nitrogen, carbon, phosphate and / or other pollution, and improve the quality of the treated water leaving the second physical separation step.

[0019] In one embodiment, a coagulant may be added to the first aqueous stream before the second separation step (c). For example, when the treated water must meet strict phosphorus discharge standards, a coagulant, for example selected from ferric chloride, alumina sulfate, polyaluminum chloride, WAC which is an aluminum polymer, or other, may be injected and thus make it possible to bind the residual phosphorus which could not be removed during the treatment of the aqueous effluent and to retain it during the second separation. Coagulation of the first aqueous stream before the second separation step (c) may make it possible to reduce the colloidal load to be filtered on the second separation step (c), which makes it possible to further reduce fouling in the case of membrane or cloth filtration.

[0020] In one embodiment, a flocculant and / or a coagulant may be added to the second thickened stream enriched with suspended biomass before the second dewatering step (d). The flocculant will improve the capture rate of free biomass, pollutants and impurities and make it easier to dewater the second thickened stream enriched with suspended biomass and also to integrate a maximum of pollutants and impurities into the cake and not into the treated water.

[0021] Preferably, the second and / or the fourth aqueous stream leaving steps (b) and (d) are sent upstream of the second separation step (c).

[0022] An advantage is to avoid reducing the hydraulic retention time (HRT) in a water treatment tank using mixed algae-bacteria granular biomass since the aqueous streams leaving steps (b) and (d) do not need to be further treated. Thus, the dimensions of such a tank 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.

[0023] 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 algae-bacteria biomass, and an aqueous effluent is produced containing treated water depleted in pollutants, at least a portion of the biomass in granular form and at least a portion of the biomass in suspension, 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.

[0024] Preferably, a portion of the first thickened stream enriched with granular biomass leaving the first separation step (a) is sent to step (B) of contacting the water to be treated with 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 "recuperative 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 the HRT.

[0025] In one embodiment, during step (B) the aqueous effluent produced may be subjected to a pre-separation step producing a first so-called sludge 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 sludge 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.

[0026] The invention also relates to a plant for separating an aqueous effluent containing mixed algae-bacteria biomass present in granular and suspended form, optionally impurities, and treated water, in particular adapted to the implementation of the separation method according to the invention. The plant comprises: (a) A first physical separation unit, in particular adapted to carry out step (a) of the separation method, comprising an aqueous effluent supply pipe, a first discharge pipe for a first thickened flow enriched in granular biomass whose granules have dimensions greater than a threshold and a second discharge 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 feed pipe connected to the first discharge pipe of the first separation unit and a discharge pipe for a second aqueous stream depleted in granular biomass; (c) A second physical separation unit, in particular adapted to carry out step (c) of the separation process, comprising a feed pipe connected to the second discharge pipe of the first separation unit, a third discharge pipe for a second thickened stream enriched in suspended biomass, and optionally in impurities and a fourth discharge pipe for a third aqueous stream forming treated water;(d) A second dehydration unit, in particular adapted to carry out step (d) of the separation process, comprising a feed pipe connected to the third discharge pipe of the second separation unit and a discharge pipe for a fourth aqueous stream depleted in suspended biomass, and optionally in impurities.;

[0027] Preferably, the first separation unit may be a screening separation unit in which the aqueous effluent passes through a perforated support chosen from a grid, a sieve or a perforated plate.

[0028] Preferably, the second separation unit can be chosen from a membrane filtration unit and a fabric filtration unit.

[0029] In one embodiment, a coagulant injection line may be connected to the feed line of the second separation unit.

[0030] In another embodiment, at least one injection line for a flocculant and / or a coagulant can be connected to the feed line of the second dehydration unit.

[0031] In one embodiment, the discharge line for a second aqueous stream depleted in granular biomass and / or the discharge line for a fourth aqueous stream depleted in suspended biomass, and optionally in impurities, can be connected to the feed line of the second physical separation unit.

[0032] In one embodiment, the separation facility may further comprise a biological treatment unit connected to the second discharge line of the first separation unit and to the supply line of the second physical separation unit.

