Patent Application
20240351925
The present invention relates to a device and a method for purifying wastewater. It applies, for example, to the field of urban wastewater purification.
The shortage of water resources and climate change call for appropriate and fine-tuned management of the available water resources. In particular, the Mediterranean regions are facing an impending water shortage and require environmental protection technologies that:
Solutions of UASB (for “Upflow Anaerobic Sludge Blanket”) anaerobic reactor type are known, developed for the treatment of wastewater in the 1970s. This reactor is supplied by upward flow with no packing medium, and is equipped in its upper part with a three-phase separation system: gas, liquid and solid. This system allows the biogas to be evacuated, and reduces the output of matter in suspension (referred to as “MES”) in the effluent. Its hydraulic behaviour is perfectly mixed except at the location of the three-phase separator, which has the hydraulic characteristics of a piston reactor. Agitation of the medium is mainly due to the production of biogas. A good distribution of wastewater is necessary, in particular for water with a low concentration of COD (for “chemical oxygen demand”) generating little gas, to prevent the formation of preferential paths.
Biomass retention in sludge bed reactors is based on the ability of anaerobic microorganisms to flocculate in order to form granules that can reach 5 mm in diameter and have good sedimentation characteristics (volumetric index <20 mL·gMVS−1; maximum settling velocity >5 m·h−1), and good mechanical resilience. This prevents their leaching from the reactor in which the upflow velocity of the liquid is generally maintained between 1 and 1.5 m·h−1.
The main problem with these systems lies in the formation and maintenance of granules. Granulation is a slow process that results in a long startup period (about six months) if unsuitable digester sludge is used as inoculum. Formation of a granular sludge is practically impossible with some types of effluent, and degranulation can be observed when the reactor that treats these effluents is seeded with a sludge that is already granulated. Sludge bed methods are also sensitive to the sludge concentration in the MES. Indeed, a low upflow velocity does not enable the leaching of particulate matter, which can cause their accumulation at the expense of the formation of granules, thus leading to a reduction in the biological activity. The resolution of these problems entails better knowledge of the intimate mechanisms of the granulation and of the factors that govern it. The granules are formed solely of microorganisms. It is generally agreed that their formation is the result of the selection of flocculated bacteria in an upflow system where the free cells in suspension are necessarily leached. The granules also have a very complex bacterial organisation. Their fermentative, syntrophic and methanogenic microorganisms are closely associated with each other, which reduces the distance between them. The inter-species transfer of hydrogen and the dispersion of metabolites along the trophic chain are more efficient.
Integrating the UASB technology as pretreatment method for the UWW has the following disadvantages:
Mixing inside the UASB reactor is a key parameter. One of the greatest technological advances in high-load anaerobic methods is the possibility of separating the hydraulic residence time (acronym “HRT”) from the sludge residence time (acronym “SRT”). Unlike aerobic systems, in anaerobic or anoxic processes the maximum acceptable load is dependent on the maximum level of reactant that can be supplied (e.g. oxygen during aerobic reactions), but in UASB digesters the performance is governed by the amount of viable anaerobic biocatalysts, i.e. the anaerobic bacterial biomass, having maximum contact with the constituents of the wastewater. There have been many changes to the UASB since its invention. Thus, some researchers have replaced the gas-solid separator in the top of the reactor by a fixed bed with ascending flow (referred to as “UASB-FA”). Other teams have adapted the fluidisation conditions of a fluidised bed (liquid and gas velocities) to sludge beds so as to obtain the expansion of the granules and handle various chemical, biochemical and biotechnological discharges at high load. This reactor, called EGSB (for “Expanded Granular Sludge Bed”), handles higher pollution loads because the dead zones are limited, which boosts the sludge/effluent contact. EGSB combines recirculation of the effluent at upflow velocities greater than 4 m·h−1 and higher reactor geometries (high height/diameter ratio).
Compartmentalisation of the reactor space has led to the internal circulation (acronym “IC”) reactor, which comprises two UASB reactors superimposed. The lower reactor operates at high load and the biogas produced is recovered for the fluidisation of the second reactor, by gas lift effect, located above and supplied at low load.
