Gas treatment method

Abstract

Methods and systems for removing a portion of at least one contaminant gas from a gas stream are disclosed. The method comprises the steps of: a) applying a voltage across a pair of electrodes in contact with the working fluid to generate a plurality of ions; and b) reacting the at least one contaminant gas with a plurality of ions to convert it into one or more reaction products to isolate at least some of the contaminant gases from the first gas stream to produce a second gas stream wherein the pH of the working fluid is substantially maintained at a predetermined setpoint for improving the efficiency of the conversion. The invention also relates to a method for treating sewage or wastewater by adding one or more reaction products to: sewage, wastewater, sludge and / or anaerobic digesters.

Description

Technical Field

[0001] The present invention relates to a method for treating sewage or wastewater and to a method for purifying a gas stream obtained from said method. In particular, the present invention relates to electrochemically treating said gas stream (such as biogas) to remove at least a portion of the pollutant gases from the gas stream to produce a gas stream that is at least partially purified (i.e. enriched in at least one target gas). The present invention also relates to a method for producing a slurry of reaction products from the electrochemical treatment of a gas stream and the use of said slurry for treating sewage, wastewater and / or sludge derived therefrom. Background Art

[0002] The following discussion of the prior art is provided to place the present invention in an appropriate technical context and to enable its advantages to be more fully understood. However, it should be understood that any discussion of the prior art throughout the specification should not be considered as an explicit or implicit admission that such prior art is well known or forms part of the common general knowledge in the art.

[0003] Microbiological processes play an important role in wastewater management. In particular, they support the biological treatment of wastewater; the most cost-effective and environmentally friendly method of treating wastewater.

[0004] Typical biological wastewater treatment plants receive wastewater from a sewer network. The wastewater is first treated to remove large particles (by screening, or passing through a primary settler, or both). The resulting liquid then enters a bioreactor where bacteria mineralize any organic carbon (commonly referred to as biochemical oxygen demand or BOD) present in the wastewater to form CO2 and convert any ammonia present into nitrates, and in some cases further into nitrogen. Some bioreactors also implement biological phosphorus removal. This method results in the formation / growth of biomass. The biomass is then separated from the liquid, typically in a secondary settler.

[0005] Typically following the thickening process, part of the sludge separated from the treated water in the secondary settler is then treated in a bioreactor (typically an anaerobic digester or an aerobic digester), sometimes together with any primary sludge that may have resulted from the settling process in the primary settler.

[0006] The proliferation of biological wastewater treatment plants (WWTPs) has resulted in the generation of increasing amounts of wastewater sludge. Anaerobic digestion is an advantageous sludge destruction method for stabilizing wastewater sludge prior to its disposal. In addition to achieving sludge stabilization, anaerobic digestion also provides the opportunity to recover organic matter from the concentrated waste stream as a renewable bioenergy source in the form of biogas.

[0007] For example, in a typical anaerobic digester, the BOD of the sludge is converted into biogas containing methane, carbon dioxide and other gases. The products from the anaerobic digester also include solid and liquid streams that can be disposed of. Biogas produced from any source by anaerobic digestion (AD) typically contains about 60% (e.g., 50%-70%) methane (CH4), about 40% (e.g., 30%-50%) carbon dioxide (CO2) and trace amounts of other gases such as hydrogen sulfide (H2S) and ammonia (NH3).

[0008] The problem with biogas

[0009] The recovery of bioenergy from wastewater by the production of biogas is well known and widely practiced in the art, particularly in large wastewater treatment facilities. The biogas produced is currently used almost exclusively locally for heat and power generation, using combined heat and power (CHP) systems, to meet the heat and power requirements of the treatment process. Limited by the price of local energy supplies, the value derived from the biogas is relatively low and is often offset by the high capital and maintenance costs of the CHP system.

[0010] Alternative uses for the collected biogas are few and far between. The most promising and valuable potential uses of biogas are as a transportation fuel or for injection into the natural gas grid. However, one of the key issues in compressing and transporting raw biogas is the conversion of the CO2 component to dry ice at high pressure. This causes significant freezing problems at metering points and valves in the compression and storage stages of CNG. Therefore, in order to use biogas as a transportable fuel, the CO2 component must be substantially removed in a cost-effective manner. Similar considerations can also be made for other impurity gases in biogas, such as hydrogen sulfide (H2S) and ammonia (NH3).

[0011] In order to compress biogas for transport purposes in a conventional manner, the CO2 component, which constitutes approximately 30%-50% of the biogas, needs to be removed to less than 5%. The economic benefit of this removal is at least twofold, since the removal of non-combustible gases such as CO2 will also increase the calorific value of the biogas and expand its application range. In this regard, the use of biogas with a typical concentration of 20 MJ / m 3 -25MJ / m 3 - Raw biogas with a calorific value in the biogas range (containing about 50%-70% CH4) can be processed to a calorific value close to pure CH4, i.e. about 36 MJ / m 3 -CH4.

[0012] Several methods of removing CO2 from raw biogas (in order to upgrade the raw biogas) are known in the art. These methods generally involve physical and / or chemical separation of the CO2 component from the raw biogas stream. These methods are relatively complex and can be expensive and inconvenient.

[0013] For example, the most widely known biogas upgrading technology in the art is high-pressure water scrubbing, which selectively dissolves CO2 from a raw biogas stream by contacting it with a high-pressure water stream. This method relies on the relatively higher solubility of CO2 than CH4 in water in order to drive the selective dissolution of the former into the water stream. In some specific implementations of this technology, organic solvents (e.g., a mixture of methanol and dimethyl ether of polyethylene glycol) are substituted for water to absorb CO2, seeking to take advantage of their higher CO2 saturation concentration, resulting in a significantly higher ability to dissolve CO2 per unit volume of solvent.

[0014] Other physical upgrading techniques known in the art include pressure swing adsorption relying on the use of materials with a high affinity for CO2 molecules (such as carbon molecular sieves, activated carbon and zeolites), membrane separation using selective permeability to separate the CO2 retentate, and cryogenic separation of liquefied CO2.

[0015] Chemical methods for removing CO2 from feed gas include contacting the feed gas with various solvents, gases and solutions (including amines, hydroxides and alkaline salts) to chemically react with CO2 and remove CO2 from the gas feed. One such system involves dissolving gaseous CO2 into solution by gas-liquid contact and removing CO2 from solution by precipitating them out of solution in the form of insoluble carbonates. In this ion reaction-based scheme, obtaining and delivering cations that promote carbonate precipitation is a key factor in the economic feasibility and practicality of biogas upgrading systems.

[0016] In addition, more direct catalyst-driven hydrogenation of CO2 can reduce this gas to CH4, thereby achieving biogas upgrading and effective methane enrichment of the feed gas. Although chemically direct, this catalytic hydrogenation method requires exposing expensive nickel and ruthenium alloy-based consumable catalysts to harsh reaction conditions including high temperature (about 300°C) and high pressure (about 5MPa-20MPa).

[0017] Methods for the biological conversion of CO2 are also known. In microbial processes, hydrogenotrophic methanogens and homoacetogens can reduce CO2 to CH4 in the presence of H2 as an electron donor. This could be a potentially cost-effective method in the future.

[0018] While the conventional physical and chemical techniques outlined can be relatively efficient, especially compared to biological methods, they often leave materials that require disposal or regeneration, require expensive catalysts or adsorbent materials, or have unacceptable energy costs. For example, disposal of some solvents (e.g., amines) can result in toxicity to humans and the environment, while regeneration of adsorbent materials such as zeolites requires large amounts of energy.

[0019] Dosing of ions in municipal wastewater systems

[0020] Dosing municipal wastewater with specific iron salts (i.e., FeCl2, FeCl3, FeSO4, and Fe2(SO4)2) is a widely accepted and utilized method of chemically conditioning municipal wastewater. In systems known in the art, the above-mentioned specific iron salts can be added for various purposes, including controlling the levels of sulfide and / or phosphate, and aiding in the clarification of wastewater by promoting coagulation and settling of sludge.

[0021] It is known in the art to add the above specific iron salts to sewer systems to effectively reduce corrosion and odor problems caused by the emission of hydrogen sulfide (H2S). 2+ ) ions and / or ferric iron (Fe 3+ ) ions to form insoluble ferrous sulfide (FeS) to effectively remove any dissolved sulfide in the wastewater. 2+ Reacts with dissolved sulfides to directly form FeS. 3+ The dissolved sulfide is first oxidized to produce elemental sulfur, while the Fe 2+ Reacts with sulfides to form FeS. In both cases, the FeS formed is substantially insoluble in water. FeS particles entrained in water form sediments or agglomerates to form sludge, which is easily removed in subsequent wastewater treatment processes.

[0022] Similar methods known in the art include treating the fluid in an anaerobic digester to oxidize and precipitate sulfide dissolved in the anaerobic digester fluid. This substantially prevents the sulfide from entering the gas phase in the form of H2S and causing problems such as corrosion of process equipment and unpleasant odors.