[0033] The invention also relates to a wastewater treatment installation comprising: (A) A pipe for 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 granular algae-bacteria biomass, comprising a supply pipe connected to the pipe for supplying water to be treated and a pipe for discharging an aqueous effluent containing treated water depleted in pollutants, at least a portion of the biomass in granular form and at least a portion of the biomass in suspension, and optionally impurities; (C) A separation installation according to the invention, said pipe for discharging an aqueous effluent being connected to the pipe for supplying aqueous effluent to the first physical separation unit of the separation installation according to the invention.

[0034] 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.

[0035] In one embodiment, the contacting unit comprises pre-separation equipment having a first outlet for discharging a first sludge effluent and a second outlet for discharging the remainder of the effluent forming a second aqueous effluent, 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.

[0036] Definitions / Abbreviations

[0037] Mixed algal-bacterial biomass is a symbiotic system between microalgae and bacteria. Bacteria are strict aerobic, microaerophilic, facultative aerobic-anaerobic, or aerotolerant anaerobic microorganisms capable of removing carbon, nitrogen, phosphorus, or organic matter from wastewater. Microalgae are unicellular photosynthetic microorganisms less than 400 micrometers in size and normally between 1 and 30 micrometers in diameter. Mixed algal-bacterial biomass may comprise a majority of eukaryotic microalgae or prokaryotic microalgae. The latter generally contain prokaryotic filamentous cyanobacteria as the main microalgae.

[0038] The mixed algae-bacteria biomass used is in granular form. Typically, the granules are in the order of 50 to 500 micrometers. As is known, the biomass grows in granular form with the help of extracellular polymeric substances present in the system, which are natural polymers secreted by microorganisms.

[0039] The size of granules of mixed algal-bacterial biomass 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 ISI 13322-2-2021, or by dynamic light scattering.

[0040] The most commonly used method for measuring granule size is sieving, which involves pouring the product to be analyzed through a series of standardized sieves (e.g., ASTM E11-22) stacked in descending order of mesh size, with the last sieve having the smallest mesh size. This method is generally applied to regularly shaped particles with a size between 40µm and 8mm.

[0041] Suspended biomass is a free-growing biomass. In general, a mixed algae-bacteria biomass in suspension forms aggregates of 3 to 30 µm.

[0042] By "wastewater" we mean urban wastewater, the origin of which is essentially domestic but some of which may be of industrial origin, or industrial wastewater, in particular that from the food industry or any other industry producing effluents loaded with carbon and nitrogen, for example.

[0043] Dryness is a unit of measurement representing the mass percentage of dry matter in a sludge.

[0044] 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

[0045] Other features and advantages of the invention will emerge from reading the description given below of several particular embodiments of the invention, given for informational purposes but not as a limitation, with reference to the appended drawings in which:

[0046] This is a schematic representation of the separation installation according to one embodiment of the invention.

[0047] This is a schematic representation of the separation installation according to a first embodiment of the invention.

[0048] This is a schematic representation of the separation installation according to a second embodiment of the invention.

[0049] This is a schematic representation of the installation according to a third embodiment of the invention.

[0050] This is a schematic representation of the installation according to a fourth embodiment of the invention. Detailed description of the invention

[0051] The alternative configurations of the different stages of the process presented below can be combined depending on the separation objective decided.

[0052] Wastewater treatment process

[0053] 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 steps beforehand. The prior treatment step(s) may, for example, be primary treatment steps that generally make it possible to reduce the solid and / or organic matter content of the water to be treated. This may in particular involve screening, desanding and / or degreasing.Advantageously, the preliminary treatment 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.

[0054] The treatment method then comprises a step (B) of bringing the water to be treated into contact with a mixed granular algae-bacteria biomass. The mixed granular algae-bacteria biomass is particularly capable of eliminating pollutants contained in the water to be treated. During the biological treatment of water by bringing it into 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 both biomass in granular form (or granular biomass) and suspended biomass (in free culture).

