System and method for purging nitrous oxide

Abstract

A system and method for removing N2O in a wastewater treatment process is provided. The method includes operating a membrane aerated bio-membrane reactor (MABR) in a high ammonia concentration activated sludge process and collecting N2O in an inner cavity of the MABR membrane. N2O is driven into the inner cavity of the membrane by pressure regulation. The pressure regulation includes a total pressure of the process gas and a partial pressure regulation of components of the process gas. The collected N2O may be recycled back to the wastewater treatment system or further treated downstream of the MABR.

Description

[0001] Related applications This application claims the benefit of Italian Patent Application No. 102023000003423, filed on February 27, 2023, which is incorporated herein by reference. Technical Field

[0002] The present invention relates to wastewater treatment processes and systems for reducing or eliminating nitrous oxide emissions from wastewater treatment systems. Background Art

[0003] As part of the Paris Climate Agreement and the resulting push toward economic decarbonization, many countries have committed to exploring ways to achieve utility-wide carbon neutrality targets. In particular, there are signs that nitrous oxide (N2O) may become a regulated component of water recycling facilities in the future. In Denmark, for example, the government has announced plans to introduce limits on N2O emissions from treatment facilities with a capacity of 30,000 PE or more.

[0004] International patent application WO 2022 / 184829 discloses a method for reducing or minimizing N2O emissions in the exhaust gas from a membrane aerated biofilm reactor (MABR) by monitoring one or more parameters of the wastewater and / or exhaust gas and / or feed gas and adjusting the supply of feed gas to the membrane based on the one or more parameters to minimize or eliminate N2O in the exhaust gas. Summary of the Invention

[0005] In one aspect of the present invention, a membrane aerated biofilm reactor is used to intentionally remove N2O produced during a wastewater treatment process. CO2 can also be removed during this process. For example, CO2 can be removed incidentally along with the N2O. Intentional removal can be achieved by driving the N2O into the lumen of the MABR. Once inside, the N2O can be sent to secondary processes for emission reduction, nitrogen recovery, enhanced liquid line treatment, energy generation, or to reduce the carbon footprint of the entire wastewater treatment facility.

[0006] The method according to the present disclosure includes operating a MABR in an anoxic zone of a wastewater treatment facility such that N2O produced in the biofilm is preferentially driven into the lumen of the membrane and concentrated in the exhaust gas. By adjusting the partial pressures of components in the process gas input to the membrane, the N2O can be preferentially driven into the lumen of the membrane, for example, to a relatively greater extent than the N2O driven into the bulk liquid. The exhaust gas containing N2O is not returned to the bulk liquid, for example as a mixing or scrubbing gas. The exhaust gas can be reused to another point in the wastewater treatment line, or to another process within the same or another treatment facility, upstream or downstream of the MABR, washed by other means, or otherwise treated to use or reduce or eliminate the N2O in the gas.

[0007] The system according to the present disclosure includes an MABR having a gas-permeable membrane capable of supporting a counter-diffusion biofilm. The MABR can be equipped with a vent valve for controlling the pressure of the process feed gas or the partial pressure of the components of the process gas fed into the lumen of the membrane. The partial pressure of the components of the gas in the lumen can be adjusted to promote or encourage the diffusion of N2O into the interior of the lumen, as opposed to diffusion into the bulk liquid. In some examples, some N2O can diffuse into the bulk liquid, as long as the denitrification capacity in the bulk liquid is sufficient to prevent the N2O from being emitted as waste gas from the bulk liquid. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 Provide a schematic diagram of a counter-diffusion biofilm.

[0009] Figure 2 Shown are example membranes for use in membrane aerated biofilm reactors. DETAILED DESCRIPTION

[0010] In one aspect, the present invention provides a system and method for reducing or eliminating NO emissions from a wastewater treatment facility, for example, to help reduce the overall carbon footprint for wastewater treatment. The methods and systems disclosed herein include removing NO from the lumen of membranes in a membrane aerated biofilm reactor (MABR) and, in another aspect, reusing the NO in upstream or downstream processes of the wastewater treatment facility, or treating the NO downstream of a MABR.

[0011] In one example, the method includes operating a membrane aerated biofilm reactor (MABR) in an unaerated zone (e.g., an anaerobic zone and preferably an anoxic zone) of a wastewater treatment system (WWTS). The unaerated zone can be, for example, an anoxic portion of an activated sludge process having an elevated ammonia concentration in the bulk liquid. In one example, the elevated ammonia concentration can be 7 mg-N / L or higher. The increased ammonia concentration can provide higher nitrification activity in the MABR and can produce more NO. As further described herein, the increased NO concentration can be removed for reuse or additional processing.