[0023] Dosing the above-mentioned specific iron salts at the inlet of the WWTP is a widely used technique for enhancing the wastewater process - a practice known in the art as chemically enhanced primary sedimentation based on iron salts. In this process, phosphates can be partially or substantially removed from the wastewater as various insoluble iron-phosphate complexes, removing excess nutrients from the wastewater and preventing eutrophication in the water receiving the effluent from the process. In addition, ferric iron (Fe 3+)'s coagulation properties can effectively accumulate any organic matter in the wastewater and subsequently separate them from the wastewater as sludge, thereby significantly reducing the organic load to downstream biological processes.

[0024] In addition to the above, specific iron salts can also be added to wastewater treatment bioreactors (including aerated bioreactors) to remove phosphate from the wastewater, or at least reduce its concentration. The reduction in phosphate concentration can enhance the settling properties of sludge entrained in the wastewater after the biological reaction, so that the separation of sludge from the treated wastewater in the secondary clarifier can be improved.

[0025] A method known in the art includes the quantitative supply of iron in the sewer that affects the wastewater treatment equipment. In this method, in addition to controlling the H2S in the sewer, adding iron salts to the sewer also produces multiple benefits to downstream wastewater treatment and sludge digestion processes. The FeS produced in the sewer is usually oxidized in the wastewater treatment reactor, wherein the regenerated iron ions are subsequently precipitated with the phosphate that may be present in the wastewater as an insoluble iron-phosphate complex, resulting in an observable reduction in phosphate concentration. The sludge containing the iron-phosphate complex can be digested in an anaerobic digester subsequently, wherein the regenerated iron ions cause the sulfide precipitation dissolved in the digestion fluid, thereby reducing the H2S discharge into the biogas produced therefrom. In addition, the quantitative supply of iron salts to wastewater can improve the aggregation or coagulation of particles via destabilizing organic matter and colloidal particles, to promote the formation of larger solid particles in the wastewater treatment reactor and in the anaerobic digester, thereby improving the sedimentation and dewatering capacity of the sludge.

[0026] Continuously obtaining and dosing inputs of various reagents and / or ions is both expensive and cumbersome, especially for municipal wastewater treatment, which is often publicly operated on a limited budget. In contrast, generating reagents and / or ions on-site at the WWTP, or even more preferably, by upgrading biogas generated from the wastewater treatment process itself, eliminates or at least reduces the need to import the reagents and / or ions from elsewhere. Considering the utility of wastewater treatment, it is desirable that any additional method of generating reagents and / or ions (such as biogas upgrading) is: (i) relatively robust, in that it can process a wide variety of sludge and wastewater inputs, (ii) self-regulating, and (iii) relatively inexpensive to operate.

[0027] While existing methods for dosing ions, particularly iron ions, are able to enhance the quality and treatability of process products from WWTPs, such as biogas and sludge, these methods require large and continuous inputs of salts, including ammonium, alkaline salts and / or the specific iron salts disclosed above, to maintain the concentration of iron ions.

[0028] In processes with long residence times such as anaerobic digestion, the requirement for continuous dosing input of salts and / or reagents can often result in large continuous amounts of expensive material inputs and the need to closely monitor reaction conditions such as pH.

[0029] Finally, the methods known in the art are significantly limited to the removal of certain chemicals such as sulfides and phosphates from sludge and biogas.Therefore, there is a need for a system that can efficiently, cheaply and systematically remove the largest incompressible component gas (i.e., CO2) from anaerobically produced biogas.

[0030] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. Summary of the invention

[0031] In a first aspect of the present invention, there is provided a method for removing a portion of at least one contaminant gas from a first gas stream, the method comprising the steps of:

[0032] a) applying a voltage across a pair of electrically connected electrodes at least partially in contact with a working fluid in an electrochemical cell to generate a plurality of ions; and

[0033] b) reacting at least a portion of the at least one contaminant gas with a plurality of ions in the working fluid to convert the portion of the at least one contaminant gas into one or more reaction products, thereby isolating at least some of the contaminant gas from the first gas stream to produce a second gas stream,

[0034] The pH of the working fluid is substantially maintained at a predetermined set point to improve the efficiency of the conversion.

[0035] One of ordinary skill in the relevant art will appreciate that for purposes of the present invention, the second gas stream comprises at least a portion of the first gas stream absent of at least one contaminant gas, and thus, is enriched in at least one target gas compared to the first gas stream.

[0036] For context, the term "target gas" as used herein relates to a gas in a form that is substantially purer than the gas in its original state due to the use of the method to remove some or all of the contaminant gases. Typically, the target gas is a desirable gas that may have commercial value, wherein the commercial value may be reduced or diminished when the target gas is contaminated with one or more contaminant gases.

[0037] The method will be described below in terms of its use in removing at least a portion of one or more contaminant gases from a raw biogas stream. However, one of ordinary skill in the relevant art will appreciate that this particular method may also be used in a range of commercial applications where a gas stream enriched with a target gas is a desired outcome. Enrichment means that the proportion of the target gas in the second gas stream is increased relative to the proportion of at least one contaminant gas remaining in the second gas stream.

[0038] The present invention provides many advantages over the prior art. For example, polluting gases in the raw biogas produced by a microbial process (such as anaerobic digestion) for wastewater treatment can be at least partially removed from the raw biogas to produce valuable gas products with commercial applications. In particular, the basic removal of incompressible pollutants (including but not limited to CO2) of biogas improves the availability and value of biogas. Applying voltage across an electrically connected electrode pair at least partially arranged in a working fluid in an electrochemical cell produces a variety of ions, which can then react with at least a portion of at least one pollutant in a first gas stream to produce one or more reaction products. At least one pollutant is isolated from the first gas stream to produce a second gas stream, which is an enriched form of the first gas stream and represents a valuable gas product for commercial applications. At least one pollutant is preferably isolated in the working fluid in the form of a precipitate, and it is relatively easy to separate from the working fluid. In the example of the present invention where at least one pollutant in the first gas stream is CO2 and multiple ions include iron (Fe) ions, the main precipitate will be FeCO3. The pH of the working fluid is substantially maintained at a predetermined set point, which improves the efficiency of the reaction of isolating at least one pollutant, thereby forming a second gas stream.

[0039] In one embodiment, the pH of the working fluid is substantially maintained at a predetermined set point by controlling the voltage and / or current across the electrically connected electrode pair. In another embodiment, the pH of the working fluid is additionally or alternatively maintained substantially at a predetermined set point by adjusting the flow rate of the first gas stream into the electrochemical cell. It should be understood that controlling the voltage and / or current and / or the flow rate of the first gas stream enables control of the composition of the second gas stream. Preferably, the flow rate of the first gas stream is controlled to maintain the concentration of at least one pollutant gas at a predetermined level. In other words, the relative concentrations of the individual components of the second gas stream can be "adjusted". Preferably, at least one pollutant gas content is sufficiently reduced so that the second gas stream contains <5% of the pollutant gas.

[0040] When used in conjunction with wastewater treatment as described above, a significant advantage of the present invention is the generation of iron carbonate (FeCO3), which can be recycled back into the wastewater treatment system. The FeCO3 produced as a result of the present invention can be obtained on site and recycled back into the sewer network, and / or just before the primary sedimentation process, and / or a wastewater treatment bioreactor, and / or just before the final clarifier, and / or recycled back into the anaerobic digestion unit. Another important advancement in the art is the quantitative supply of FeCO3 for municipal wastewater, regardless of the source of FeCO3. The limited solubility of FeCO3 particles has limited the use of FeCO3 in the past. To explain, as far as the inventors are aware, only specific, more soluble iron salts FeCl2, FeCl3, FeSO4 and Fe2(SO4)2 have been used to chemically regulate municipal wastewater.

[0041] The use of FeCO3 is a significant advance in the art because FeCO3 has an advantage that these other specific iron salts do not have, namely its ability to maintain and provide additional alkalinity to the wastewater / sludge, as opposed to the iron salts currently used which deplete the alkalinity in the wastewater / sludge. In this regard, the maintenance of wastewater / sludge alkalinity maintains and / or enhances conditions that are favorable for coagulation and precipitation of materials suspended therein.

[0042] According to this particular embodiment of the invention, the immediate and efficient introduction of electrochemically generated ions into the working fluid allows for in-situ isolation of pollutant components from the feed biogas. This eliminates the need to remotely acquire, transport and introduce ions by dissolving reactive salts into the electrolyte, thereby providing an alternative use for the products of pollutant gas purification. The combination of gas purification and byproduct use makes the electrochemical pollutant gas isolation method of the invention economical and commercially viable.

[0043] In various embodiments of the present invention, the pollutant gas in the first gas stream is selected from the group consisting of CO2, H2S and ammonia (NH3) and / or combinations thereof, and the target gas enriched in the second gas stream comprises methane (CH4) and / or hydrogen (H2).

[0044] In one embodiment, the predetermined set point is maintained by adjusting at least one of: the flow rate of the first gas flow; the voltage; and the current applied across the electrodes.

