Method and plant for treating water by ozone injection

Abstract

The invention relates to a method for treating wastewater in a wastewater treatment plant (2), the method comprising: - a step of biological treatment of the wastewater in an activated sludge bioreactor (4), - a step of transferring mixed liquor from the bioreactor (4) to a device (6) for separating the solids and liquids of the mixed liquor, the transfer being carried out via a hydraulic circuit (8) connecting the bioreactor (4) to the device (6) for separating the solids and liquids, and - a step of separating, in the separation device (6), the solids and liquids of the mixed liquor, characterized in that it comprises a step of in-line introduction of ozone into the hydraulic circuit (8).

Description

Process and installation for water treatment by ozone injection

[0001] The invention relates to the treatment of wastewater such as urban and / or industrial wastewater. More specifically, the invention relates to a process and installation for wastewater treatment including the treatment of micropollutants by ozone injection. Such micropollutants include, for example, household and industrial chemicals and their by-products, pesticides, pharmaceuticals, and personal care products.

[0002] The need to conserve water resources has led to the deployment of wastewater treatment systems focused on overall parameters. Today, the European Union aims to restore the quality of aquatic environments. The constraints applicable at the European level regarding wastewater treatment, and in particular Directive 2024 / 3019 / EC of the European Parliament and of the Council of 27 November 2024 establishing a framework for Community action in the field of water policy, aim to progressively reduce, in the long term, the discharge of contaminants from wastewater into receiving aquatic environments. Sewage sludge is also produced during wastewater treatment. New standards for urban wastewater treatment aim to encourage the reuse of this sludge whenever possible.The objective of good chemical and ecological status for all European water bodies implies the adoption of measures to control discharges, emissions and losses for chemicals identified at European level as priority substances and priority hazardous substances and fixed at local / river basin / national level for relevant substances determined on the basis of inventories.

[0003] Wastewater treatment plants are the primary point of collection and subsequent discharge of micropollutants into the environment. These micropollutants fall into various categories that reflect all water uses and their dispersal in the environment: domestic and industrial chemicals and their by-products, pesticides, pharmaceuticals, and personal care products. Published results from various national inventories indicate that the standard design and operation of biological wastewater treatment plants allow for a limited reduction in the overall micropollutant load. In particular, the load of priority substances under the Water Framework Directive can be reduced by 80%, while nearly 80% of the total pharmaceutical load entering urban wastewater treatment plants due to domestic or hospital emissions can be discharged into surface waters.Persistent pollutants are characterized by high hydrophilicity and low biodegradability (Techniques Sciences Méthodes, number 3, 2015, Dossier “The elimination of micropollutants in domestic wastewater treatment plants”).

[0004] The behavior of micropollutants during conventional biological wastewater treatment depends on their physicochemical properties, which provide several elimination pathways: volatilization during mixing and aeration, adsorption onto suspended solids and sludge, biodegradation, and photodegradation at the water surface. Currently, adsorption onto sludge appears to be the primary elimination pathway for the monitored micropollutants, which raises the issue of subsequent sludge treatment.

[0005] It is accepted that several technologies provide a certain degree of effectiveness in removing persistent organic compounds from wastewater. These include secondary biological processes specific to older sludge ages, such as membrane bioreactors, and tertiary treatments such as membrane separation, adsorption, and chemical oxidation using ozone, or advanced oxidation processes. In this latter area, US patent 2011 / 0259832 A1 describes an advanced oxidation process combining ozonation and ultraviolet irradiation, applied as a final treatment for wastewater to achieve micropollutant removal and disinfection.

[0006] Physical separation treatments have the major drawback of transferring pollutants in concentrated streams for membrane technologies, or onto materials for adsorption technologies, which necessitates subsequent energy-intensive waste disposal. Regarding treatments involving biological or chemical reactions, it is essential to control the formation of potentially toxic byproducts.

[0007] Various applications of ozone are known for wastewater treatment upstream or downstream of biological treatment (Degrémont Water Treatment Handbook, 2005).

[0008] The main application of ozone at present is tertiary, meaning it is applied after a biological treatment stage, such as the removal of nitrogen and, where applicable, phosphorus pollution, and a liquid-solid separation stage. The aim is to apply ozone to the cleanest possible water to minimize competition with soluble organic matter, thereby maximizing the effectiveness of the ozone treatment while keeping operational costs to a minimum. It is also important to limit the formation of byproducts, as nitrification is a crucial step in the removal of precursors whose oxidation produces potentially toxic byproducts.