[0055] The contacting step thus produces an aqueous effluent containing treated water depleted of 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.

[0056] In one embodiment, during step (B), the aqueous effluent resulting from the contacting of water with the biomass may be subjected to a pre-separation step producing a first so-called sludge effluent comprising at least 90% by mass of the granular biomass, and optionally a portion of the suspended biomass, and a second so-called aqueous effluent containing the remainder of the biomass. The first sludge 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 the granular biomass, and optionally a portion of the suspended biomass. The second aqueous effluent then contains the remainder of the biomass, namely predominantly (generally more than 90% by mass) the suspended biomass, and, where appropriate, the remainder of the granular biomass.This second aqueous effluent may include a quantity of TSS solids of 10 mg / L to 2 g / L, of 100 mg / 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.

[0057] This pre-separation step is typically a settling or clarification step. The quantities of biomass present in each stream can be modulated according to 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 can comprise a decanter or clarifier integrated or not into the contacting unit in which step (B) is implemented. For example, the contacting unit can comprise at least one sequenced biological reactor (also designated by the acronym “SBR”) operating sequentially in mixing and rest modes to separate the first sludge effluent from the second aqueous effluent.Alternatively, the contacting unit may comprise a contacting reactor connected to a clarifier or decanter for separating the first sludge effluent from the second aqueous effluent.

[0058] 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.

[0059] When step (B) includes a pre-separation step, during step (C), the first sludge effluent is sent to step (a) of the separation process according to the invention, the second aqueous effluent being sent to step (c) of the separation process. This makes it possible on the one hand to reduce the volume of effluent sent to 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).

[0060] The second aqueous effluent, containing mainly 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 pollutants 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, a biological activated sludge reactor, a sequenced biological reactor (SBR), a fluidized fixed culture biological reactor (also referred to by the acronym "MBBR" for "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 disc reactor also called "Rotating Biological Contactor" in English. Since the biological treatment unit is carried out on a smaller volume compared to the volume initially treated during the contact 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 dewatering stage (d).

[0061] Process for separating an aqueous effluent

[0062] First step (a) of physical separation

[0063] The aqueous effluent produced during step (B) of the wastewater treatment process, or the first pre-separated sludge effluent, is then sent to a first physical separation step (a) without prior addition of a separation aid compound, for example of the coagulant and / or flocculant type. The first physical separation step (a) produces a first thickened stream enriched in granular biomass whose granules have dimensions greater than a threshold and a first aqueous stream depleted in granular biomass whose granules have dimensions less than the threshold and enriched in suspended biomass.

[0064] The objective of this step (a) is to separate the granular biomass from the treated water, and optionally also from the pollutants and impurities, to obtain a flow enriched in 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 according to the dimensions of the biomass granules. In general, a threshold of at least 50 micrometers and optionally at least 1 millimeter makes it possible to recover the majority of the granular biomass. The threshold may also be at least 70µm, in particular for separating biomass from silts and clays, or at least 125µm or at least 250 micrometers, in particular for separating biomass from very fine or fine sand.

[0065] Step (a) is typically a screening separation step in 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.

[0066] Step (a) may be carried out by means of a device selected 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 sieve, belt filter or vibrating filter is a sieve or filter having openings through which the aqueous effluent circulates. The vibrating sieve or filter will vibrate to filter and separate the biomass. For this separation, the threshold may be achieved by choosing a sieve whose openings have a diameter corresponding to the threshold.

[0067] A portion of the first thickened stream enriched in granular biomass leaving step (a) may optionally be returned to step (B) of the wastewater treatment process. Recirculation of the biomass makes it possible to return a biomass-rich fraction to the reactor of step (B) and thus increase the efficiency of the wastewater treatment by improving the performance of the biological treatment.

[0068] In one embodiment, the first aqueous stream depleted in granular biomass produced by step (a) is subjected to an optional biological treatment step before being sent to the physical separation step (c). This biological treatment may 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 biological treatment unit or separate biological treatment units may be used to treat this second aqueous effluent and the first aqueous stream depleted in granular biomass produced by step (a).