[0045] In another embodiment, the predetermined set point falls within the range of pH 7.5 to pH 9.0, preferably pH 8.0 to pH 9.0, and even more preferably about pH 8.5. For example, in some embodiments, the predetermined set point falls within the range of pH 7.5 to pH 7.6, pH 7.6 to pH 7.7, pH 7.7 to pH 7.8, pH 7.8 to pH 7.9, pH 7.9 to pH 8.0, pH 8.0 to pH 8.1, pH 8.1 to pH 8.2, pH 8.2 to pH 8.3, pH 8.3 to pH 8.4, pH 8.4 to pH 8.5, pH 8.5 to pH 8.6, pH 8.6 to pH 8.7, pH 8.7 to pH 8.8, pH 8.8 to pH 8.9, and pH 8.9 to pH 9.0.

[0046] In a certain embodiment, the voltage across the electrodes is adjusted between the range of 0.5 V and 50.0 V, preferably between 1.0 V and 10.0 V, and even more preferably between 2.0 V and 5.0 V. For example, the voltage across the electrodes is adjusted between the range of 0.5 V and 50.0 V, between 0.6 V and 45.0 V, between 0.7 V and 40.0 V, between 0.8 V and 35.0 V, between 0.9 V and 30.0 V, between 1.0 V and 25.0 V, between 1.2 V and 20.0 V, between 1.4 V and 15.0 V, between 1.6 V and 10.0 V, between 1.8 V and 10.0 V, or between 2.0 V and 5.0 V.

[0047] In some embodiments, the predetermined set point falls within the range of pH 7.5 to pH 7.6, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 7.6 to pH 7.7, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 7.7 to pH 7.8, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. pH 7.8 to pH 7.9, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 7.9 to pH 8.0, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V.In some embodiments, the predetermined set point falls within the range of pH 8.0 to pH 8.1, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 8.1 to pH 8.2, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. pH 8.2 to pH 8.3, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 8.3 to pH 8.4, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 8.4 to pH 8.5, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V.pH 8.5 to pH 8.6, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 8.6 to pH 8.7, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 8.7 to pH 8.8, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some embodiments, the predetermined set point falls within the range of pH 8.8 to pH 8.9, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V. In some 2 embodiments, the predetermined set point falls within the range of pH 8.9 to pH 9.0, and the voltage across the electrodes is adjusted between the range of 0.5V and 50.0V, between 0.6V and 45.0V, between 0.7V and 40.0V, between 0.8V and 35.0V, between 0.9V and 30.0V, between 1.0V and 25.0V, between 1.2V and 20.0V, between 1.4V and 15.0V, between 1.6V and 10.0V, between 1.8V and 10.0V, or between 2.0V and 5.0V.

[0048] Preferably, the voltage is kept relatively low to save energy consumption. However, in some cases, it may be preferred to utilize a higher voltage to save material costs (by, for example, using recycled iron) or to reduce capital costs at the expense of energy costs.

[0049] In one embodiment, at least one of the electrodes comprises a metal selected from the group consisting of iron (Fe), magnesium (Mg), zinc (Zn), nickel (Ni), copper (Cu), aluminum (Al), titanium (Ti), or alloys thereof. The plurality of ions electrochemically generated from these metal electrodes in the electrochemical cell are reactive to at least one pollutant gas.

[0050] In an embodiment of the present invention, at least one of the electrodes comprises a metal selected from the group consisting of iron (Fe), magnesium (Mg), zinc (Zn), nickel (Ni), copper (Cu), aluminum (Al) and titanium (Ti), wherein the selected electrode produces a plurality of corresponding ions selected from the group consisting of iron (Fe) ions, magnesium (Mg) ions, zinc (Zn) ions, nickel (Ni) ions, copper (Cu) ions, aluminum (Al) ions and titanium (Ti) ions.

[0051] Therefore, in an embodiment of the present invention in which the first gas stream contains a pollutant gas which is carbon dioxide (CO2), one or more of the reaction products produced by the method of the present invention can be selected from the group consisting of magnesium carbonate (MgCO3), iron (II) carbonate (FeCO3), zinc carbonate (ZnCO3), nickel (II) carbonate (NiCO3), copper (II) carbonate (CuCO3), aluminum carbonate Al2(CO3)3 and titanium (III) carbonate Ti2(CO3)3, depending on the selected metal electrode.

[0052] In an embodiment, at least one of the electrodes comprises iron (Fe) and at least one contaminant gas is carbon dioxide (CO2), wherein one or more reaction products formed comprise a slurry of iron (II) carbonate (FeCO3) separated from the working fluid.

[0053] Preferably, the second gas stream has less than 30 wt.%, 20 wt.%, 10 wt.%, 5 wt.%, or 1 wt.% CO. For example, in some embodiments, the second gas stream has less than 30 wt.%, 29 wt.%, 28 wt.%, 27 wt.%, 26 wt.%, 25 wt.%, 24 wt.%, 23 wt.%, 22 wt.%, 21 wt.%, 20 wt.%, 19 wt.%, 18 wt.%, 17 wt.%, 16 wt.%, 15 wt.%, 14 wt.%, 13 wt.%, 12 wt.%, 11 wt.%, 9 wt.%, 8 wt.%, 7 wt.%, 6 wt.%, 5 wt.%, 4 wt.%, 3 wt.%, 2 wt.%, or 1 wt.% CO. In one embodiment, at least one of the electrodes comprises iron (Fe) and at least one contaminant gas is hydrogen sulfide (H2S), wherein one or more reaction products formed include iron (II) sulfide (FeS) separated from the working fluid.

[0054] In other embodiments, at least one of the electrodes comprises magnesium (Mg) and at least one of the contaminant gases is hydrogen sulfide (H2S), wherein one or more reaction products formed comprise magnesium sulfide (MgS) separated from the working fluid.

[0055] In other embodiments, at least one of the electrodes comprises zinc (Zn) and at least one of the contaminant gases is hydrogen sulfide (HS), wherein one or more reaction products formed comprise zinc sulfide (ZnS) separated from the working fluid.

[0056] In other embodiments, at least one of the electrodes comprises copper (Cu) and at least one of the contaminant gases is hydrogen sulfide (H2S), wherein one or more reaction products formed comprise copper (I) sulfide (Cu2S) separated from the working fluid.

[0057] In other embodiments, at least one of the electrodes comprises nickel (Ni) and at least one of the contaminant gases is hydrogen sulfide (H2S), wherein one or more reaction products formed comprise nickel sulfide (NiS) separated from the working fluid.

[0058] In other embodiments, at least one of the electrodes comprises aluminum (Al) and at least one of the contaminant gases is hydrogen sulfide (H2S), wherein one or more reaction products formed comprise aluminum sulfide (Al2S3) separated from the working fluid.

[0059] In other embodiments, at least one of the electrodes comprises titanium (Ti) and at least one of the contaminant gases is hydrogen sulfide (H2S), wherein one or more reaction products formed comprise titanium sulfide (TiS) separated from the working fluid.

[0060] Preferably, the second gas stream has less than 1000 ppmv, 500 ppmv, 300 ppmv, 200 ppmv, 100 ppmv or 10 ppmv H2S. For example, in some embodiments, the second gas stream has less than 950ppmv, 900ppmv, 850ppmv, 800ppmv, 750ppmv, 700ppmv, 650ppmv, 600ppmv, 550ppmv, 500ppmv, 450ppmv, 400ppmv, 350ppmv, 300ppmv, 250ppmv, 200ppmv, 150ppmv, 100ppmv, 95ppmv, 90ppmv, 85ppmv, 80ppmv, 75ppmv, 70ppmv, 65ppmv, 60ppmv, 55ppmv, 50ppmv, 45ppmv, 40ppmv, 35ppmv, 30ppmv, 25ppmv, 20ppmv, or 10ppmv of H2S.

[0061] In certain embodiments, at least one of the pollutant gases is ammonia (NH3), and wherein the one or more reaction products formed comprise aqueous ammonium (NH4 + ), removing at least some of the aqueous ammonium from the working fluid by purging at least a portion of the aqueous ammonium from the electrochemical cell. Preferably, the second gas stream has less than 1000 ppmv, 100 ppmv, 10 ppmv or 1 ppmv of NH3. For example, in some embodiments, the second gas stream has less than 950ppmv, 900ppmv, 850ppmv, 800ppmv, 750ppmv, 700ppmv, 650ppmv, 600ppmv, 550ppmv, 500ppmv, 450ppmv, 400ppmv, 350ppmv, 300ppmv, 250ppmv, 200ppmv, 150ppmv, 100ppmv, 95ppmv, 90ppmv, 85ppmv, 80ppmv, 75ppmv, 70ppmv, 65ppmv, 60ppmv, 55ppmv, 50ppmv, 45ppmv, 40ppmv, 35ppmv, 30ppmv, 25ppmv, 20ppmv, 15ppmv, 10ppmv, 5ppmv, 4ppmv, 3ppmv, 2ppmv, or 1ppmv of NH3.

[0062] In embodiments, at least a portion of the second gas stream is recycled back into the first gas stream.

[0063] In an embodiment, the first gas stream contains at least one pollutant gas selected from the group consisting of CO2, H2S, NH3, NOx and halogens. For example, halogens typically include chlorine (Cl) gas, but may also include fluorine (F) gas.

[0064] In another embodiment, the first gas stream is biogas, and wherein the biogas is produced during anaerobic digestion of organic matter. The organic matter may be derived from the group consisting of sewage / wastewater, agricultural waste, municipal waste, manure, plant material, green waste and food waste. Alternatively, the organic matter is sludge derived from sewage or wastewater.