[0009] Such an implementation presents the following drawbacks: First, it is necessary to construct a facility dedicated to this treatment, enabling the transfer of ozone (by bubbling gaseous ozone) and contact between the dissolved ozone and the water (in a contactor). The doses required for treatment are significantly higher than the instantaneous ozone demand (IOD) of the effluent. This implies substantial production costs and a considerable land footprint. It also implies maintaining contact between the water and the ozone for a sufficiently long period, which necessitates reactors of a certain size. The application of ozone in tertiary treatment prevents the treatment of some of the micropollutants that may be present in the sludge, as explained above.The application of ozone in tertiary treatment does not allow us to benefit from the positive effects that ozone can have on sludge, particularly in terms of settling and limiting the risk of the appearance of filamentous bacteria that cause settling difficulties.

[0010] It is also known to inject ozone into the recirculation loop of a biological reactor, such as an activated sludge reactor. However, this has the following drawbacks: Applying ozone to a recirculation loop significantly limits achievable performance because it does not treat the entire flow of water passing through the biological reactor, unless a very high recirculation rate is applied. By performing a mass balance on a system, the maximum achievable removal of micropollutants can be evaluated based on the local removal achieved in the recirculation loop where ozone is injected and the recirculation rate of that injection loop. It is observed that to increase the overall removal rate, a high local removal rate in the injection loop and a high recirculation rate are necessary.For example, to achieve an overall 80% removal rate, a 95% local removal rate is required, with a 300% recirculation flow rate compared to a fixed daily water treatment flow rate of 100%. This leads to very high ozone consumption to obtain the desired removal rate. It is difficult, if not impossible, to have any flexibility in the amount of ozone injected to account for the quantity and quality of the water being treated. Indeed, since ozone is typically injected directly into the mixed liquor via a venturi hydro-ejector, the technical constraints associated with this injection method are limited, namely the inability to vary the liquor flow rate feeding the venturi. When ozone is injected into an internal recirculation loop, the ozonated mixed liquor must be reinjected into the biological reactor in an aerated area to avoid disrupting the anoxic or anaerobic treatment process within the reactor.This can be complex or even impossible in facilities where treatment is carried out with intermittent aeration. In this case, ozone must only be operated when the basin aeration is running, which degrades performance by creating hydraulic bypasses of non-ozonated mixed liquor when the basin is not aerated.

[0011] Finally, it is known to inject ozone at the inlet of a biological reactor, such as an activated sludge reactor. However, this leads to a drawback similar to the one mentioned earlier regarding the limitation of achievable performance. More precisely, in this case, it is not the injection into a recirculating system that degrades performance, but rather the fact that the amount of dissolved organic matter is significantly greater because the biodegradable fraction (i.e., by the biological reactor) has not yet been removed. Since ozonation performance is directly related to the amount of ozone transferred divided by the concentration of dissolved organic matter, enormous quantities of ozone would have to be injected to compensate for the large amount of dissolved organic matter present. Furthermore, the biological treatment removes a large portion of the precursors of oxidation byproducts. This leads to the need to add ozone after nitrification.

[0012] The invention aims to provide a wastewater treatment process enabling the treatment of micropollutants by the use of ozone with great flexibility while minimizing the ground footprint of a treatment facility and obtaining all the benefits of the action of ozone on the water to be treated and on the purifying biomass present in the biological basin.

[0013] To this end, the invention relates to a wastewater treatment process in a wastewater treatment plant, the process comprising: a biological treatment step of wastewater in an activated sludge biological reactor, a mixed liquor transfer step from the biological reactor to a device for separating solids and liquids from the mixed liquor, the transfer being carried out via a hydraulic circuit connecting the biological reactor to the device for separating solids and liquids, and a separation step in the device for separating solids and liquids from the mixed liquor, the process further comprising a step of introducing ozone into the hydraulic circuit.

[0014] According to the invention, the step of introducing ozone into the hydraulic circuit is carried out by inline injection of ozone into the hydraulic circuit.

[0015] Thus, ozone is injected between the biological treatment stage and the separation stage. Compared to previous methods involving tertiary ozone treatment, a dedicated ozone treatment unit (ozone diffusion and contact) is not required. This reduces the land footprint and production costs of the wastewater treatment plant. Furthermore, the ozone is injected into the mixed liquor at the outlet of the biological reactor, but before separation. The reactivity and decay kinetics of ozone are greater when it is in contact with the mixed liquor than when it is in contact with the water at the outlet of the solids / liquid separator, resulting in significantly shorter required contact times.Furthermore, the dose of ozone to be introduced locally is lower than during ozonation following a solids / liquids separation step in the mixed liquor (this is because a lower local reduction than in the prior art is acceptable to achieve the same overall reduction). Finally, the residence time in the hydraulic circuit and in a solids / liquid separator is much longer than what is required to achieve the desired ozone decay. Therefore, there is no need for a dedicated ozone contactor, only a degassing step to remove excess gas.

[0016] Finally, ozone injected upstream of the solids-liquid separation stage can modify the structure of the solids present in the mixed liquor, thereby improving solid / liquid separation within the separation device. The presence of ozone in the mixed liquor also helps prevent or eliminate the growth of filamentous bacteria that cause foaming and / or poor settling.