[0012] In one example, the systems and methods disclosed herein can be used for high-intensity nitrogen removal, such as in mainstream or sidestream wastewater treatment. Minimizing the airflow for post-treatment in the sidestream may be easier to do than in the mainstream environment. For example, the oxygen demand for ammonia oxidation can be reduced by about half using an anaerobic ammonium oxidation pathway. As a result, the sidestream ratio of oxygen demand to nitrogen load can be much lower than in the mainstream. MABR can further promote post-treatment of N2O emissions by operating at very high oxygen transfer efficiencies, even exceeding 95%, which can result in less airflow required to transfer the same amount of oxygen and can minimize the amount of post-treatment gas required to remove N2O. For example, the systems and methods disclosed herein can be used to treat centrifuge liquor from digested sludge (i.e., after aerobic or anaerobic digested sludge dewatering), or any high ammonia waste stream, such as food and industrial waste with low carbon. For example, MABR can be operated for a short-cut nitrogen removal process for treating wastewater with a low carbon-nitrogen ratio.

[0013] Membrane aerated biofilm reactors include counter-diffusion biofilms. Counter-diffusion biofilms can, for example, be grown on a permeable membrane, where the electron acceptor and electron donor substrates are supplied from opposite sides of the biofilm (i.e., from the bulk liquid and from the lumen of the membrane). In contrast, co-diffusional biofilms grow on an impermeable matrix where both the electron donor and acceptor substrates are supplied by the bulk liquid. Figure 1 A representative counter-diffusion biofilm 100 is provided, comprising an aerobic / nitrifying layer 102 and an anoxic / denitrifying layer 104, showing substrate movement into and out of the biofilm. Figure 1 Also shown is the distribution of N2O produced in the biofilm, for example, N2O can diffuse into the membrane lumen 106 containing the exhaust gas or diffuse into the bulk liquid 108. Figure 1 In the example of FIG, oxygen diffuses from the membrane lumen 106 through the membrane wall 110 to the aerobic / nitrifying layer 102 of the biofilm. The N2O produced in the nitrification process flows from the aerobic layer 102 back to the membrane lumen 106 and flows to the bulk liquid 108 (which is also anoxic and provides denitrification). NH4 can move from the bulk liquid to the aerobic biofilm layer, while COD moves to the anoxic biofilm layer. NO from the aerobic layer x Move to the anoxic biofilm layer, and N2 from the anoxic biofilm layer moves to the bulk liquid.

[0014] Figure 2 An example membrane 202 is shown that can be used in a MABR. Figure 2As shown, process feed air can be input into the lumen 204 of the membrane at a first end, and exhaust air can be exhausted from a second end. The second end of the membrane can, for example, be at the end of the membrane opposite the input end of the membrane. The process feed gas can include oxygen, nitrogen, and / or other components. In one example, the process feed gas can be air. The MABR containing the membrane is preferably located in the anoxic zone of the reactor, but can also be located in the anaerobic zone or other unaerated zone. The oxygen-depleted zone drives oxygen from the lumen of the membrane into the bulk liquid, where oxygen is unavailable. This allows a biofilm to form at the interface between the outer membrane wall and the bulk liquid as oxygen flows from the high oxygen concentration region in the lumen to the low oxygen concentration region in the bulk liquid. In other words, oxygen from the process gas can diffuse through the permeable membrane wall 206 toward the counter-diffusion biofilm 208 growing on the outside of the membrane wall. The biofilm can comprise several layers, including an aerobic layer closest to the membrane wall and an anoxic layer closest to the bulk liquid. This type of biofilm can promote both nitrification and denitrification pathways in the aerobic and anoxic layers, respectively. N2O is a byproduct of secondary wastewater treatment that can be formed during nitrification and denitrification pathways within biofilms. N2O can diffuse out of the biofilm, either toward the lumen or toward the bulk liquid. In another example, other greenhouse gases, such as CO2, can be produced and diffuse out of the biofilm in a similar manner to N2O.