Although the UWW is considered to be diluted effluents (COD<1000 mg·L−1), it has the characteristics of a complex effluent, with a low temperature (fluctuating between 15 and 25° C.) and a high MES ratio of approximately 50-65%, i.e. a low ratio of soluble COD to the total COD. Consequently, the total conversion of the COD is limited by the hydrolysis phase of solid compounds. The management and yields of this methanation technology are highly dependent on the temperature. As with all chemical and biochemical transformations, the speed of the reactions that occur in methanation increases with the temperature. However, these systems are very dependent on their operating temperature.
Patent application US 2020 392 025 and patent U.S. Pat. No. 8,721,877 are known, which disclose devices for treating wastewater solids and gas. However, such devices operate at atmospheric pressure and therefore do not make it possible to boost the evaporation of the biogas partially dissolved in the treated water and thus an optimum recovery of the biogas. In addition, such devices do not make it possible to obtain treated water having a maximised COD value, in particular associated to a reduced concentration of residual pollutants. Lastly, the treated water obtained by these devices has a level of floats that is still high, this treated water consequently not having a good quality that makes it possible to comply in particular with the required specifications.
Therefore, there is no satisfactory system making it possible to optimise mixing inside the reactor while enabling the facilitated extraction of biogas coming from the reactor.
The present invention aims to remedy all or part of these drawbacks.
To this end, according to a first aspect, the present invention envisages a purification device according to claim 1.
Thanks to these provisions, it is possible to create a first step of energy recovery in an urban wastewater (UWW) cleaning chain.
This makes it possible to clarify the UWW and transform the pollution into methane. These provisions offer three advantages, in particular:
Mixing inside the reactor improves as biogas extraction increases. If the mixing is optimum, the better the performance of the reactor in terms of converting the pollution into methane. In addition, the presence of a buffer column makes it possible, in particular, to optimise the operation of the digester. In effect, such a buffer column makes it possible to start a flow of wastewater towards the digester based on hydrodynamic constraints imposed by the digestion process. In particular, such a flow, or hydraulic circulation, is triggered according to the principle of communicating vessels when a vacuum is applied in the upper part of the digester. In addition, the buffer column, with the flow it triggers during the implementation of the digestion process performed by the digester, contributes to the application of a sufficient vacuum, and in particular a negative pressure, in the upper part of the digester and above the supernatant water.
In some embodiments, the digester comprises a bottom outlet for treated water, connected to the weir and positioned at a third height starting from the lower part, said third height being lower than the first height of the weir. When the treated water outlet is placed at the bottom of the digester, the treated water can be evacuated without air suction by this treated water outlet. In this case, such an evacuation is performed despite a negative pressure, or vacuum, applied in the upper part of the digester.
In some embodiments, the third height of the treated water outlet is lower than or equal to the level of treated water in the buffer column. Thanks to these provisions, the evacuation of treated water is easily performed, for example, in the lower part of the digester. In particular, such an evacuation is optimised by the hydraulic pressures generated in the device.
In some embodiments, the treated water outlet also comprises an extraction valve for extracting water from the weir. Thanks to these provisions, the evacuation of treated water is performed during the opening of the extraction valve. In this way, an adjustable flow rate of treated water is generated, in particular based on wastewater treatment constraints, such as the volume of this wastewater to be treated or the volume of treated water to be generated.
In some embodiments, the device that is the subject of the invention comprises a sensor for detecting the presence of water in the weir, at least the extraction valve being activated based on the detected presence of water. These embodiments enable a transition from one configuration, in which the pressure of the digester evolves towards a nominal pressure, to a configuration in which the device is in nominal operation. These embodiments allow the valve to be kept in the closed position when water has not been detected at this level, to prevent air from entering the reactor.
In some embodiments, the device that is the subject of the invention also comprises:
These embodiments make it possible to boost sludge mixing in the lower part of the digester.
In some embodiments, the device that is the subject of the invention also comprises a valve for activating the recirculation pipe. These embodiments, when the device comprises a valve for extracting water from the weir, allow the device to have distinct modes of operation based on the opening and closing of valves.
In some embodiments, the device that is the subject of the invention has a mode of operation in which the valve for activating the recirculation pipe and the valve for extracting water are closed. These embodiments enable a reduction in the pressure of the reactor suitable for preparing the device for nominal operation, making possible a maximum extraction of the biogas and water.
In some embodiments, the device that is the subject of the invention also comprises:
These embodiments enable the device to optimally achieve an operating working pressure.
In some embodiments, the digester also comprises an open tank, the riser column for raising water by suction of biogas of water comprising, in the upper part of the digester, an outlet for the raised biogas, said outlet being connected to the open tank and positioned at a fourth height lower than the first height. These embodiments make it possible to separate the sludge, the water and the biogas obtained in the digester.