[0017] Compared to ozone injection in a recirculation loop, the process according to the invention makes it possible to improve treatment performance with the same quantity of ozone injected, or to reduce the quantity of ozone injected while maintaining unchanged treatment performance compared to implementation in a recirculation loop. Indeed, injection into a recirculation loop means that, mechanically, a large portion of the flow is not directed towards the ozone injection point, and even if performance at the injection point is very good, an overall performance limit is observed between the inlet and outlet of the biological treatment. In the context of the invention, all or a large majority (>75%) of the mixed liquor is brought into direct contact with ozone before entering the separation device. This is then referred to as an "ozonated" mixed liquor.Any fraction of mixed liquor not brought into direct contact with ozone (or "non-ozonated" fraction) will be so by mixing with the ozonated fraction which will act as an ozone carrier.

[0018] Furthermore, the need to inject ozonated mixed liquor into a specific area of ​​the biological reactor at a specific time during its operation is eliminated. Finally, for the same quantity of ozone injected, the dose at the injection point is lower, leading to a reduction in the local formation of unwanted byproducts such as bromates. Moreover, any oxidation byproducts generated occur in the presence of purifying biomass that can degrade them, considering that a portion of the ozonated mixed liquor is recirculated to the head of the biological basin after clarification, typically with a recirculation rate of 100% or more.

[0019] Compared to ozone injection at the inlet of a biological reactor, the process according to the invention also makes it possible to improve treatment performance with the same quantity of ozone injected, or to reduce the quantity of ozone injected while maintaining unchanged treatment performance, because the ozone injection is carried out after the majority of the dissolved biodegradable fraction has been degraded. Finally, the requirement for ozone injection after nitrification is met by injecting ozone between the outlet of the aerated zone of the biological treatment, which performs nitrification and thus eliminates a large portion of the oxidation by-product precursors, and the separation of solids and liquids.

[0020] Depending on other optional characteristics of the wastewater treatment process taken alone or in combination: the solids and liquids separation stage is carried out in a clarifier or in a membrane separation unit; the mixed liquor transfer stage from the biological reactor to the solids and liquids separation device includes a mixed liquor residence stage in a collection tank; the ozone introduction stage at the hydraulic circuit level includes the following steps: passing at least a portion of the mixed liquor from the biological treatment stage through at least one gas / liquid mixing device, for example a hydro-ejector; enriching the mixed liquor with ozone by injecting gaseous ozone into the gas / liquid mixing device; and injecting the ozone-enriched mixed liquor into the hydraulic circuit.The process includes a degassing step of the mixed liquor after enrichment of the mixed liquor with ozone by injecting gaseous ozone into the gas / liquid mixing unit. The mixed liquor from the biological treatment step is divided into several sub-volumes, each sub-volume passing through a different gas / liquid mixing unit.

[0021] According to features not covered by the present invention: the process includes an additional treatment step downstream of the solids and liquids separation step, in which the ozone introduction step into the hydraulic circuit comprises the following steps: taking a liquid fraction at the outlet of the separation step and / or at the outlet of an additional treatment step, passing the liquid fraction through at least one gas / liquid mixing device, for example a hydro-ejector, and enriching the liquid fraction by injecting gaseous ozone into the gas / liquid mixing device,The process includes a step of degassing the mixed liquor after the liquid fraction has passed through at least one gas / liquid mixing unit and enriching the liquid fraction by injecting gaseous ozone into the gas / liquid mixing unit. The ozone-enriched liquid fraction and the mixed liquor from the biological treatment stage are then mixed in a liquid / liquid mixer, for example a static mixer, and the mixture of the mixed liquor and the ozone-enriched liquid fraction is then injected into the hydraulic circuit.

[0022] The invention also relates to a wastewater treatment plant (2) comprising: a biological reactor for activated sludge treatment, a device for separating solids and liquids from a mixed liquor at the outlet of the biological reactor, a hydraulic circuit arranged between the biological reactor and the separation device so as to allow the passage of the mixed liquor from the biological reactor to the separation device, the plant comprising means for introducing ozone into the hydraulic circuit.

[0023] According to the invention, the means for introducing ozone into the hydraulic circuit are configured so as to achieve an in-line injection of ozone into the hydraulic circuit.

[0024] Depending on other optional features of the wastewater treatment plant, taken alone or in combination: the solids and liquid separation device is formed by a clarifier; the hydraulic circuit includes a mixed liquor collection tank located between the biological reactor and the separation device and downstream of an ozone introduction point; the introduction means include an injection unit, the injection unit comprising: at least one conduit for at least a portion of the mixed liquor located between the biological reactor and the hydraulic circuit, at least one gas / liquid mixing device, for example a hydro-ejector, located on the conduit, and at least one gaseous ozone injection device in the liquid / gas mixing device; the plant includes several mixed liquor conduits located between the biological reactor and the hydraulic circuit.Each flow pipe is equipped with a gas / liquid mixing device and a valve configured to allow or prevent the passage of mixed liquor through the gas / liquid mixing device.