[0015] Driving NO from the biofilm into the lumen of the membrane can include using a process feed gas with little or no NO, such as using a process feed gas, such as ambient air pressurized by a blower without NO, or another gas with less than 25 ppm NO. In this way, the lumen of the membrane can operate at a lower NO concentration than the biofilm, so that NO moves from a high concentration where it is produced in the biofilm to a low concentration area of ​​NO in the lumen of the membrane. For example, during operation, the lumen can have a feed gas or be fed with a feed gas with a lower concentration of NO than the biofilm. Some NO can be allowed to flow from the biofilm to the bulk liquid, where it can be denitrified if there is enough readily biodegradable carbon. In another example, some NO that diffuses into the bulk liquid can also be reduced to N in the biofilm (e.g., in the anoxic outer layer of the biofilm). If there is not enough denitrification capacity, NO can be stripped to the atmosphere by aeration of the bulk liquid, which is undesirable. N2O and / or other components (e.g., CO2) can be driven into the lumen by selecting a feed or input gas (i.e., process gas) that is low in or devoid of such components. For example, the partial pressure of N2O and / or CO2 in the process gas can be selected or adjusted to be less than the partial pressure of N2O and / or CO2 in the bulk liquid. In one example, the process feed gas is selected such that N2O and / or CO2 are absent.

[0016] In particular, as the pressure difference between the biofilm and the lumen membrane approaches equilibrium, some greenhouse gases (such as N2O or CO2) can flow into the bulk liquid. Some N2O or CO2 flowing into the bulk liquid is acceptable, as long as there is sufficient denitrification capacity to remove N2O from the bulk liquid before it is discharged into the environment. The N2O partial pressure at the second end of the lumen (i.e., the end where the waste gas is discharged) can be adjusted by reducing the partial pressure of O2 in the process feed gas. The oxygen partial pressure can be reduced to just enough to balance the limitations of nitrification and denitrification reactions in the biofilm to produce less N2O while maintaining an effective MABR system. In one example, the partial pressure of N2O is minimal or non-existent at the input end of the lumen and is highest at the discharge end of the lumen, for example to the equilibrium point between the lumen and the biofilm. In this way, for greenhouse gases, the waste gas can be discharged from the lumen at its maximum capacity while limiting the amount of N2O that diffuses back into the bulk liquid.

[0017] In one example, as NO diffuses from the biofilm into the lumen of the membrane, the concentration of NO can vary along the length of the membrane. For example, a higher concentration of NO can be present at the second end closer to the lumen, closest to where the exhaust is discharged. In one example, a process according to the present disclosure can include monitoring the components of the exhaust (e.g., NO, CO, and O) and, based on these parameters, controlling the process by adjusting the pressure of the process feed gas or the back pressure entering the lumen.

[0018] In one example, the pressure of the process feed gas entering the lumen of the membrane can be adjusted. Increasing the pressure of the feed gas can be used to reduce the diffusion of greenhouse gases into the lumen of the membrane, while reducing the pressure can increase the diffusion of greenhouse gases into the lumen of the membrane.

[0019] In one aspect, the total flow rate of the input process feed gas can be reduced to minimize the amount of exhaust air that may need to be redirected to other processes, or the change in the total flow rate over time. In this case, the same kg of N2O or kg of CO2 is produced in the system and can be driven into the inner chamber, but the amount of gas that needs to be processed is lower, with lower O2, which can result in easier to process components. Therefore, reducing the input air can reduce the amount of exhaust gas that needs to be further processed and redirected from the inner chamber.

[0020] In another aspect, NO in the flue gas can be measured or monitored to determine what, if any, adjustments need to be made to the process feed gas. For example, if NO in the flue gas increases above an upper threshold, the process feed gas flow rate can be reduced. The reduced flow rate of the process feed gas into the lumen of the membrane reduces the availability of O in the biofilm and increases the chances of NO being reduced to N by the denitrifier in the biofilm. When NO measurement or monitoring indicates a decrease in NO concentration in the flue gas, e.g., below a lower threshold, the process feed gas flow rate can be increased to increase the O concentration in the biofilm, promoting further nitrification.