[0069] First stage (b) of dehydration

[0070] The first thickened stream enriched in mixed algae-bacteria granular biomass leaving step (a), or optionally a portion of this first thickened stream enriched in granular biomass, is then dehydrated. During step (b), a first high-value-added cake, without added chemical compound, is produced as well as a second aqueous stream depleted in granular biomass.

[0071] This first dewatering step (b) may be preceded by a thickening step to reduce the volume of the sludge. The thickening step may be a gravity settling thickening step with recovery of the granules at the bottom of the thickener while the liquid is discharged by an overflow or a dynamic thickening step. Dynamic thickening may be achieved by microbubble flotation (also called dissolved air flotation), by dewatering (e.g., by means of a dewatering or thickening grid, a dewatering table, or a dewatering drum or thickening 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 density difference.The thickening step may also include gravity settling thickening and dynamic thickening, advantageously gravity settling thickening followed by dynamic thickening.

[0072] Dehydration reduces the water content of biomass to achieve a dryness of approximately 10% to 20%. Dehydration can be carried out by filtration (filter press, piston press, screw press) and / or by centrifugation in the usual manner.

[0073] 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 such as flocculants and / or coagulants usually used to improve the recovery of mixed biomass. Due to its low content of pollutants, and optionally of impurities, this cake is therefore particularly suitable for use in agriculture, particularly in organic farming.

[0074] Preferably, the second aqueous stream depleted in granular biomass produced by step (b) is sent entirely or in part to the second separation step (c), and optionally to the wastewater treatment step (B) described above.

[0075] Second stage (c) of physical separation

[0076] During the first stage (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 of algae and bacteria whose granules have dimensions below the threshold and (ii) enriched in suspended biomass.

[0077] 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 suspended biomass and a third aqueous stream depleted in mixed algae-bacteria biomass, generally also depleted in pollutants and impurities. This third stream forms treated water.

[0078] The objective of this step (c) is to carry out a more thorough 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 according to target levels of pollutants and / or impurities in the treated water, these target levels being able to be set according to the standards for discharges of the treated water. 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.).

[0079] The second stage (c) of physical separation is typically chosen from membrane filtration (c1) and fabric filtration (c2).

[0080] Membrane filtration (c1) is a membrane separation process that involves separating several compounds from a liquid using one or more membranes. Depending on the difference in membrane permeability to the compounds present in the liquid and the size of the membrane pores, it is possible to separate biomass, pollutants, and impurities from an aqueous effluent. Such membrane filtration (c1) will be preferred when the treated water must meet a strict target solids content. For example, microfiltration and / or nanofiltration techniques may be used to implement this step.

[0081] Cloth filtration (c2) involves circulating the first biomass-enriched stream through a filter cloth that will retain particles that are 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 low-restrictive target solids content.

[0082] Optionally, before the second stage (c) of physical separation, a coagulant can be added to the first aqueous stream enriched with granular biomass. The addition of coagulant can in particular make it possible to bind the residual phosphorus that could not be eliminated during the treatment of the wastewater 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).

[0083] Optionally, the treated water leaving the second physical separation stage may, before being discharged, reinjected into the groundwater, or reused as bathing water, irrigation water, process water, or to be the source of water for subsequent direct or indirect treatment aimed at producing drinking water, undergo a treatment stage, for example a biological treatment stage of the type previously described, and / or a treatment stage for eliminating micropollutants or microorganisms. This elimination stage may be a disinfection, for example using oxidants (e.g. chlorine, peracids), ultraviolet rays, or ozonation.

[0084] Second stage (d) of dehydration

[0085] The second thickened stream enriched in suspended biomass leaving step (c) is then dewatered. During step (d), a second low-value cake is produced as well as a fourth aqueous stream depleted in suspended biomass.