[0065] In another embodiment, the working fluid comprises an electrolyte solution of an alkali metal salt or an alkaline earth metal salt. Preferably, the alkali metal salt or the alkaline earth metal salt is selected from the group of alkali metal salts or alkaline earth metal salts consisting of sodium (Na), potassium (K), lithium (Li), magnesium (Mg), calcium (Ca) or mixtures thereof.

[0066] In one embodiment, the electrolyte is an aqueous NaCl solution, preferably the concentration of the NaCl solution is between about 0.1% to about 5%, preferably in the range of about 0.5% to about 3%, and even more preferably in the range of about 1% to about 2%. For example, in some embodiments, the concentration of the NaCl solution is between about 0.1% to about 5%, about 0.2% to about 4.5%, about 0.3% to about 4%, about 0.4% to about 3.5%, about 0.5% to about 3%, about 0.6% to about 2.5%, about 0.7% to about 2.3%, about 0.8% to about 2.2%, about 0.9% to about 2.1%, or about 1% to about 2%.

[0067] Preferably, the electrodes are formed from iron or at least partly from iron.The iron used may be manufactured iron (from iron ore or recycled iron) or recycled iron scrap.

[0068] In a certain embodiment, the method further comprises the following steps before step a) or before step b):

[0069] a1) fluidly communicating at least a portion of a first gas stream having at least one contaminant gas from a gas source to an electrochemical cell.

[0070] In one embodiment, a gas scrubbing process may be utilized, or alternatively a bubble column process may be employed, as discussed further below.

[0071] In another embodiment, step b) includes fluidly connecting at least a portion of the first gas stream having at least one pollutant gas to at least a portion of the working fluid in a gas-liquid reactor, the gas-liquid reactor being in fluid communication with an electrochemical cell circulation fluid, wherein multiple electrochemically generated ions from the electrochemical cell and at least one pollutant gas are fluidly contacted in the gas-liquid reactor. Preferably, the gas-liquid reactor is selected from a gas scrubber and a bubbling tower. In addition, preferably, multiple electrochemically generated ions and at least a portion of the first gas stream are fluidly contacted in a countercurrent arrangement. Preferably, the first gas stream is compressed to flow through the gas-liquid reactor.

[0072] In a certain embodiment, the method further comprises the step of thickening / dehydrating one or more of the reaction products.

[0073] In a second aspect of the invention, a second gas stream produced by the above method is provided, wherein the second gas stream has a lower concentration of at least one pollutant gas than the first gas stream. In this embodiment, the second gas stream is enriched in hydrogen (H2), and the molar increase of hydrogen is proportional to the molar decrease of at least one pollutant gas.

[0074] In another embodiment, the amount of H2 produced (in moles) is approximately equal to the amount of CO2 and H2S removed (in moles each). This means that the second gas contains a high concentration of H2.

[0075] In a third aspect of the invention there is also provided use of the second gas stream as described above, including as a transport fuel, as a fuel for injection into a natural gas network to supplement the natural gas supply, for further purification and / or separation of its component gases.

[0076] In a fourth aspect of the present invention, there is provided a system for removing a portion of at least one contaminant gas from a first gas stream, the system comprising:

[0077] an electrochemical cell comprising an electrically connected pair of electrodes disposed at least partially in a working fluid, wherein, when a voltage is applied across the electrodes, a plurality of ions are electrochemically generated in the working fluid for reacting with at least a portion of at least one contaminant gas to convert the portion of the at least one contaminant gas into one or more reaction products, thereby isolating at least some of the contaminant gases from the first gas stream, thereby generating a second gas stream,

[0078] The pH of the working fluid is substantially maintained at a predetermined set point to improve the efficiency of the conversion.

[0079] In one embodiment, the predetermined set point is maintained by adjusting at least one of: the flow rate of the first gas flow; the voltage; and the current applied across the electrodes.

[0080] In another embodiment, the predetermined set point falls within the range of pH 7.5 to pH 9.0, preferably pH 8.0 to pH 9.0, and even more preferably about pH 8.5. For example, in some embodiments, the predetermined set point falls within the range of pH 7.5 to pH 7.6, pH 7.6 to pH 7.7, pH 7.7 to pH 7.8, pH 7.8 to pH 7.9, pH 7.9 to pH 8.0, pH 8.0 to pH 8.1, pH 8.1 to pH 8.2, pH 8.2 to pH 8.3, pH 8.3 to pH 8.4, pH 8.4 to pH 8.5, pH 8.5 to pH 8.6, pH 8.6 to pH 8.7, pH 8.7 to pH 8.8, pH 8.8 to pH 8.9, and pH 8.9 to pH 9.0.

[0081] In further embodiments, the voltage across the electrodes is adjusted between the range of 0.5 V and 50.0 V, preferably between 1.0 V and 10.0 V, and even more preferably between 2.0 V and 5.0 V. For example, the voltage across the electrodes is adjusted between the range of 0.5 V and 50.0 V, between 0.6 V and 45.0 V, between 0.7 V and 40.0 V, between 0.8 V and 35.0 V, between 0.9 V and 30.0 V, between 1.0 V and 25.0 V, between 1.2 V and 20.0 V, between 1.4 V and 15.0 V, between 1.6 V and 10.0 V, between 1.8 V and 10.0 V, or between 2.0 V and 5.0 V.

[0082] In embodiments, at least one of the electrodes comprises iron (Fe), and at least one of the pollutant gases is carbon dioxide (CO2), wherein the one or more reaction products formed comprise a slurry of iron (II) carbonate (FeCO3) separated from the working fluid. Preferably, the second gas stream has less than 30 wt.%, 20 wt.%, 10 wt.%, 5 wt.%, or 1 wt.% CO2. For example, in some embodiments, the second gas stream has less than 30 wt.%, 29 wt.%, 28 wt.%, 27 wt.%, 26 wt.%, 25 wt.%, 24 wt.%, 23 wt.%, 22 wt.%, 21 wt.%, 20 wt.%, 19 wt.%, 18 wt.%, 17 wt.%, 16 wt.%, 15 wt.%, 14 wt.%, 13 wt.%, 12 wt.%, 11 wt.%, 9 wt.%, 8 wt.%, 7 wt.%, 6 wt.%, 5 wt.%, 4 wt.%, 3 wt.%, 2 wt.%, or 1 wt.% CO2.

[0083] In a certain embodiment, at least one of the electrodes comprises iron (Fe) and at least one of the contaminant gases is hydrogen sulfide (H2S), wherein the one or more reaction products formed comprise a slurry of iron (II) sulfide (FeS) separated from the working fluid. Preferably, the second gas stream has less than 1000 ppmv, 500 ppmv, 300 ppmv, 200 ppmv, 100 ppmv or 10 ppmv of H2S. For example, in some embodiments, the second gas stream has less than 950ppmv, 900ppmv, 850ppmv, 800ppmv, 750ppmv, 700ppmv, 650ppmv, 600ppmv, 550ppmv, 500ppmv, 450ppmv, 400ppmv, 350ppmv, 300ppmv, 250ppmv, 200ppmv, 150ppmv, 100ppmv, 95ppmv, 90ppmv, 85ppmv, 80ppmv, 75ppmv, 70ppmv, 65ppmv, 60ppmv, 55ppmv, 50ppmv, 45ppmv, 40ppmv, 35ppmv, 30ppmv, 25ppmv, 20ppmv, or 10ppmv of H2S.

[0084] In one embodiment, at least one pollutant gas is ammonia (NH3), and wherein the one or more reaction products formed comprise aqueous ammonium (NH 4+ ). Preferably, the second gas stream has less than 1000 ppmv, 100 ppmv, 10 ppmv or 1 ppmv NH3. For example, in some embodiments, the second gas stream has less than 950ppmv, 900ppmv, 850ppmv, 800ppmv, 750ppmv, 700ppmv, 650ppmv, 600ppmv, 550ppmv, 500ppmv, 450ppmv, 400ppmv, 350ppmv, 300ppmv, 250ppmv, 200ppmv, 150ppmv, 100ppmv, 95ppmv, 90ppmv, 85ppmv, 80ppmv, 75ppmv, 70ppmv, 65ppmv, 60ppmv, 55ppmv, 50ppmv, 45ppmv, 40ppmv, 35ppmv, 30ppmv, 25ppmv, 20ppmv, 15ppmv, 10ppmv, 5ppmv, 4ppmv, 3ppmv, 2ppmv, or 1ppmv of NH3.

[0085] In another embodiment, at least a portion of the second gas stream is recycled back into the first gas stream.

[0086] In other embodiments, the first gas stream comprises at least one pollutant gas selected from the group consisting of CO2, H2S, NH3, NOx and halogens. For example, halogens typically include chlorine (Cl) gas, but may also include fluorine (F) gas.

[0087] In a certain embodiment, the first gas stream is biogas, and wherein the biogas is produced during anaerobic digestion of organic matter. Preferably, the organic matter is derived from the group consisting of sewage / wastewater, agricultural waste, municipal waste, manure, plant material, green waste and food waste. Alternatively, the organic matter is sludge derived from sewage or wastewater.