[0021] The process feed gas that does not diffuse through the membrane wall into the bulk liquid and any components that diffuse from the biofilm into the lumen of the membrane can be concentrated in the lumen as waste gas. The waste gas will have a higher NO concentration than the process feed gas. For example, the waste gas can contain NO and other components that diffuse from the bulk liquid and / or biofilm into the lumen of the membrane. The waste gas can be sent for additional treatment or redirected to another part of the wastewater treatment system. In one example, redirecting the waste gas can be used to balance the denitrification capacity in the bulk liquid surrounding the MABR. The NO in the waste gas can be used as a means to determine the ability of the anoxic zone to denitrify any NO produced by the MABR and diffused into the bulk liquid. For example, the denitrification capacity in the anoxic zone of the bulk liquid can be used to determine the NO denitrification capacity. The NO in the waste gas can indicate the driving force for NO to enter the lumen of the membrane, where the higher the driving force for NO to enter the lumen, the higher the NO concentration in the bulk liquid. Higher concentrations of NO in the bulk liquid may be associated with nitrate and nitrite returning to the anoxic zone, for example, due to excess nitrate and nitrite. By controlling the return of nitrate and nitrite to the anoxic zone, more denitrification capacity can be used to remove NO produced by the MABR in the bulk liquid. In one example, a target exhaust NO band (e.g., a defined upper limit) can be selected such that if NO exceeds the upper limit, the MABR system can be controlled to achieve one or more of the following: i) a lower percentage of internal mixed liquor recirculation, ii) addition or adjustment of an intermittent aeration schedule in the bulk liquid, and iii) an increased percentage of carbon added to the bulk liquid.

[0022] In one example, at least a portion of the NO-rich exhaust gas can be directed to downstream treatment. Downstream treatment can include a wet scrubber, which can be used, for example, with raw sewage or primary effluent as the liquid source, or with supplemental carbon, such as in the form of acetate or methanol, where carbon is not available, such as in some food waste applications. Other downstream treatments can include NO abatement or reduction treatments, catalysis, NO absorption, and NO adsorption.

[0023] In another example, the N2O-rich exhaust gas can be used to enhance energy production through combustion. For example, the fuel-air ratio can be adjusted to minimize nitrogen oxide emissions associated with N2O from the wastewater treatment process. In another example, the exhaust gas can be treated by adsorption, such as by treating the N2O using titanium-coated carbon and UV light (e.g., from the sun), or in another example, using a catalyst and heating the exhaust gas to 200+°C. In another example, the exhaust gas can be diffused into a denitrification submerged biological process, such as a fixed bed or denitrification filter.

[0024] On the other hand, the N2O-rich waste gas can be used to control the bulk liquid in the anoxic zone, aerobic zone and / or other zones in the wastewater treatment system. For example, the waste gas can be used to control the return activated sludge recirculation rate or the internal mixed liquor recirculation rate. The N2O-rich waste gas can be used to control the oxidation-reduction potential (ORP) in the anoxic zone of the wastewater treatment system or to control intermittent aeration in the aerobic zone. In another example, the N2O-rich waste gas can be used to control the target effluent nitrate / nitrite at the end of the aerobic zone or the plant effluent, for example in an attempt to limit the amount of nitrogen oxides returned in the return activated sludge. Balancing the potential of N2O in the waste gas may also be beneficial to maintaining the denitrifying capacity of the bulk liquid, for example to treat any N2O that flows from the biofilm into the bulk liquid (rather than into the lumen of the membrane). For example, by controlling the mixed liquor recirculation, ORP and dissolved oxygen in the anoxic zone and / or aerobic zone of the wastewater treatment plant, the denitrifying capacity in the bulk liquid can be maintained.

[0025] In one aspect, the waste gas is separated from the flushing or mixing gas dedicated to the MABR operation (e.g., the flushing or mixing gas directed to the bulk liquid). In this way, the N2O or CO2 or other greenhouse gases that are driven into the lumen of the membrane are not returned to the liquid phase of the system. The waste gas rich in undesirable greenhouse gases can be removed from the system and / or treated or reused to balance the potential of N2O, for example as described herein. The mixing or flushing gas can be received from an alternative source. In this way, the denitrification capacity of the bulk liquid only needs to consider the N2O that diffuses from the biofilm into the bulk liquid, rather than the N2O that is reintroduced via the bulk liquid aeration (i.e., mixing or flushing) system.

[0026] In another aspect, the intensity or frequency of the flushing or mixing gas in the bulk liquid can be controlled to manipulate biofilm thickness. Thicker biofilms can provide larger anoxic and aerobic layers, which can be used to directly process NO in the biofilm, which can help reduce the total amount of NO flowing out of the biofilm (in either direction). In another example, the frequency or intensity of the flushing or mixing gas can be adjusted to promote substrate renewal in the liquid phase to promote further growth of the biofilm.