[0086] This second dewatering step (d) may be preceded by a thickening step to reduce the volume of the sludge. This thickening step may be carried out by gravity settling and / or by dynamic thickening as described above. In one embodiment, the thickened stream leaving this thickening step may be subjected to an anaerobic digestion step producing biogas and digested sludge, which is then sent to the second dewatering step (d). This makes it possible to recover the second thickened stream enriched in suspended biomass leaving step (c) as biogas and to reduce the volume of dewatered effluent during step (d). This anaerobic digestion step may be implemented in a methanization reactor in free or mixed culture.

[0087] Dehydration reduces the water content of the biomass to achieve a dryness of approximately 10% to 30%. Dehydration can be carried out by filtration and / or centrifugation, as usual.

[0088] The cake thus produced cannot be used in agriculture since the second cake generally includes, in addition to biomass, impurities from the aqueous effluent entering the separation process such as sand and pollutants, or even chemical compounds that had to be added to facilitate separation. It will therefore be disposed of in a landfill, for example.

[0089] 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 above.

[0090] Optionally, a flocculant and / or a coagulant may be added to the second thickened stream enriched in biomass before the second dehydration step (d) to flocculate the maximum amount of free biomass, pollutants and impurities in order to dehydrate this stream more easily and also to integrate a maximum of pollutants and impurities into the cake and not into the fourth aqueous stream depleted in biomass.

[0091] Since 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 envisage the use of less expensive and less restrictive techniques in terms of maintenance than if the aqueous effluent leaving the biological reactor were directly treated in steps (c) and (d). It is also possible to use, during this second separation step, 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, which has low added value due to the presence of these chemical reagents and the pollutants and / or 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 recovery of the biomass, in particular in granular form, by obtaining the first cake, and on the other hand to optimize the treatment of the water, in particular by reducing the CAPEX and OPEX of the second stages of separation and dehydration.

[0092] Installation Description

[0093] With 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 implement step (a) of the method, a first dehydration unit 120 adapted to implement step (b) of the method, a second physical separation unit 130 adapted to implement step (c) of the method and a second dehydration unit 140 adapted to implement step (d) of the method.

[0094] The first separation unit 110 is a unit configured to separate granules of mixed algae-bacteria granular biomass with dimensions greater than a threshold. The first separation unit 110 is, for example, a sieving separation unit. For example, a draining table, a sieve, a vibrating sieve, a grid, a belt filter, a filter, a vibrating filter may be used. The first unit 110 comprises a supply pipe 1 for an aqueous effluent, a first discharge pipe 2 for a thickened flow enriched in mixed algae-bacteria granular biomass whose granules have dimensions greater than a threshold, and a second discharge pipe 3 for a first aqueous flow depleted in mixed algae-bacteria granular biomass whose granules have dimensions less than the threshold.

[0095] The first dehydration unit 120 is a unit selected from a centrifugation unit, a filtration unit and a combination of these units, making it possible to produce a high added value mixed algae-bacteria granular biomass cake. The first unit 120 comprises a feed pipe 4 connected to the first discharge pipe 2 of the first separation unit 110 and a discharge pipe 5 for a second aqueous flow depleted in mixed algae-bacteria granular biomass. The first unit 120 also comprises a discharge pipe for the dehydrated biomass not shown in the figures.

[0096] 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 for a second thickened flow enriched in mixed algae-bacteria biomass in suspension and a fourth discharge pipe 8 for a third aqueous flow depleted in mixed algae-bacteria biomass in suspension and forming treated water.

[0097] The second dehydration unit 140 is a unit selected from a centrifugation unit, a filtration unit and a combination of these units, making it possible to produce a low added value mixed algae-bacteria biomass cake also containing pollutants and impurities. The second unit 140 comprises a feed pipe 9 connected to the third discharge pipe 7 of the first separation unit 130 and a discharge pipe 10 for a fourth aqueous stream depleted in suspended mixed algae-bacteria biomass. The second unit 140 also comprises a discharge pipe for the dehydrated biomass not shown in the figures.