[0088] In embodiments, the working fluid comprises an electrolyte. Preferably, the electrolyte is an aqueous NaCl solution, preferably a concentration of the NaCl solution between about 0.1% to about 5%, preferably in the range of about 0.5% to about 3%, and even more preferably in the range of about 1% to about 2%. For example, in some embodiments, the concentration of the NaCl solution is between about 0.1% to about 5%, about 0.2% to about 4.5%, about 0.3% to about 4%, about 0.4% to about 3.5%, about 0.5% to about 3%, about 0.6% to about 2.5%, about 0.7% to about 2.3%, about 0.8% to about 2.2%, about 0.9% to about 2.1% or about 1% to about 2%. In another embodiment, the system also includes a gas-liquid reactor connected to an electrochemical cell circulation fluid for fluidly receiving a working fluid comprising a plurality of electrochemically generated ions and a first gas stream, wherein the gas-liquid reactor is suitable for contacting a plurality of electrochemically generated ions from an electrochemical cell with at least one pollutant gas fluid. The gas-liquid reactor is preferably selected from a gas scrubber and a bubble column.Preferably, the gas-liquid reactor fluidly contacts the plurality of electrochemically generated ions with at least a portion of the first gas stream in a counter-current arrangement.

[0089] In further embodiments, the system further comprises a compressor for compressing the first gas stream.

[0090] In certain embodiments, the system further comprises a purge stream for removing at least a portion of the working fluid. The purge stream is preferably suitable for removing aqueous ammonium (NH4 + ) in at least some aqueous ammonium.

[0091] In one embodiment, the system further comprises a solid-liquid separation device for thickening / dewatering one or more reaction products.

[0092] In a fifth aspect of the present invention, there is provided a method for treating sewage or wastewater, the method comprising the steps of:

[0093] Obtaining sewage, wastewater and / or sludge derived therefrom;

[0094] fluidly communicating at least a portion of the sewage, wastewater, and / or sludge derived therefrom to an anaerobic digester to produce a first gas stream comprising at least one pollutant gas;

[0095] fluidly connecting at least a portion of the first gas stream to the system to convert at least a portion of the at least one pollutant gas into one or more reaction products; and

[0096] adding at least a portion of the one or more reaction products to one or more of:

[0097] (a) sewage and / or wastewater,

[0098] (b) sludge derived from sewage and / or wastewater, or

[0099] (c) anaerobic digesters,

[0100] by:

[0101] (i) reacting with one or more contaminants therein to facilitate at least partial removal of the contaminants therefrom; and / or

[0102] (ii) Enhance the settling and / or dewatering performance of sludge.

[0103] The present invention provides numerous advantages over the prior art, such as by combining the ion generation function with the isolation and removal of contaminant gas components of the biogas.

[0104] By utilizing electrochemical generation, a readily available and continuous source of reaction products is available for use in forming sludge for use in wastewater treatment processes. This enhances process efficiency for settling and water clarification, as well as providing synergy with electrochemical biogas purification processes that produce reaction products that are subsequently used in wastewater treatment processes.

[0105] In one embodiment, at least one of the electrodes comprises iron (Fe) and at least one of the pollutant gases is carbon dioxide (CO2), wherein the one or more reaction products formed comprise a slurry of iron (II) carbonate (FeCO3). The sewage and / or wastewater is preferably derived from a sewer network, and at least one of the pollutants identified in the sewer network is sulfide. Similarly, at least one of the one or more pollutants identified in the anaerobic digester is sulfide.

[0106] In another embodiment, the FeCO slurry reacts with sulfide to produce iron (II) sulfide (FeS). Advantageously, the removal of dissolved sulfide from sewage / wastewater eliminates / reduces the emission of H 2 S from sewage / wastewater to the sewer atmosphere, reducing sewer corrosion and odor emissions, or digesters resulting in a reduction of H 2 S in biogas.

[0107] In other embodiments, the wastewater or sludge originates from the inlet of a wastewater treatment plant (WWTP) or from a bioreactor of a WWTP, and at least one of the one or more pollutants at least partially removed therefrom is phosphate. The FeCO slurry preferably reacts with phosphate to produce one or more insoluble iron phosphate complexes.

[0108] In certain embodiments, the settling and / or dewatering performance of sludge in wastewater treatment bioreactors and / or anaerobic digesters is enhanced by FeCO3-induced sludge coagulation.

[0109] In a sixth aspect of the present invention, there is also provided a method for treating sewage or wastewater, the method comprising the following steps:

[0110] Treatment of sewage, wastewater and / or sludge derived therefrom with iron(II) carbonate (FeCO3) to:

[0111] (i) reacting with one or more contaminants therein to facilitate at least partial removal of the contaminants therefrom; and / or

[0112] (ii) Enhance the settling and / or dewatering performance of sludge.

[0113] In one embodiment, the sewage and / or wastewater is derived from a sewer network, and at least one of the one or more pollutants partially removed therefrom is sulfide or phosphate. The wastewater is preferably derived from a landfill, and at least one of the one or more pollutants at least partially removed therefrom is sulfide. Alternatively, the sludge is derived from an anaerobic digester, and at least one of the one or more pollutants at least partially removed therefrom is sulfide. Preferably, FeCO3 reacts with sulfide to produce iron (II) sulfide (FeS).

[0114] In an alternative embodiment, the wastewater or sludge originates from the inlet of a wastewater treatment plant (WWTP) or from a bioreactor of a WWTP, and at least one of the one or more pollutants at least partially removed therefrom is phosphate. Preferably, FeCO3 reacts with phosphate to produce one or more insoluble iron phosphate complexes.

[0115] In an alternative preferred embodiment, the sludge originates from an anaerobic digester or a WWTP bioreactor, and the settling and / or dewatering properties of the sludge are enhanced by FeCO3-induced coagulation of the sludge.

[0116] definition

[0117] In describing and claiming the present invention, the following terms will be used according to the definitions set forth below. It should also be understood that the terms used herein are only for the purpose of describing specific embodiments of the present invention and are not intended to be limiting. Unless otherwise defined, all technical terms and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs.

[0118] Unless the context clearly requires otherwise, throughout the specification and claims, the words "comprising," "including," and the like should be construed in an inclusive sense rather than an exclusive or exhaustive sense; that is, in the sense of "including but not limited to."

[0119] As used herein, the phrase "consisting of excludes any element, step, or ingredient not specified in the claim. When the phrase "consisting of" (or variations thereof) appears in a clause of the body of a claim rather than immediately following a preamble, it is limited only to the elements set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phrase "consisting essentially of limits the scope of a claim to the specified elements or method steps, plus those elements or method steps that do not materially affect the basic and novel characteristics of the claimed subject matter.

[0120] With respect to the terms "comprising," "consisting of," and "consisting essentially of," where one of these three terms is used herein, the subject matter disclosed and claimed herein may include the use of either of the other two terms. Thus, in some embodiments not expressly stated otherwise, any instance of "comprising" may be replaced by "consisting of," or alternatively, by "consisting essentially of."

[0121] These examples are not intended to limit the scope of the present invention.Hereinafter, or where otherwise indicated, "%" will mean "volume %", "ratio" will mean "volume ratio", and "part" will mean "part by volume".

[0122] Unless otherwise indicated, the term "substantially" as used herein shall mean greater than 50% by volume, mass or weight, depending on the context in which it is used. Preferably, it is intended to mean greater than 75%. Even more preferably, it is intended to mean greater than 90%. Most preferably, it is intended to mean 100% or close to 100%.

[0123] The recitation of numerical ranges using endpoints includes all numbers subsumed within that range (eg, 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

[0124] The terms "preferred" and "preferably" refer to embodiments of the present invention that may provide certain benefits in certain circumstances. However, other embodiments may also be preferred under the same or other circumstances. In addition, the recitation of one or more preferred embodiments does not mean that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the present invention.

[0125] It must also be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0126] As used herein, the phrase "contaminant gas" relates to a gas that is considered to be an impurity or undesirable component or unwanted component present in the first gas stream (i.e., the natural gas and / or biogas stream), which may be the result of anaerobic digestion of organic matter. Preferably, the contaminant gas is at least partially soluble in the working fluid, and preferably reacts with free metal ions in the working fluid to form a substance that is at least partially insoluble in the working fluid.

[0127] As used herein, the phrase "first gas stream" relates to a gas stream that may be a natural gas stream, an industrial gas stream, or more preferably a biogas stream produced as a result of anaerobic digestion of organic matter, wherein the gas stream is a gas mixture comprising one or more contaminant gases which, once at least partially removed, produce a second gas stream suitable for enrichment for commercial applications.

[0128] As used herein, the phrase "second gas stream" refers to a gas stream that is enriched after at least partial removal of one or more contaminant gases from a first gas stream.

[0129] As used herein, the phrase "working fluid" refers to a liquid medium containing an electrolyte used in an electrochemical cell.