[0027] A method for reducing or eliminating NO emissions from a wastewater treatment system according to aspects disclosed herein is provided. For example, the method can include operating an MABR at a location within the wastewater treatment facility where ammonia concentration is high or highest, such as in an anoxic zone upstream of an aerobic zone or in a side stream treatment of a centrifuge from a dewatered digester digestate. The MABR membrane comprises a hollow internal lumen having an inlet for introducing a process gas as a feed gas into the system and an outlet for discharging the exhaust gas. The feed gas can provide, for example, oxygen, which diffuses through the walls of the lumen to opposite sides of the membrane wall where a biofilm is formed. The biofilm can contain both anoxic and aerobic zones that promote both nitrification and denitrification. Both nitrification and denitrification pathways produce NO. The higher the nitrogen or ammonia content in the bulk liquid surrounding the MABR, the more NO can be produced in the biofilm. The feed gas is low in NO and preferably completely depleted in NO. The NO can be driven by partial pressure regulation into the lumen of the membrane. Some NO can flow into the bulk liquid. As previously described herein, NO driven into the exhaust gas can be reused in the wastewater treatment system to maintain or balance sufficient denitrification capacity in the bulk liquid. This recirculation can be used to treat any NO flowing into the bulk liquid before it diffuses into the atmosphere (e.g., in the exhaust gas from bulk aeration). The NO-rich exhaust gas can alternatively or additionally be treated to reduce or eliminate NO, for example, by using a wet scrubber, adsorption, or other submerged biological processes.

[0028] In another aspect of the present invention, a system for removing N2O from a wastewater treatment plant is provided. The system comprises a membrane aerated biofilm reactor adapted to grow a counter-diffusion biofilm that produces N2O in a nitrification and / or denitrification pathway. The membrane used in the MABR comprises an inner cavity and a permeable membrane wall, for example the membrane may be a hollow fiber membrane. The inner cavity may be used to concentrate N2O that diffuses into the inner cavity from the biofilm grown on the outer membrane wall. The inner cavity may comprise a process feed gas input and an exhaust gas output. The exhaust gas output may be connected to a downstream N2O treatment step or a recycle stream to redirect the exhaust gas to another part of the wastewater treatment system.

[0029] In one example, the MABR includes a vent valve for controlling the partial pressure of components of the process feed gas introduced into the lumen of the membrane. The partial pressure of the components can be adjusted to promote the diffusion of NO from the high concentration of NO in the biofilm to the low partial pressure environment in the lumen of the membrane. For example, the NO partial pressure in the lumen can be adjusted so as to maintain or increase the driving pressure of NO from the bulk liquid. In one example, the process gas introduced into the lumen is completely depleted of NO. In one example, the lumen of the membrane can include a partial pressure gradient, wherein the partial pressure of NO is lowest at the input end of the lumen and highest at the output end.

[0030] Downstream N2O treatment can include treatment with a wet scrubber, for example, using raw sewage or primary effluent as the liquid source, or with supplemental carbon treatment. In another example, treatment of the waste gas containing N2O can be achieved by adsorption or by diffusion of the waste gas into a denitrifying submerged biological process. Analysis of the waste gas rich in N2O can include a recirculation control system that manipulates the performance of the activated sludge process to have lower N2O concentrations in the bulk liquor. Recirculation or reuse of the waste gas rich in N2O can include a recirculation conduit, for example to allow the waste gas to be used to i) control the return activated sludge rate or internal mixed liquor recirculation rate, ii) control ORP in the anoxic or aerobic zone, iii) control aeration in the aerobic zone and / or iv) control target effluent nitrate / nitrite at the end of the aerobic zone or plant effluent to limit the amount of nitrogen oxides returned in the return activated sludge.

[0031] According to the above disclosure, a system and method are provided for driving NO produced in a biofilm into the lumen of an MABR to facilitate downstream processing. Furthermore, as disclosed herein, the system and method further provide for the reuse of NO in various processes. Thus, the invention disclosed herein can be beneficial in limiting greenhouse gas emissions from the wastewater industry while also promoting more efficient processes within the wastewater industry.

Claims

1. A method for treating wastewater, the method comprising: operating a membrane aerated biofilm reactor (MABR) in an unaerated zone of a wastewater treatment system (WWTS), comprising introducing a process feed gas into a first end of an inner lumen of a membrane of the MABR and discharging a waste gas from a second end; producing N2O in a biofilm grown on a membrane of the MABR; driving N2O into the lumen of the membrane; The exhaust gas is exhausted from the lumen of the membrane, wherein the exhaust gas has a higher concentration of N2O than the process feed gas.

2. The method of claim 1, further comprising generating and driving CO2 in the same manner as N2O.

3. The method of claim 1 or 2, wherein driving N2O and / or CO2 into the lumen of the membrane comprises one or more of: i) adjusting the partial pressures of components of the process feed gas and ii) adjusting the pressure of the process feed gas introduced into the interior of the lumen of the membrane.