[0098] The present invention presents an embodiment of a wastewater treatment plant 250 comprising a separation plant 200. The units 210, 220, 230 and 240 are identical to the units 110, 120, 130 and 140 described with reference to the, the numbering of the pipes remains the same for these units.

[0099] With reference to the, the wastewater treatment installation 250 comprises a supply pipe 11 for water to be treated containing pollutants, and optionally impurities, capable of carrying out step (A) of the treatment method, a unit 251 for bringing the water to be treated into contact capable of carrying out step (B) of the treatment method and a discharge pipe 12 for an aqueous effluent capable of carrying out step (C) of the treatment method.

[0100] Unit 251 is a unit for bringing the water to be treated into contact with a mixed granular algae-bacteria biomass capable of eliminating pollutants. It comprises a supply line 13 connected to the supply line 11 of the water to be treated and a discharge line 12 of an aqueous effluent connected to the supply line 1 of the unit 210 of the separation installation 200. Unit 251 may 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.

[0101] Optionally, a recirculation pipe 14 connects the discharge pipe 2 of the separation unit 210 of the installation 200 and the unit 251 of the treatment installation 250 so that a portion of the first flow enriched in mixed algae-bacteria granular biomass leaving step (a) is returned to step (B) of the wastewater treatment process.

[0102] Preferably, the discharge 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 discharge pipes 5, 10 can also be connected to the pipe 11 of the installation 250 by a pipe 15.

[0103] The present invention presents another embodiment of the wastewater treatment plant 350 comprising a separation plant 300. The units 310, 320, 330, 340 and 351 are identical to the units 210, 220, 230, 240 and 251 described with reference to the, the numbering of the pipes remains the same for these units.

[0104] With reference to the, the separation installation 300 comprises a coagulant injection pipe 16 connected to the supply pipe 6 of the second separation unit 330.

[0105] The separation installation 300 also comprises a flocculant and / or coagulant injection line 17 connected to the feed line 9 of the second dehydration unit 340.

[0106] The installation 300 finally comprises, downstream of the second separation unit 330, a unit 360 for disinfecting the treated water comprising a supply pipe 18 connected to the discharge pipe 8 of the unit 330 and a discharge pipe 19 for disinfected water. This unit 360 may also be a biological treatment unit or comprise both a disinfection unit and a biological treatment unit.

[0107] The present another embodiment of the wastewater treatment plant 450 comprising a separation plant 400. The units 410, 420, 430, 440 and 451 are identical to the units 310, 320, 330, 340 and 351 described with reference to the, the numbering of the pipes remains the same for these units.

[0108] In the embodiment of the, the separation plant 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 optional biological treatment unit 470 can also be provided in the embodiments shown in Figures 1-3. The separation plant 400 also comprises an anaerobic digestion unit 480 supplied with effluent via the discharge 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 recovered biogas via a pipe 21. This optional anaerobic digestion unit 480 can also be provided in the embodiments shown in Figures 1-3.

[0109] The present another embodiment of the wastewater treatment plant 550 comprising a separation plant 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 numbering of the pipes remains the same for these units.

[0110] In the embodiment of the, the contacting unit 551 is equipped with pre-separation equipment 552, integrated or not into a reactor of the contacting unit, as previously described. This pre-separation equipment 552 comprises a first outlet 552a for discharging a first sludge effluent and a second outlet 552b for discharging the remainder 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 sludge effluent into the first separation unit 510. The second outlet 552b is connected either directly to the feed pipe 6 of the second separation unit 530, or to 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.It should also be noted that the first aqueous flow leaving the first separation unit 510 via the discharge pipe 3 could be sent directly to the second separation unit 530 without passing through the biological treatment unit 570.

[0111] Such pre-separation equipment 552 may also be provided in the contacting units of the previously described embodiments.

[0112] Generally speaking, screening and / or degreasing and / or desanding units may be provided upstream of units 251, 351, 451, 551 to treat the incoming wastewater. Preferably, these units are not preceded by a settling unit.