In some embodiments, the riser column for raising biogas is at least partially surrounded by a settling zone.
In some embodiments, a fifth height defining the limit of the settling zone is lower than the first height of the weir. Thanks to this disposition, the settling zone makes it possible to limit the amount of particles in suspension, in particular floats, reaching the first height of the weir for treated water. The COD of the treated water is therefore lowered. As a result, the quality of the treated water is improved.
In some embodiments, the settling zone comprises a lamella settler. Thanks to this disposition, the settling zone makes it possible to further limit the amount of pollutant particles in suspension in the treated water. The settling is therefore optimised, thus making it possible to improve the quality of the treated water.
In some embodiments, the digester also comprises a gas-liquid-solid separator communicating with a lower end of the water riser column, the separator being substantially conical in revolution and with a top arranged inside the water riser column, and being configured to carry at least biogas in the water riser column. Thanks to these provisions, the separator makes it possible to direct the biogas and water to the water riser column while restricting the raising of solid particles.
In some embodiments, the conical separator has a base, and the digester also comprises a deflector:
Thanks to these provisions, the deflector enables biogas to be accumulated over a lower part and then the biogas flow to be directed towards the separator. In particular, the excess biogas not retained by the lower part rises towards the base of the separator, being guided by the end of the deflector, and is then carried towards the water riser column to achieve the raising of water drawn by the gas, also called the “gas lift” effect. The cooperation between the separator and the deflector therefore enables an optimised gas lift effect. As a result, the homogenisation of the mixture in the digester is optimum.
In some embodiments, the device that is the subject of the invention comprises:
These embodiments make it possible to automatically regulate the amount of wastewater in the buffer column. In such embodiments, a “PID” type of regulation can be utilised, in which the operating frequency of the pump is servoed to the liquid level of the buffer column.
In some embodiments, the device that is the subject of the invention comprises, downstream from the vacuum pump:
These embodiments enable the biogas to be treated in order to comply with recovery specifications set.
In some embodiments, the digester comprises:
These embodiments make it possible to automatically regulate the operating temperature of the digester.
According to a second aspect, the present invention envisions a method for purifying wastewater according to claim 19. As the particular features, advantages and aims of this method correspond to those of the device that is the subject of the invention, they are not repeated here.
Other advantages, aims and particular features of the invention will become apparent from the non-limiting description that follows of at least one particular embodiment of the device and method that are the subjects of the invention, with reference to drawings included in an appendix, wherein:
The present description is given in a non-limiting way, in which each characteristic of an embodiment can be combined with any other characteristic of any other embodiment in an advantageous way. Hereafter, “valve” refers to any known type of valve suitable for the use indicated contextually. Such a valve is, for example, a motorised valve. Note that the figures are not to scale.
The device 100 also comprises:
Anaerobic digester 105 refers to a tank used in the methanation process that produces biogas by means of a method for the anaerobic digestion of organic matter from various sources. Here, preferably, the organic matter comes from urban wastewater. The digester 105 is referred to as a “vacuum” digester, i.e. its preferred operating conditions are at a pressure generally below atmospheric pressure. The digester 105 is formally divided into two parts: a lower part 110 and an upper part 120, whose relative proportions can vary. The term “lower part 110” refers generally to the part of the digester 105 generally close to the base, i.e. to the ground, when the digester 105 is in operating condition. The lower part 110 therefore refers to the area that receives the gravity flows occurring in the digester 105. This digester 105 can utilise different internal devices able to interact with the flow streams of biogas, water and/or organic waste. For example, as shown in
In the lower part 110, the device 105 comprises an inlet 115 for wastewater. Such an inlet 115 corresponds to an opening, preferably connected to a tube, and optionally associated to a means for controlling a valve governing the opening and/or closing of the inlet. Such a valve can be actuated manually or automatically, using a programmable logic controller, according to preferred operating values of the digester 105. The lower part 110 of the digester 105 also comprises a bed of microorganisms, such as bacteria, selected for their ability to digest the UWW and produce biogas. In particular, such a digestion by the bacteria is carried out without oxygen and therefore corresponds to an anaerobic digestion. Such microorganisms can be supported by granules 121, for example bacterial granules referred to as “UASB”, arranged at the bottom of the tank (i.e. in the lower part 110) of the digester 105.