[0130] All prior art mentioned herein is incorporated herein by reference. Although exemplary embodiments of the disclosed technology are explained in detail herein, it should be understood that other embodiments are contemplated. Therefore, it is not intended that the scope of the disclosed technology be limited to the details of the construction and arrangement of the components set forth in the following description or illustrated in the accompanying drawings. The disclosed technology can have other embodiments and can be practiced or implemented in various ways. BRIEF DESCRIPTION OF THE DRAWINGS

[0131] The present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0132] Figure 1 is a schematic diagram of a process flow diagram of an exemplary embodiment of the present invention utilizing a single-stage reaction vessel;

[0133] Figure 2a is a schematic diagram of a process flow of a preferred embodiment of the present invention, including a two-stage reaction scheme across an electrochemical cell and a bubble column. Figure 2b shows a similar two-stage reaction scheme, wherein a scrubber replaces the bubble column of Figure 2a;

[0134] Figure 3 is a simplified process flow diagram of one embodiment of the present invention, wherein a gas feed including a pollutant gas and N2 as a substitute for CH4 is electrochemically treated to produce an enriched gas stream;

[0135] Figure 4 is a graphical representation of test results obtained from electrochemical purification of a feed gas comprising N2 and CO2 at pH 7.5;

[0136] Figure 5 is a graphical representation of test results obtained from electrochemical purification of a feed gas comprising N2 and CO2 at pH 8.0;

[0137] Figure 6 is a graphical representation of test results obtained from electrochemical purification of a feed gas comprising N2 and CO2 at pH 8.5;

[0138] Figure 7 is a graphical representation of test results obtained from electrochemical purification of a feed gas comprising N2 and CO2 at pH 9.0;

[0139] Figure 8 is a graphical representation of test results obtained from electrochemical purification of feed gases including N2 and CO2 at various pH levels;

[0140] Fig. 9 a to Fig. 9 f is a graphical representation of test results obtained from electrochemical purification of a feed gas similar to raw biogas at pH 8.5. Fig. 9 g to Fig. 9 i is a graphical comparison of the results obtained from electrochemical purification of a feed gas containing mainly CH4 and N2, showing that N2 can be used as a substitute for CH4;

[0141] Fig.10 is a graphical representation of test results obtained from electrochemically purifying feed gases including N2 and CO2 at various feed gas flow levels;

[0142] Fig.11 is a graphical representation of the XRD crystallographic profile of FeCO3 particles electrochemically produced by purified feed gas at pH 8.5;

[0143] Fig.12is a graphical representation of the particle size distribution and sedimentation properties of various FeCO3 particles electrochemically produced at pH 8.5 and stored for different amounts of time;

[0144] Fig.13 a to Fig.13 d is a graphical representation of the effect on the sulfide concentration in the wastewater when FeCO3 slurries of different ages are dosed at different concentrations. Fig.13 e and Fig.13 f is a graphical representation of the effect on wastewater pH when FeCO3 slurries of different ages are under-dosed and over-dosed, respectively;

[0145] Fig.14 is a graphical representation of the effect on wastewater pH of dosing FeCO3 slurry into sulfide-containing wastewater under sewer-like conditions;

[0146] Fig.15 is a graphical representation of the effect of dosing FeCO3 slurry to aerated phosphate-containing sludge under simulated biological wastewater treatment conditions;

[0147] Fig.16 is a graphical representation of the effect of dosing FeCO3 slurry to sulfide-containing sludge in a simulated anaerobic digester;

[0148] Figures 17a and 17b are graphical representations of the effect of dosing a FeCO slurry to sewer-like conditions and the flow effect on aerated biological treatment in a simulated wastewater treatment plant, respectively;

[0149] Fig.18 a and Fig.18 b is when there is no quantitative supply of FeCO3 ( Fig.18 a) and in the case of a quantitative supply of FeCO3 ( Figure 8 b), graphical comparison of the conversion of nitrogenous compounds in aerated sludge receiving wastewater;

[0150] Fig.19 is a graphical representation of the flow effects of dosing FeCO3 slurry in a simulated sewer on a simulated anaerobic digester;

[0151] Fig. 20 a is a graphical comparison of the settleability (measured as the sludge volume index (SVI)) of wastewater sludge with and without FeCO3 dosing, and Fig. 20 b is a graphical comparison of the dewatering capacity (measured as suction resistance force (SRF)) of anaerobically digested sludge with and without FeCO3;

[0152] Fig.21is a graphical representation of the effect of dosing FeCO3 slurry directly into the simulated anaerobic digester on the simulated anaerobic digester;

[0153] Fig. 22 is a process flow diagram of one embodiment of the present invention showing a wastewater treatment plant comprising a cell for purifying raw biogas from an anaerobic digester and dosing the FeCO3 slurry produced thereby into a sewer network leading to said plant; and

[0154] Fig.23 is a process flow diagram of another embodiment of a wastewater treatment plant in which FeCO3 electrochemically produced from purified anaerobic digested biogas is directly metered in several unit operations of the plant. DETAILED DESCRIPTION

[0155] Those skilled in the art will appreciate that the present invention includes the embodiments and features disclosed herein and all combinations and / or permutations of the disclosed embodiments and features.

[0156] Example 1 - Single stage purification

[0157] First reference Figure 1 An exemplary embodiment of the present invention shown in , provides a system for purifying a gas stream containing biogas from an anaerobic digester by removing at least one of the contaminant gases CO2, H2S and NH3 by precipitating / ionizing the contaminant gases using electrochemically generated iron (Fe) ions.

[0158] The electrochemical cell 1 uses a power source 4 to generate an electrical potential between an anode 2 and a cathode 3. The cell 1 generates Fe at the anode 2 by sacrificially oxidizing the iron-containing anode. 2+ ions, and produces H2 and OH- at the cathode 3 by decomposing water containing electrolyte 5. The biogas is compressed by compressor 6 and introduced into cell bubbler 7 to form fine or micron- or nano-sized bubbles 8. The upwardly travelling bubbles 8 are mixed with Fe-containing 2+ and OH - The gas bubbles are in contact with the electrolyte 5, wherein the pollutant gas, mainly CO2, in the bubbles diffuses into the electrolyte 5 to form ions including bicarbonate, carbonate, sulfide and ammonium under alkaline pH conditions.

[0159] The dissolved ions in electrolyte 5 and Fe 2+The precipitates together to form an insoluble iron salt including FeCO3, which is precipitated onto a rake substrate 9 of the battery 1 in the form of a slurry and subsequently removed from the substrate 9. The slurry is thickened in a liquid-solid separator 10, wherein the thickened slurry is transported for on-site dosing operations or stored in suitable containers for further use. The electrolyte 5 removed from the slurry in the liquid-solid separator is recycled back to the electrolyte replenishment feed 11 of the battery 1.

[0160] The remaining CH4, now substantially comprising biogas bubbles 6, has a low solubility in the electrolyte 5 and therefore remains entrained in the bubbles travelling upward to enter the head space 12 of the cell 1 along with the H2 gas formed at the cathode 3. The substantially enriched biogas overhead product is removed from the head space 8 for pumping through or storage in a suitable container.

[0161] In an optional embodiment where the enriched biogas in the head space 12 still contains CO2 above the desired level, at least a portion of the enriched biogas is recycled back to the gas bubbler 7 for further purification in the gas recycle stream 13. In an optional embodiment including an electrolyte recycle stream 14, the electrolyte 5 of the cell 1 can similarly be recycled.

[0162] In this optional embodiment, the two recycle streams are combined with their respective biogas and electrolyte feed streams to adjust the incoming concentration of each feed and improve the efficiency of steady state operation of the unit. In this optional embodiment, both or either of the streams can be turned on or off, their operation responsive to manual user intervention or via electronic control input from a controller such as a PID controller.

[0163] In another optional embodiment, the cell 1 includes an electrolyte purge stream 15 to purge a portion of the recycled electrolyte separated from the slurry in the liquid-solid separator 10. The purge stream 15 is generally adapted to remove at least a portion of the electrolyte containing non-deposited precipitates and / or other soluble species of gaseous contaminants from the biogas feed stream, including sulfides such as FeS and ammonium salts formed in the cell 1. These precipitates tend to be smaller and less deposited, making them difficult to remove from the electrolyte 5. When turned on by manual input or electronic controller input, the purge stream removes at least a portion of the electrolyte 5 while it removes dissolved salts and smaller non-deposited precipitates from further circulation in the system.

[0164] Example 2 - Two-stage purification

[0165] In certain embodiments of the present invention, a two-stage process is provided for: 1) electrochemically generating ions; and 2) using the ions to purify a contaminated gas feed. The two stages are performed in separate reaction vessels using various methods to contact the gas feed with the dissolved electrochemically generated ions. The contacting methods include, but are not limited to, gas bubbling and / or mist-based gas scrubbing.

[0166] Example 2.1

[0167] refer to Figure 2A , system 100 includes an electrochemical cell 101 including a sacrificial iron-containing anode 102 and a cathode 103 in electrical communication with a power source 104 that provides an electrical potential to an electrolyte 105. Cell 101 generates metal ions such as Fe at sacrificial anode 102. 2+ , while the electrolyte is reduced at the negative electrode 103 to form H2 gas and OH - Ions. The electrochemically generated ions remain in the electrolyte 105, while H2 gas bubbles through the electrolyte to the headspace 106 of the cell 101 and is subsequently pumped out of the cell for storage or reuse.