[0113] It will also be possible to 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 settling tank, a flotation reactor, a centrifuge, a draining drum or even a draining table or grid. When present, the anaerobic digestion unit 480, 580, can be located downstream of an optional thickening unit.

[0114] The different embodiments presented with reference to figures 1 to 5 can be combined depending on the treatment and separation objective decided.

Claims

Method for separating an aqueous effluent containing mixed algae-bacteria biomass present in granular and suspended form and treated water, the method comprising:(a) A first step of physical separation of at least a portion 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 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 suspended biomass;(b) A first step of dehydration of the first thickened stream enriched in granular biomass producing a first cake and a second aqueous stream depleted in granular biomass,(c) A second step of physical separation of the first aqueous stream producing a second thickened stream enriched in 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 method 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 screening 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 implemented by means of a device chosen from a draining table, a vibrating or non-vibrating sieve, a grid, a belt filter, a vibrating or non-vibrating filter; - the second separation step (c) is chosen from membrane filtration and fabric filtration. Separation process according to any one of claims 1 to 3, characterized in that the first aqueous stream is subjected to a biological treatment step before the second separation step (c). Separation method 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 stream before the second separation step (c);- a flocculant and / or a coagulant is added to the second thickened stream 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 the fourth aqueous stream 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 algae-bacteria biomass, 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 of the biomass in suspension;(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 a part of the first thickened flow enriched in granular biomass leaving the first separation step (a) is sent to step (B) of bringing the water to be treated into contact with a 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 sludge 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 sludge effluent is sent to step (a) of the separation method,- the second aqueous effluent is sent to step (c) of the separation method, 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 in granular biomass whose granules have dimensions greater than a threshold and a second discharge pipe (3) for 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 feed line (4) connected to the first discharge line (2) of the first separation unit (110, 210, 310, 410, 510) and a discharge line (5) for a second aqueous stream depleted in granular biomass; A second physical separation unit (130, 230, 330, 430, 530), comprising a feed line (6) connected to the second discharge line of the first separation unit (110, 210, 310, 410, 510), a third discharge line (7) for a second thickened stream enriched in suspended biomass and a fourth discharge line (8) for a third aqueous stream forming treated water;A second dehydration unit (140, 240, 340, 440, 540), comprising a feed line (9) connected to the third discharge line of the second separation unit (130, 230, 330, 430, 530) and a discharge line (10) for a fourth aqueous stream depleted in suspended biomass.; Separation installation (300, 400, 500) according to claim 10, characterized in that it comprises at least one of the following characteristics: - a pipe (16) for injecting a coagulant connected to the supply pipe (6) of the second separation unit (330) - at least one pipe (17) for injecting a flocculant and / or a coagulant connected to the supply pipe (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 discharge pipe (5) of the second aqueous flow depleted in granular biomass and the discharge pipe (10) of the fourth aqueous flow depleted in suspended biomass is connected to the supply pipe (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 discharge pipe (3) of the first separation unit (410, 510) and to the supply pipe (6) of the second physical separation unit (430, 530). Wastewater treatment plant (250, 350, 450, 550), 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 algae-bacteria biomass, comprising a supply pipe (13) connected to the pipe (11) for supplying water to be treated and a discharge pipe (12) for an aqueous effluent containing treated water depleted in pollutants, at least a portion of the biomass in granular form and at least a portion of the biomass in suspension;A separation plant (100, 200, 300, 400, 500) according to any one of claims 10 to 13, said discharge pipe (12) for an effluent aqueous being connected to the aqueous effluent supply line (1) of 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 discharge pipe (2) for the first thickened flow enriched in granular biomass is connected to a recirculation pipe (14) connected to the unit (251, 351, 451, 551) for bringing the water to be treated into contact with a granular biomass, - the contacting unit (551) comprises equipment for pre-separating (552) the aqueous effluent having a first discharge outlet (552a) for a first sludge effluent and a second discharge outlet (552b) for the remainder of the effluent forming a second aqueous effluent, the first outlet (552a) being connected to said discharge 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).,

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