It is noted that a bacteria bed in a digester 105 is generated using a method known to the person skilled in the art, for example by introducing bacteria into the digester 105, also called digester inoculation, or by progressively introducing wastewater to be treated that contains bacteria which will, over time, create UASB granules. The wastewater therefore exhibits an upward flow in the transverse surface of the reactor, except in the column 141 where the water descends.
The lower part 110 and upper part 120 of the digester 105 are connected by a riser column 140 for raising water by suction of the biogas from the lower part 110 to the upper part 120. The lower part 110 and upper part 120 of the digester 105 are also connected by a downer column 141 for bringing down untreated wastewater from the upper part to the lower part. The columns 140 for raising water by suction of the biogas and 141 for bringing down water are, for example, coaxial.
In some variants, the digester 105 comprises a means for collecting wastewater raised by the effect of the suction of the biogas through the riser column 140. This wastewater is directed towards the downer column 141 for bringing down untreated wastewater to the lower part of the digester 105.
During the utilisation of the vacuum pump 145, the water in the lower part 110 is sucked up to the upper part of the digester 105. The riser 140 and downer 141 columns increase the mixing within the reactor by channelling the biogas towards the centre.
In the upper part 120, the digester 105 mainly comprises outlets for the different species of interest generated. Firstly, an outlet 130 for water supplied by a weir 125 whose function consists of allowing water to exit that exceeds a predefined height, called the first height 131, a function of the vacuum level, which is set as a function of the total height of the reactor, chosen to correspond to the height reached by the water once treated by the microorganisms.
“Outlet” of the weir 125 refers here, for example, to a wall for retaining liquids, wherein the crossing of the wall constitutes the outlet 130, in a similar way to a hydraulic weir.
“Height” refers to a value of a physical quantity representative of the distance between the base and a given point of the digester 105 along a gravitational vertical axis. In addition, the height can also be measured from the highest point of the tank or any other reference point located at a higher elevation than the point of interest whose height is measured when the digester 105 is in operating conditions.
In some preferred variants, the weir 125 is connected to a pipe for the vertical flow of treated water towards the lower part 110, gravitationally or by means of a pump, so as to counteract the effect of the negative pressure in the upper part 120 of the digester 105 when the water exits from the digester 105.
In some embodiments, such as those shown in
In some embodiments, such as the one shown in
Note that the device 100 shown in
In some embodiments, such as those shown in
In other words, when:
the water extraction valve 165 is open for the evacuation of the treated water.
Preferably, such an opening is carried out following a command sent by a sensor for detecting the presence of water in the weir 125. At that time, in the pipe 130, the value of hydrostatic pressure will be greater than the value of the vacuum and the flow of treated water will take place with no suction of outside air. In particular, when the outlet 130 for water from the weir 125, such as a pipe 130 connected to the weir 125, is filled with water, such filling corresponds to a liquid seal and prevents the suction of air that can occur when the water extraction valve 165 is open.
In some embodiments, such as that shown in
The sensor 170 is, for example, a capacitive sensor activated by the presence of water in the weir 125. The detection of water can correspond to a change of operational regime of the device 100, from a start-up phase to a phase of nominal operation. These changes of regime are described below.
Secondly, the digester 105 comprises a biogas outlet 135, comprising for example a biogas extraction tube, an opening of which is positioned at the second height. The outlet 135 is configured to suck up the biogas located above the purified water, and water sucked up by the downer column 141.
The movement of the water in the digester 105 is induced by the action of the vacuum pump 145 configured to generate a low pressure, at the location of the biogas outlet 135, below the pressure of the UWW on input to the digester 105.
In some embodiments, such as that shown in
The biogas inlet 150 is, for example, structurally similar to the wastewater inlet 115 in the different variants presented. The recirculation pipe 155 is, for example, a tube configured to connect the downstream of the vacuum pump 145 and the biogas inlet 150 in the lower part 110 of the digester 105. This recirculation pipe 155 can be associated to a set of valves whose selective activation makes it possible to force all or part of the biogas flow towards said recirculation pipe 155. In a minimalist variant, the vacuum pump 145 is connected to a biogas evacuation pipe and the recirculation pipe 155 is a branch on this evacuation pipe, the branch and evacuation pipe each being associated to a separate valve, the opposing activation of which results in the biogas passing through either the evacuation pipe or the recirculation pipe 155.