[0168] In this embodiment, the cell 101 is fluidically connected to a bubbling tower 107 filled with an electrolyte 105 and comprising a bubbler 108 at the bottom of the tower suitable for delivering a feed of contaminated biogas compressed by a compressor 109 into the electrolyte 105 in the form of fine or micrometer- or nanometer-sized bubbles 110.

[0169] The biogas bubbles 110 generated by the bubbler 108 are mixed with the gas containing metal ions and OH. - The biogas bubbles 110, now substantially free of pollutant gases, are bubbled into the overhead space 111 of the bubbling column 107, where the enriched biogas (at Figure 2A The overhead product (labeled "purified gas") is pumped out of the headspace for immediate use elsewhere or stored in a container for future use.

[0170] The electrolyte 105 containing metal ions produced in the cell 101 by sacrificial oxidation of the iron-containing anode 102 is pumped to a bubble column 107, where the dissolved contaminant gas ions (particularly carbonate) in the electrolyte 105 are precipitated with the metal ions to form a metal carbonate precipitate, which settles in the form of a slurry onto the rake base 112 of the bubble column 107. In a manner similar to Example 1, the slurry is then removed from the base 112 for thickening in a liquid-solid separator 113, where the thickened slurry is transported for on-site dosing operations or stored in a suitable container for further use. The electrolyte 106 removed from the slurry in the liquid-solid separator 113 is recycled back to the electrolyte replenishment feed 114 for the cell 101. Similarly, the electrolyte 105 not contained in the slurry formed at the rake base 112 is also recycled back to the cell 101 for further electrolysis.

[0171] Similar to the single tank implementation scheme of Example 1, Figure 2A Optional embodiments of the system 100 illustrated in Example 1 may also include various recycle streams as well as purge streams to remove non-deposited reaction products. In this regard, optional embodiments may include a gas recycle stream 115, wherein the enriched biogas in the tower overhead space 111 still contains CO2 above a desired level so that at least a portion of the enriched biogas can be recycled back to the gas bubbler 108. According to the single cell-only embodiment of Example 1, the various recycle streams of this embodiment are controlled to optimize the reaction efficiency during steady-state operation and to manage the material balance across the system 100.

[0172] In another optional embodiment, the system 100 includes an electrolyte purge stream 116 to purge a portion of the recycled electrolyte separated from the slurry in the liquid-solid separator 113. Similar to Example 1, the purge stream 116 is generally adapted to remove at least a portion of the electrolyte 105 containing non-deposition materials of other gaseous contaminants, including sulfides such as FeS and ammonium salts formed in the bubble column 107, from the biogas feed stream.

[0173] Example 2.2

[0174] refer to Figure 2B , another embodiment of the gas purification system is provided, wherein the gas-liquid contacting is carried out in a spray scrubber.

[0175] In this embodiment, system 100a includes a scrubber 107a in place of the bubble column 107 of system 100, wherein compressed biogas feed passes through a gas dispersion outlet 108a to be evenly dispersed across scrubber 107a. Electrochemical cell 101a contains metal ions such as Fe 2+ and OH -The electrolyte 105a is pumped to the top of the scrubber tower 107a where it enters the tower through nozzles 117a forming droplets.

[0176] The upwardly travelling biogas and the downwardly travelling droplets of electrolyte 105a achieve countercurrent contact, through which pollutant gases such as CO2 are absorbed into the droplets, in which OH is present. - ions are converted into ions such as bicarbonate and carbonate. In this case, carbonate and Fe 2+ The insoluble FeCO3 precipitate is collected at the rake base of the scrubber 107a. In particular, metal carbonate particles such as FeCO3 precipitate settle to form a slurry, which is then removed as a bottom product, while the supernatant of the slurry is recycled to the electrochemical cell 101a to replenish the electrolyte 105a.

[0177] The CH4 containing biogas feed has a low solubility in the droplets and therefore travels to the top of the scrubber 107a for collection. The head space 111a may still contain some CO2.

[0178] According to the embodiment illustrated in Example 2.1, Figure 2B The optional embodiment of the system 100a illustrated in FIG. 1 may also include a gas recycle stream 115a and a purge stream 116a to remove non-deposited reaction products from the recycled electrolyte 105a.

[0179] Example 3 - CO2 Removal

[0180] This embodiment relates to an integrated method for removing a portion of a contaminant gas from a first gas stream using electrochemically generated ions to react with the contaminant gas and isolate the contaminant gas at a set pH level. In particular, this embodiment relates to removing CO2 from a feed gas similar to raw biogas.

[0181] like Figure 3 As illustrated in , CO2 removal was carried out in a modified glass bottle with a total volume of 325 mL in a fume hood in a temperature-controlled (22°C ± 1°C) laboratory. The reactor was sealed to ensure airtightness and mixed at 300 rpm by a magnetic stirrer. Two iron plates (mild steel, hard steel) used as anode and cathode, respectively, were placed in parallel and fixed to the lid of the bottle, with a gap of 1.0 cm between the electrodes. The dimensions of the iron plates were 15 cm × 1.4 cm × 0.3 cm. Each iron plate was immersed in the electrolyte at a depth of 3.5 cm to achieve 11.9 cm 2The iron oxidation was achieved by controlling the current of the electrochemical cell via a benchtop power supply (72-2685, TENMA, China). The pH in the reaction vessel was monitored with a portable pH meter (miniCHEM, Labtek). Figure 3 As illustrated in FIG, the reactor has sampling ports for gas, liquid and solid sampling.

[0182] In each test, tap water was used as the electrolyte to prepare 200 mL of a 2 g / L NaCl solution. The electrolyte was sparged with simulated biogas (N2 was used as a substitute for CH4 in most tests to reduce laboratory risk; N2 and CH4 have similar low solubility) at a flow rate of 0.1 L / min for about 30 min to remove dissolved oxygen (DO) before testing.

[0183] The simulated biogas was delivered from the cylinder to the reactor via a gas flow controller (EL-FLOW, Bronkhorst). The air diffuser used to deliver air into the electrolyte was a needle with a diameter of 0.5 mm in the reactor. The feed gas was a combination of about 60% N2 and about 40% CO2, which was used to simulate the composition of raw biogas, which typically contains about 60% CH4 and about 40% CO2. Since the electrolysis process reacts neither with CH4 nor with N2, the results obtained are similar to those of feeding raw biogas. The flow rate of the feed gas was controlled by a gas flow controller (Bronkhorst, the Netherlands), and the "modified gas" was collected with a 5L gas bag connected to the outlet of the reactor.

[0184] Example 3 - Experimental:

[0185] Figure 4 a presents an overview of the gas composition (i.e., H2, N2, and CO2) in the headspace of a batch test vessel, pH, total iron concentration, and reactor current and voltage when tested at pH 7.5. After sparging with feed gas before time 0, the headspace contained approximately 60% N2 and approximately 40% CO2, consistent with the known composition of simulated biogas. As shown in Figure 4 b and Figure 4As seen in Fig. d, the pH at time 0 is 4.5 ± 0.1, which is lower than the initial pH of the electrolyte (about 5.8), due to acidification caused by CO2 dissolution. During the run-up phase, a current of 0.12 A was applied, resulting in H2 formation, accompanied by a pH increase due to the simultaneous production of hydroxide at the cathode, reaching 7.3 ± 0.1 within one hour, and 7.5 ± 0.0 at the end of the run-up phase. CO2 in the headspace diffused back into the liquid phase, as indicated by the decrease in its concentration in the headspace, and was probably retained in the liquid as dissolved CO2-bicarbonate and carbonate, the latter reacting with Fe2 produced at the anode. 2+ Sediment together.

[0186] The N2 content in the headspace gradually decreased during the preparation phase due to dilution caused by H2 produced at a rate higher than the CO2 removal rate. At the end of the preparation phase, the total iron concentration reached 1.2±0.1 g / L, demonstrating the generation of iron ions at the anode.

[0187] During the experimental phase, feed gas was supplied to the reactor at a rate of 5 mL / min from the start of the experimental phase. To maintain pH 7.5, the current of the electrochemical cell was also increased from 0.12 A to 0.27 A ± 0.01 A while the operating voltage was increased from 2.44 V ± 0.01 V to 6.03 V ± 0.01 V.

[0188] After a short period of two hours, the levels of H2, N2 and CO2 reached a steady state and remained stable for the last two hours.

[0189] Example 3 - Results:

[0190] As summarized in Table 1 below, once steady state was reached, the CO2 content in the headspace was 9.1±0.1%, significantly lower than the value of the original gas (ie, about 40%). Therefore, the CO2 removal efficiency was calculated to be 76.6±0.2%.

[0191] Table 1. Summary of CO2 removal results at pH 7.5

[0192]

[0193]

[0194] like Figure 4 As shown in c, the total iron concentration increased linearly over the duration of the experimental period and reached 6.0 ± 0.1 g / L, including 5.8 ± 0.1 g / L and 0.2 ± 0.1 g / L of Fe 2+ and Fe 3+ .

[0195] The iron utilization efficiency, calculated as the ratio between the total amount of CO2 removed (in moles) and the total amount of iron produced (also in moles), was 87.1 ± 0.2%, indicating that most of the Fe produced 2+ For CO2 removal. Iron consumption is supported by the TIC (on a molar basis) to total iron (on a molar basis) ratio in the solids collected from the batch reactor, which was 84.1 ± 1.8%.