[0025] According to features not covered by the present invention: the installation includes an additional treatment device downstream of the separation device, the introduction means comprising an injection unit, the injection unit comprising: means for sampling a liquid fraction at the outlet of the separation device and / or at the outlet of the additional treatment device, a gas / liquid mixing element configured to enrich the liquid fraction by injecting gaseous ozone, means for supplying the ozone-enriched liquid fraction into the hydraulic circuit, and the means for supplying the ozone-enriched liquid fraction into the hydraulic circuit include: a liquid / liquid mixer, preferably a static mixer, configured to mix the ozone-enriched liquid fraction with the mixed liquor at the outlet of the biological reactor,a supply line for the mixture of the ozone-enriched liquid fraction and the mixed liquor into the hydraulic circuit. Brief description of the figures

[0026] The invention will be better understood upon reading the following description, given solely by way of example and made with reference to the accompanying drawings in which:

[0027] This is a schematic representation of a wastewater installation according to an embodiment of the invention.

[0028] This is a schematic representation of an ozone injection unit for a wastewater treatment plant, according to one embodiment variant of the...

[0029] This is a schematic representation of a wastewater installation according to an example not covered by the present invention.

[0030] This is a schematic representation of an ozone injection unit for a wastewater treatment plant, according to an example not covered by the present invention. Detailed description

[0031] We now refer to the illustration of a wastewater treatment plant 2, such as urban and / or industrial wastewater, according to an embodiment of the invention.

[0032] The wastewater treatment plant 2 includes a free-floating biomass biological reactor 4. A "free-floating biomass biological reactor" is defined as a biological reactor in which the bioreactor biomass is kept in suspension within the reactor in the form of flocs. This is also referred to as a free culture.

[0033] Purifying biomass is classically composed of a mixture of living or dead microorganisms (bacteria), plant and / or mineral debris, colloids, and small animal species such as microfauna of a few micrometers to millimeters, specific to the site.

[0034] Biological reactor 4 is an activated sludge treatment reactor.

[0035] The biological reactor allows water treatment by decomposition of BOD (biological oxygen demand) and COD (chemical oxygen demand) under aerobic or anoxic conditions.

[0036] Once the biological treatment has been carried out, a mixed liquor is recovered at the outlet of the biological reactor 4. The term "mixed liquor" refers to the mixture consisting of sludge and purified water.

[0037] It is known to direct the mixed liquor to a separation device 6 for the solids and liquids of the mixed liquor at the outlet of the biological reactor 4. The separation device 6 can consist of a clarifier (or decanter) or a filtration unit, for example, a membrane filtration unit. The separation device 6 allows the mixed liquor to be separated into a clarified final effluent with a suspended solids content generally less than 20-30 mg / L (i.e., a separation efficiency greater than 98%) and a solid phase (concentrated sludge). The latter is then partially recirculated to the biological reactor 4 via a sludge recirculation loop 11, thus maintaining the concentration of the reactor's purifying biomass, while the excess sludge is removed.

[0038] The wastewater treatment plant 2 further includes a hydraulic circuit 8. This circuit is located between the biological reactor 4 and the separation device 8 so as to allow the mixed liquor to flow from the biological reactor 4 to the separation device 6. The hydraulic circuit 8 includes at least one pipe or a set of pipes enabling the aforementioned flow. The hydraulic circuit 8 may include, in its upstream section, a structure integrated into the biological reactor that collects the mixed liquor at the outlet of the biological reactor 4 and can be used for the ozone injection described below.

[0039] The hydraulic circuit 8 may include a mixed liquor collection tank disposed between the biological reactor 4 and the separation device 6. The collection tank allows for the recovery and storage of the mixed liquor before it enters the separation device 6.

[0040] The hydraulic circuit 8 may also include a device for degassing gases dissolved in the mixed liquor, for example, oxygen injected into the biological reactor 4, or methane or nitrogen, in order to avoid sudden pressure variations that could impair the proper functioning of the separation device 6. Degassing therefore does not occur during the separation of solids and liquids, which would interfere with said separation. These gases can be destroyed or recovered.

[0041] As can be seen in the diagram, it is possible to provide an additional treatment device 10 downstream of the separation device 6. This is called tertiary treatment, which can be of different types depending on the objective. For example, it could involve the reduction of suspended solids and / or phosphorus and / or further treatment of micropollutants.

[0042] The wastewater treatment plant 2 further includes means for introducing ozone into the hydraulic circuit 8, including an injection unit 14 configured to inject ozone into the hydraulic circuit 8. This enables an oxidation reaction as soon as the mixed liquor leaves the biological reactor 4, during its passage through the hydraulic circuit 8 and even during its passage through the separation device 6.