In some variants, the recirculation pipe 155 is associated to a gas pump and/or a check valve upstream from the inlet 155.
In some embodiments, such as that shown in
In some embodiments, such as that shown in
Such a mode of operation corresponds to a start-up or initialisation phase of the device 100 in which the vacuum is created in the digester 105. For the vacuum to be created, the outlets and inlets of the digester 105 must be in the closed position. As the vacuum is created, the water raises along the digester 105, from the lower part 110 to the upper part 120, until the treated water enters the weir 125. When the water enters the weir 125, a valve 165 is opened, allowing the treated water to exit. Other operating regimes are described below.
In some embodiments, such as that shown in
The pressure sensor 175 can be any type known to the person skilled in the art that matches the operating conditions of the device 100, in particular in terms of temperature, pressure or humidity. The PID, for “proportional-integral-derivative”, regulator 180 is a control system making it possible to improve the performance of a servocontrol, i.e. a closed-loop system or method. Here, the vacuum pump 145 is servoed to the pressure detected by the sensor 175.
In some embodiments, such as those shown in
In some embodiments, such as those shown in
This height differential is due to the level of gas retention in the zone above the riser column 140 for raising water by suction of the biogas.
This makes it possible to observe a double imbalance:
In other words, the column 140 enables water to be raised, through the gas lift effect, with the biogas, corresponding to a gas-liquid two-phase mixture. At the location of the outlet 185 for water from the column 140 inside the open tank 303, the two-phase mixture will settle:
In some embodiments, such as those shown in
In some embodiments, the settling zone 305 comprises a lamella settler. It is noted that a lamella settler has plates arranged in parallel in order to increase the settling surface area. The blades are arranged obliquely so as to guide the sliding of settled matter and floats towards the bottom of the lamella settler.
In some embodiments, such as those shown in
In some embodiments, such as those shown in
In these embodiments, a portion of the water initially present in the lower part of the digester 105 circulates towards the settling zone 305, passing between the space delimited by the base 310 of the separator 307 and the end of the deflector 308.
In some embodiments, such as that shown in
The main function of the buffer column 190 is to serve as a tank from which, by suction, water to be treated is sucked towards the digester 105. The inlet 195 is, for example, similar to the inlet 115 of the digester 105. The atmospheric air intake 200 is, for example, an opening to the environment outside the device 100, optionally able to be closed or not by means of a plug or valve. When the buffer column 190 is present, the wastewater inlet 115 can be reduced to a pipe, not being blocked or able to be closed, between the digester 105 and the buffer column 190.
In some embodiments, such as that shown in
The sensor 205 is, for example, an ultrasound sensor. When the detected fill level is below a predefined limit value, or a dynamic value calculated based on the flow rate of the vacuum pump 145 or the pressure detected inside the digester 105, the pump 210 is, for example, configured to inject wastewater into the buffer column 190. Conversely, when the detected fill level is above a predefined limit value, or a dynamic value calculated based on the flow rate of the vacuum pump 145 or the pressure detected inside the digester 105, the pump 210 is, for example, configured to stop injecting wastewater.
In some embodiments, such as that shown in
The dehumidifier 215 is, for example, a condenser of residual water vapour present in the biogas. The adsorption means 220 is, for example, an adsorption column with activated carbon or another porous adsorption medium such as silica or zeolites. In some variants, if the biogas quality required on output must be higher, the adsorption means 220 can also be more complex, for example “PSA” (for “Pressure Swing Adsorption”) type. The storage 225 can be a transport channel or a tank, for example.
Preferably, the device 100 also comprises:
In some embodiments, the digester 105 comprises:
Two operating regimes are presented below. In the two examples, the startup condition is achieved when the liquid level is at a height of one metre in the buffer column and in the digester 105.
In the first operating regime, referred to as “regulated operation”, the steps of the following process are carried out:
All or part of this example can be implemented in an advantageous embodiment benefiting, individually or synergistically, from the unitary advantages conferred by these parts.
In some embodiments, the method 400 also comprises, before the outflow step 420, a settling step for separating the water and solid matter. These steps have been described, mutatis mutandis, with reference to
Preferably, the means and the elements of the device 100 are configured to implement the steps of the method 400 and their embodiments as described above, and the method 400 and its different embodiments can be implemented by the means and elements of the device 100.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2108719 | Aug 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/072874 | 8/16/2022 | WO |