4. The method of claim 3, wherein adjusting the partial pressures of the components of the process feed gas comprises reducing the partial pressure of N2O and / or CO2 in the process feed gas to less than the partial pressure of N2O and / or CO2 in the bulk liquid surrounding the MABR.

5. The method of any one of claims 1 to 4, further comprising monitoring the N2O concentration in the exhaust gas until an upper threshold is reached, and then reducing the O2 introduced into the lumen of the membrane by the process feed gas.

6. The method of claim 5, comprising increasing the O2 introduced into the lumen of the membrane by the process feed gas when the N2O concentration in the exhaust gas decreases below a lower threshold.

7. The method of any one of claims 1-6, further comprising regulating the frequency of the mixing or flushing gas directed to the bulk liquid surrounding the MABR.

8. The method of claim 7, wherein the exhaust gas is separated from the mixed or flush gas and has a higher concentration of N2O than the mixed or flush gas.

9. The method of any one of claims 1 to 8, wherein the unaerated zone is part of an activated sludge process having an elevated ammonia concentration in the bulk liquid.

10. The method according to claim 9, wherein the ammonia concentration is 7 mg-N / L or higher.

11. The method of any one of claims 1-10, further comprising reducing a flow rate of the process feed gas introduced into the inner cavity at the first end to reduce a flow rate of the exhaust gas output from the second end of the inner cavity.

12. The method according to any one of claims 1 to 11, wherein the non-aerated zone is an anoxic zone.

13. The method of any one of claims 1 to 12, further comprising directing at least a portion of the flue gas containing N2O to downstream treatment, or recycling to another portion of the WWTS, wherein the downstream treatment or recycling comprises any one or more of the following: a) N2O emission reduction treatment; b) treatment with a wet scrubber using either i) raw sewage or primary effluent as the liquid source, or ii) supplemental carbon; c) treating N2O by catalysis; d) treating said exhaust gas containing N2O by adsorption; e) diffusing the waste gas into a denitrification submerged biological process; and, f) using the exhaust gas to enhance energy production by combustion by adjusting the fuel-air ratio to minimize nitrogen oxide emissions associated with N2O from the WWTS.

14. The method according to any one of claims 1 to 12, comprising using N2O in the lumen of the membrane to control: a) Return activated sludge recirculation rate or internal mixed liquor recirculation rate; b) controlling the oxidation-reduction potential (ORP) in the unaerated zone; c) intermittent aeration in the aerobic zone; d) Target effluent nitrate / nitrite at the end of the aerobic zone or plant effluent to limit the amount of nitrogen oxides returned in the return activated sludge.

15. A system for removing N2O from a wastewater treatment system (WWTS), the system comprising, a membrane aerated biofilm reactor (MABR) located in the unaerated zone of an activated sludge process having high ammonia concentrations, the MABR comprising a membrane adapted to grow a biofilm that produces N2O when in use; The membrane further comprises an inner cavity adapted to collect N2O-rich exhaust gas, the inner cavity comprising an input end and an output end; and, A membrane off-gas output connects the output end of the lumen to (i) downstream N2O treatment or (ii) a recycle conduit to another portion of the WWTS.

16. The system of claim 15, wherein the biofilm is a counter-diffusion biofilm.

17. A system according to claim 15 or 16, comprising a vent valve for controlling the pressure of gas introduced at the input end of the lumen.

18. The system of claim 17, wherein the exhaust valve is adapted to control the partial pressure of components in the gas introduced at the input end of the lumen.

19. The system of any one of claims 15-19, wherein the unaerated zone is an anoxic zone.

20. The system of any one of claims 15-19, wherein the downstream N2O processing comprises: a) N2O emission reduction treatment; b) by catalytic treatment; c) treatment with a wet scrubber using either i) raw sewage or primary effluent as the liquid source, or ii) supplemental carbon; d) treating said exhaust gas containing N2O by adsorption; e) diffusing the waste gas into a denitrification submerged biological process.

21. The system of any one of claims 15 to 20, wherein the recirculation conduit to another portion of the WWTS is used to provide the exhaust gas to: a) Control the return activated sludge recirculation rate or the internal mixed liquor recirculation rate; b) controlling the oxidation-reduction potential (ORP) in the anoxic zone; c) Controlled intermittent aeration in aerobic zones; d) Control of target effluent nitrate / nitrite at the end of the aerobic zone or plant effluent to limit the amount of nitrogen oxides returned in the return activated sludge.

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