[0043] According to the invention, the means for introducing ozone into the hydraulic circuit 8 are configured to perform in-line ozone injection into the hydraulic circuit 8. "In-line injection" means the introduction of ozone directly into a flowing pipe, that is, without interrupting the water flow. Therefore, there is no intermediate structure, such as a dedicated ozonation reactor in which ozonation is performed during a stay in said dedicated structure.

[0044] Ozone injection is carried out using air or concentrated oxygen as a carrier gas (for example, a mixture of 10% ozone with 90% dioxygen), which saturates the mixed liquor with oxygen before it enters the separation device 6. The risks of causing anoxia in the sludge at the bottom of the separation device 6, and therefore the creation of nitrogen bubbles by uncontrolled denitrification which disrupts clarification, are limited.

[0045] Furthermore, injecting ozone into the mixed liquor potentially allows some of the ozone to react with compounds that can hinder oxygen transfer during aeration, such as surfactants. Since this mixed liquor is partially recirculated, this theoretically reduces the concentration of these compounds in biological reactor 4 and thus improves oxygen transfer.

[0046] Finally, injecting ozone into the mixed liquor between biological treatment and separation leads to mixing with the incoming water during recirculation after separation, and a dilution of the micropollutant concentrations reaching the biological reactor, as the recirculated liquor will have been treated to achieve the treatment objective. This results in a lower ozone abatement requirement at the injection point than if there were no recirculation. Thus, a lower local dose of ozone (mg / L) is applied, which limits by-products and dissolved ozone after injection (closer to the limit at which 100% of the ozone is consumed instantaneously after injection, known as DIO). Furthermore, dissolved ozone consumption is faster due to the presence of biomass. All of this allows for the elimination of the tertiary contact structure and a much more compact design, while still allowing the biological treatment to benefit from ozone.

[0047] Furthermore, and in the case where a degassing device is present, it is possible to use the latter to degas the excess gas which may contain traces of untransferred ozone, in order to avoid the construction of a degassing structure dedicated to ozone, which reduces the ground footprint of the wastewater treatment plant 2. This existing degasser must nevertheless be covered in order to collect the gas and send it to a system for the destruction of residual ozone.

[0048] According to the embodiment of the invention illustrated in figures 1 and 2, the injection unit 14 comprises: at least one through-pipe 16 for at least a portion of the mixed liquor present in the biological reactor disposed between the biological reactor 4 and the hydraulic circuit 8, at least one gas / liquid mixing element 18, for example a hydro-ejector, disposed on the through-pipe 16, and at least one gaseous ozone injection element 20 in the gas / liquid mixing element 18. This ozone injection element 20 can allow the production and supply of ozone in the gas / liquid mixing element 18.

[0049] A hydro-ejector is known from the prior art. A hydro-ejector is a suction device that uses a pressurized liquid flow (here, the mixed liquor) passing through a narrowed nozzle, creating a Venturi effect that generates a vacuum which causes the water to be pumped out.

[0050] According to Figures 1 and 2, at least a portion of the mixed liquor is taken from the biological reactor 4 and passes through at least one hydro-ejector 18. As it passes through the hydro-ejector, gaseous ozone is injected directly into the mixed liquor. This mixed liquor, exiting the injection unit 14, is injected into the hydraulic circuit 8, which may be of the form described above, with, for example, a residence time in the collection tank. If gaseous ozone is injected into only a portion of the mixed liquor, this ozone-enriched (or ozonated) portion is mixed with the mixed liquor that did not pass through the injection unit 14 to allow for ozone treatment of the mixed liquor as soon as it enters the hydraulic circuit 8.

[0051] In variant A, several hydro-ejectors (five for illustrative purposes) are arranged in parallel on the flow line, and therefore between the biological reactor 4 and the hydraulic circuit 8. Each hydro-ejector is preceded by a valve 22, for example a solenoid valve, configured to allow or prevent the passage of mixed liquor through the hydro-ejector located downstream of it. An electronic control unit allows the opening configuration of the valves 22 to be controlled according to various parameters described below.

[0052] The opening configuration of the valves 22 is preferentially a function of the volume of mixed liquor that we wish to pass through the injection unit 14, a volume which itself may depend on the quantity of water treated at a time T by the biological reactor 4. The greater the quantity of mixed liquor at the outlet of the biological reactor 4, the more valves 22 can be opened in order to use a greater number of hydro-ejectors.

[0053] In other words, the flow rate of mixed liquor through the hydro-ejector is constant. It is possible to vary the amount of ozone injected by the injection device 14 by varying the gas flow rate and / or the ozone concentration of the gas.

[0054] It should also be noted that the number of hydro-ejectors used may depend on the percentage of mixed liquor that is to be passed through injection unit 14 within a given time interval. Advantageously, at least 75%, and preferably at least 90%, of the mixed liquor can be passed through injection unit 14. Passing less than 90% is possible, but this will degrade performance.

[0055] Regarding the amount of ozone to be injected, this is theoretically known for a given volume of mixed liquor exiting a known biological reactor. The relationship between micropollutant abatement and the injected ozone dose is generally linear up to a target abatement level, after which a slowdown in the increase of abatement with dose is observed. If the local target abatement level is lower than the target abatement level and the percentage of mixed liquor that is ozonated is sufficiently large, applying the amount of ozone that would be required to achieve the target if all of the mixed liquor were ozonated to a fraction of the mixed liquor does not change the ozone requirements. In this case, the increase in local dose resulting from applying the ozone flow to a smaller flow rate of mixed liquor compensates for the small fraction of unozonated mixed liquor.This will depend on the percentage of ozonated mixed liquor. If this percentage reduces the local dose increase, applying the ozone to this percentage of mixed liquor, rather than to the entire liquor, will be greater, and the dose at which the performance increase is no longer linear will be reached more quickly. Conversely, if higher performance is desired, the curve is no longer linear, and it is necessary to slightly increase the ozone injected into the fraction of mixed liquor passing through injection unit 14 to compensate for the fact that some of the mixed liquor does not pass through injection unit 14. For example, if 90% of the mixed liquor is ozonated and 10% is not, and the goal at the injection point is to achieve 80% abatement, the total quantity of ozone to be injected into the 90% is 10% greater than the quantity required if it were injected into 100% of the mixed liquor flow.

[0056] Thus, and knowing the flow rate of mixed liquor to be treated and the theoretical quantity of ozone to be injected for a given volume of mixed liquor at the outlet of a known biological reactor, it is possible to modulate the number of hydroejectors used and the quantity of ozone injected at each hydroejector in order to treat a volume of mixed liquor in a given time.

[0057] The choice of the number of hydro-ejectors and their flow rate depends on the plant's hydraulic profile and its recirculation flow rate. The objective is to achieve the highest possible percentage of the daily mixed liquor flow rate passing through ozonation. Hydro-ejectors with varying flow rates can be selected. Choosing hydro-ejectors with lower individual flow rates allows for finer control of the injection unit but requires a larger number of hydro-ejectors, while choosing hydro-ejectors with higher individual flow rates allows for a smaller injection unit but increases the fraction of mixed liquor that will not pass through the hydro-ejectors. The choice may also depend on the configuration of a water treatment plant, particularly the inlet flow rate, the recirculation flow rate, and daily flow rate variations.

[0058] Advantageously, the ozone injection element 20 is connected to the smallest diameter section of the hydro-ejector(s). This allows for better mixing and dissolution of the ozone in the mixed liquor by taking advantage of the velocity effect and turbulence within the hydro-ejector.

[0059] Regarding the wastewater treatment process according to the invention, the latter comprises the following steps: a biological treatment step of the wastewater in the biological reactor; a transfer step of mixed liquor from the biological reactor 4 to the solids and liquids separation device 6 via the hydraulic circuit 8; a separation step of the solids and liquids from the mixed liquor by the separation device 6 (for example in a clarifier).

[0060] The process according to the invention may further include a step of the mixed liquor remaining in the collection tank.

[0061] The process according to the invention further includes a step of introducing ozone inline (as defined above) into the hydraulic circuit 8, by means of the injection unit 14. In the embodiment of the invention shown in Figures 1 and 2, this injection is carried out via the following steps: Passing at least a portion of the mixed liquor from the biological treatment step into at least one gas / liquid mixing element 18. Enriching the mixed liquor with ozone by injecting gaseous ozone into the gas / liquid mixing element 18. Injecting the mixed liquor enriched with dissolved ozone into the hydraulic circuit 8.

[0062] As described above, the number of hydro-ejectors used or the quantity of ozone injected per hydro-ejector can vary depending on the volume of mixed liquor to be passed through and enriched with ozone by the injection unit 14.

[0063] Finally, as described previously, the process according to the invention may further include an additional treatment step downstream of the solids and liquids separation step. This may be, for example, filtration through a sand filter, dephosphatation by a lamellar clarifier, flotation, adsorption on activated carbon, or tertiary ozonation.

[0064] In one embodiment, where the clarifier recirculation rate is 100% of the inlet flow, ozone is injected into biological reactor 4 at 200% of the inlet flow, with a dissolved organic carbon (DOC) level equal to that of the treated water. Three parts per million (ppm) of ozone are injected at the injection point, corresponding to an ozone concentration of 0.3 milligrams per milligram of DOC. The injection occurs upstream of a degassing device where the mixed liquor remains for approximately two minutes, followed by several hours in the separation unit. This quantity allows for an 80% reduction in diclofenac, for example. Conversely, and with an injection according to the prior art in the recirculation loop, the injection of an identical ozone flux leads to an injection of 6 parts per million on 100% of the recirculated flow, which allows a 95% reduction on the recirculated flow, and therefore a 60% reduction in diclofenac overall.The reduction is therefore less for the same quantity of ozone used. The invention makes it possible, compared to an ozone injection into a recirculation loop, to obtain at least the same reduction for a smaller quantity of ozone, to obtain a better reduction with the same quantity of ozone, or even, as is the case in the example, to obtain a better reduction for a smaller quantity of ozone.

[0065] We now refer to the illustration of an embodiment not covered by this invention. In what follows, only the differences with the embodiment of the invention shown in Figures 1 and 2 will be described.

[0066] In the example illustrated in the figure, the ozone introduction means include an injection unit 14' comprising: Means for sampling a liquid fraction at the outlet of the separation device 6 and / or at the outlet of the additional treatment device 10. These means may consist of at least one pump and at least one sampling line 25. A gas / liquid mixing device configured to enrich the liquid fraction by injecting gaseous ozone, A liquid / liquid mixer, preferably a static mixer, configured to mix the ozone-enriched liquid fraction with the mixed liquor at the outlet of the biological reactor 4, A supply line for the mixture of the ozone-enriched liquid fraction 26 and the mixed liquor into the hydraulic circuit 8.

[0067] In other words, and in this example, the goal is to charge a fraction of liquid with ozone and inject it into the mixed liquor, unlike the first embodiment in which all or part of the mixed liquor passes through at least one gas / liquid mixing device.

[0068] Laillent illustrates a variant not covered by the present invention in which a specific mixer is provided to mix the ozone-enriched liquid fraction and the mixed liquor at the outlet of the biological reactor 4 before the mixture returns to the hydraulic circuit 8. Direct injection of the ozone-enriched liquid fraction into the hydraulic circuit 8 is also possible, as is the integration of the mixer into the hydraulic circuit 8. The wastewater treatment plant therefore includes means for supplying the ozone-enriched liquid fraction into the hydraulic circuit, these means being able to take the form of a liquid / liquid mixer combined with a supply line for the mixture of the ozone-enriched liquid fraction 26 and the mixed liquor into the hydraulic circuit 8, a non-limiting possibility.

[0069] Advantageously, the mixer is a static mixer 28 known from the prior art. It is a continuous fluid mixing device capable of mixing liquids, gases, or, as in this case, a gas and a liquid. In wastewater treatment, static mixers are used to mix chemicals, gases, and reagents with water. They are commonly used for disinfection, pH control, coagulation, flocculation, and sludge mixing.

[0070] The liquid fraction is taken from the outlet of the separation device 6 or the outlet of the additional treatment device 10. This is therefore treated water that does not contain flocs and has a limited particle content. For this type of water, the instantaneous ozone demand (IOD), which is the amount of ozone required to oxidize all species with very high reactivity to ozone, and the decay kinetics of dissolved ozone are limited and significantly lower than those of the mixed liquor. This allows for a residual dissolved ozone concentration, with limited ozone "losses" (due to IOD consumption), that is sufficiently stable to be subsequently mixed with the mixed liquor. This small pumped flow rate is the source of the dissolved ozone flux necessary to apply the desired dose to the mixed liquor.

[0071] The liquid fraction can represent up to 30% of the treated water, into which a quantity of ozone can be injected, depending on the ozone solubility threshold in water as described above. A recirculated fraction of 30% therefore leads to an increase in the flow rate at the inlet of the separation device. Depending on the circumstances, a more complex ozone injection system can be used in the recirculated flow (by modifying the pressure and / or pH of the water) to maximize ozone solubility in this water and reduce the required water flow rate. However, the effect of ozone on the mixed liquor, and in particular the improvement in its settling capacity, compensates for this increased flow rate.

[0072] Alternatively, the fraction of liquid taken comes from the biological reactor 4. It is therefore a question of using a fraction of the mixed liquor used as a carrier to supply the mixed liquor with ozone between the biological reactor 4 and the separation device 6.

[0073] Regarding the process described above, it comprises the following steps: A liquid fraction is taken from the outlet of the separation stage or from a further treatment stage. The liquid fraction is then passed through at least one gas / liquid mixing device, for example, a hydro-ejector, and the liquid fraction is enriched by injecting gaseous ozone into the gas / liquid mixing device. All of the ozone is injected to treat the mixed liquor. Ozone is injected up to the solubility limit, which is approximately 30 mg / L at 25°C. The ozone-enriched liquid fraction and the mixed liquor from the biological treatment stage are mixed in a liquid / liquid mixer, for example, a static mixer. A supply line 27 from the mixed liquor to the injection unit 14' is provided for this purpose. The mixture of the mixed liquor and the ozone-enriched liquid fraction is then injected into the hydraulic circuit 8.

[0074] Here again, the use of a liquid / liquid mixer is a possible implementation. The ozone-enriched liquid fraction can be injected upstream of the liquid / liquid mixing unit (as shown in the figure) or perpendicularly to the liquor flow in a pressurized pipeline.

[0075] When a mixer is used, the ozone-enriched liquid fraction and the mixed liquor can be injected into the same pipe and pass through static elements, such as blades. Each blade is curved to create turbulence in the fluid(s) flowing along it. The combination of the flow in the pipe and the turbulence ensures that the two liquids are mixed.

[0076] The ozone-enriched liquid fraction can advantageously be degassed to remove excess gas. Degassing can be carried out before or after mixing with the mixed liquor, preferably after.

[0077] In this example, it is also possible to vary the volume of the extracted fraction and / or the amount of ozone injected into the liquid fraction to optimize the treatment of the mixed liquor, based on variations in the wastewater flow rate. Specifically, it will be possible to ensure 100% ozonation of the mixed liquor flow rate at all times and to more easily control the ozone injection to adjust performance as needed. List of references

[0078] 2: Wastewater treatment plant

[0079] 4: Biological reactor

[0080] 6: Separation device

[0081] 8: Hydraulic circuit

[0082] 10: Additional treatment device

[0083] 11: Sludge recirculation loop

[0084] 14, 14': injection unit

[0085] 16: Passage driving

[0086] 18: Gas / liquid mixing device

[0087] 20: Ozone injection unit

[0088] 22: valve

[0089] 25: Sampling procedure

[0090] 26: Supply line for the mixture of the ozone-enriched liquid fraction and the mixed liquor

[0091] 27: Supply line for the mixed liquor to the injection unit

[0092] 28: Static mixer

Claims

Wastewater treatment process in a wastewater treatment plant (2), the process comprising: a biological wastewater treatment step in an activated sludge biological reactor (4), a mixed liquor transfer step from the biological reactor (4) to a solids and liquids separation device (6) of the mixed liquor, the transfer being carried out via a hydraulic circuit (8) connecting the biological reactor (4) to the solids and liquids separation device (6), and a separation step in the separation device (6) of the solids and liquids of the mixed liquor, characterized in that it comprises a step of introducing ozone into the hydraulic circuit (8), the step of introducing ozone into the hydraulic circuit (8) being carried out by inline injection of ozone into the hydraulic circuit (8). Wastewater treatment process according to claim 1, wherein the step of transferring the mixed liquor from the biological reactor (4) to the solids and liquids separation device (6) includes a step of the mixed liquor remaining in a collection tank. Wastewater treatment process according to any one of the preceding claims, wherein the step of introducing ozone into the hydraulic circuit (8) comprises the following steps: passing at least a portion of the mixed liquor from the biological treatment step through at least one gas / liquid mixing element (18), for example a hydro-ejector, enriching the mixed liquor with ozone by injecting gaseous ozone into the gas / liquid mixing element (18), and injecting the mixed liquor enriched with dissolved ozone into the hydraulic circuit (8). Wastewater treatment process according to the preceding claim, wherein the mixed liquor from the biological treatment step is divided into several sub-volumes, each sub-volume passing through a different gas / liquid mixing element (18). Wastewater treatment plant (2) comprising: a biological reactor (4) for activated sludge treatment, a device for separating solids and liquids from a mixed liquor at the outlet of the biological reactor (4), a hydraulic circuit (8) arranged between the biological reactor (4) and the separation device (6) so as to allow the passage of the mixed liquor from the biological reactor (4) to the separation device (8), characterized in that it comprises means for introducing ozone into the hydraulic circuit (8), the means for introducing ozone into the hydraulic circuit (8) being configured so as to carry out an in-line injection of ozone into the hydraulic circuit (8). Wastewater treatment plant (2) according to claim 5, wherein the hydraulic circuit (8) includes a mixed liquor collection tank disposed between the biological reactor (4) and the separation device (6) and downstream of an ozone introduction point. Wastewater treatment plant (2) according to any one of claims 5 or 6, wherein the introduction means comprise an injection unit (14), the injection unit (14) comprising: at least one through pipe (16) for at least a portion of the mixed liquor disposed between the biological reactor (4) and the hydraulic circuit (8), at least one gas / liquid mixing element (18), for example a hydro-ejector, disposed on the through pipe (16), and at least one gaseous ozone injection element (20) into the liquid / gas mixing element (18). Wastewater treatment plant (2) according to the preceding claim, comprising several mixed liquor passage lines (16) arranged between the biological reactor (4) and the hydraulic circuit (8), each passage line (16) being equipped with a gas / liquid mixing element (18) and a valve (22) configured to allow or prohibit the passage of mixed liquor through the gas / liquid mixing element (18).

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