[0196] The results demonstrate the feasibility of achieving CO2 removal by using an iron oxidation electrochemical cell. The results also show that a portion of the produced ferrous ions were not combined with carbonate (about 15%) - indicating that some other iron compounds, such as ferrous hydroxide, may have been formed during the experimental period.

[0197] Example 4 - CO2 removal at different pH

[0198] CO2 removal was also evaluated at pH 7.5, pH 8.0, pH 8.5 and pH 9.0 in a series of batch tests. Initially, 200 mL of oxygen-free electrolyte was added to the reactor, leaving 125 mL as headspace. In the preparatory stage, the battery was supplied with current without gas supply. Due to the continuous production of ferrous ions and hydroxide (and H2) in the battery, the pH in the reactor gradually increased to a pre-specified level (i.e. 7.5, 8.0, 8.5 or 9.0). The gas reforming reaction was started when the pH set point was reached, during which the polluted feed gas was fed to the reactor at a rate of 5 mL / min. The current of the experimental stage was further manually adjusted so that the pH was maintained at the set point (i.e. 7.5, 8.0, 8.5 or 9.0). Once the reaction reaches a steady state under the corresponding current and / or voltage, the pH of the reactor becomes substantially self-regulated because the total concentration of hydroxide ions is maintained due to the following reactions occurring simultaneously:

[0199] Fe+2H + →Fe 2+ +H2 and CO2+H2O+Fe 2+ →FeCO3+2H +

[0200] Gas samples were taken from the head space of the reactor using a 100 μL syringe every hour for the first 4 h and then every half hour for the last 2 h. Liquid and solid samples were taken every hour for analysis of iron concentration.

[0201] Example 4 - Results

[0202] Following the results shown at pH 7.5 Figure 4 ,exist Figure 5 , Figure 6 and Figure 7 The results from electrochemical feed gas purification at three additional target pH levels (i.e., pH 8.0, pH 8.5, and pH 9.0) are illustrated in FIG. Figure 8 A comparison between different pH levels is provided in which the headspace gas composition (i.e., H2, N2, and CO2), CO2 removal efficiency, total iron concentration, coulombic efficiency, molar ratio of TIC to total iron, iron utilization efficiency, cell voltage, and current are compared. The gas composition and test results are also summarized in Table 2 below.

[0203] Table 2. Summary of CO2 removal results at various pH levels

[0204]

[0205] Compared to the results at pH 7.5, a lower CO2 composition in the headspace gas was achieved, from 76.6±0.2% at pH 7.5 to 6.6±0.5% at pH 8.0, 5.9±0.2% at pH 8.5, and 5.0±0.0% at pH 9.0. This resulted in a corresponding increase in CO2 removal efficiency, as summarized in Table 2 above.

[0206] In contrast, iron utilization efficiency decreases with increasing pH, which is supported by the decrease in the ratio of TIC to total iron in the iron compounds produced. These results support the view from Example 3 that as pH increases, the proportion of ferrous ions that bind to hydroxide rather than carbonate increases. In particular, a sharp drop in iron utilization efficiency is observed between pH 8.5 and pH 9.0. Similarly, the cell voltage and current increase with increasing cell pH.

[0207] Overall, pH 8.5 appeared to be a favorable condition with relatively high CO2 removal efficiency (i.e., 85.1±0.4%) and Coulombic efficiency (i.e., 95.8±1.9%) as well as satisfactory iron utilization efficiency (i.e., 80.1±0.8%).

[0208] Example 5 - Removal of NH3 and H2S from biogas containing methane

[0209] Another test was conducted using a gas containing about 60% CH4 and about 40% CO2 as well as trace amounts of NH3 and H2S as the feed gas to the electrochemical cell. The content of the feed gas essentially simulates the content of raw biogas that can be obtained from a wastewater treatment process. The operating procedure for this test was similar to that of the CO2 removal experiment conducted at pH 8.5 described above, with the only difference being the feed gas.

[0210] Example 5 - Results

[0211] like Fig. 9As depicted in a, CO2 in the raw gas was efficiently removed with a removal efficiency of 87.9 ± 1.3%, while the content of CH4 hardly changed during the electrochemical purification. Fig. 9 g. Fig. 9 h and Fig. 9 Overlaying the results obtained from the electrochemical purification of a methane-containing feed gas as performed in Example 1 with the equivalent tests performed at pH 8.5 in Example 4, the differences across the various performance factors (including purification performance) were insignificant.

[0212] The above results obtained for a methane-containing feed gas indicate that the results obtained in the batch electrochemical purification of a N2-containing feed gas represent the corresponding purification of the raw biogas from a wastewater treatment process. Fig. 9 e and Fig. 9 f shows that toxic NH3 and H2S in the raw gas can also be removed efficiently by the electrochemical method performed at pH 8.5. The average concentrations of NH3 and H2S in the feed gas are 267.5±26.1ppmv and 884.1±63.5ppmv, compared to 54.3±12.3ppmv and 46.2±6.8ppmv in the outlet gas. These represent a 78.7±10.2% reduction in NH3 and a 94.8±6.1% reduction in H2S between the inlet and outlet gases.

[0213] Example 6 - Effect of Gas Flow Rate

[0214] One of them aims to evaluate the effect of gas flow rate on cell performance. The test lasted 9 hours, including a 2-hour preparation phase in which the pH was raised to 8.5 without gas supply, and a 7-hour experimental phase during which the gas flow rate was gradually increased from 2 mL / min (3 hours), 5 mL / min (2 hours), and 10 mL / min (2 hours). Gas samples were obtained every hour for the first 4 hours, and then every half hour for the next 5 hours.

[0215] Example 6 - Results

[0216] The CO2 removal efficiency of the electrochemical system was studied with different feed gas flow rates at pH 8.5. After the transport period following each change in feed gas flow rate, Fig.10 a shows that CO2 concentration increases with gas flow rate from 3.2±0.3% at 2 mL / min to 6.2±0.1% at 5 mL / min, and then to 9.5±0.5% at 10 mL / L. Fig.10 In b, the CO2 removal efficiency decreased significantly from 92.0±0.8% to 83.8±0.4% and finally to 74.0±0.3%.

[0217] The results show that an increase in gas flow rate leads to a decrease in gas residence time and, therefore, a decrease in CO2 reaction time. The reduction in the time for gaseous CO2 to dissolve and react with iron ions leads to a decrease in CO2 removal efficiency. Therefore, it is speculated that CO2 removal from biogas can be maximized by complementary reactor design and gas flow rate, so that a satisfactory gas retention time is achieved.

[0218] Example 7 - Particle Analysis and Characterization

[0219] In this example, according to the present invention, FeCO3 precipitate formed by electrochemical purification of CO2-containing feed gas at pH 8.5 was subjected to particle analysis. XRD analysis was performed on about 50 mL of Fe-containing slurry using an X-ray diffractometer (Bruker D8). Prior to XRD measurement, the Fe-containing slurry was dried under vacuum conditions (-50°C, 0.1 mbar) and then ground into powder under anaerobic conditions to prevent oxidation.

[0220] In addition, particle size measurements were performed using 3 mL samples for freshly produced slurries and slurries stored for 1, 2 and 4 weeks.Particle size was measured using dynamic light scattering (Zetasizer Nano ZS, Malvern Instruments).

[0221] Example 7 - Results

[0222] Fig.11 Figure 2 shows the XRD pattern of the Fe-containing slurry produced at pH 8.5. Three crystalline iron species were identified in the Fe-containing slurry, including siderite (FeCO3), goethite (α-FeO(OH)), and hematite (Fe2O3). 2+ Goethite and hematite were also detected in the slurry, which is consistent with previous results, indicating that the slurry contains some Fe generated during electrolysis in the cell. 3+ ion.

[0223] Fig.12 Figure 2 shows the particle size distribution and suspension properties of FeCO3 precipitates (produced in pH 8.5) at different ages after precipitation. The particles are stable with almost the same particle size distribution profile ( Fig.12 a) and D representing the 10th, 50th, and 90th percentile granularity 10 , D 50 and D 90 value( Fig.12 b). These results indicate that long-term storage of the product does not result in significant particle aggregation or breakage. The results of the settling performance tests confirm that the FeCO3 slurry does not settle substantially under typical turbulent flow conditions in sewers ( Fig.12 c), regardless of their age. These results indicate that after dosing in the sewer, the FeCO3 precipitates remain essentially suspended in the sewer, making sewer dosing upstream of the wastewater treatment plant feasible and effective.

[0224] Example 8 - Sulfide / Phosphate Removal

[0225] In this example, the suitability of FeCO3 produced in biogas upgrading to support municipal wastewater management was investigated in several batches. The FeCO3 slurry collected from the reactor after upgrading the feed gas and / or biogas was dosed directly in batches, simulating a sewer, aerated biological treatment processes, and anaerobic digesters to determine its effect on sulfide and phosphate concentrations.

[0226] Sewer conditions are established by using the wastewater collected from the local domestic wastewater pumping station (Brisbane, Australia). The wastewater initially contains the total COD of 400mg / L-600mg / L, the soluble COD of 220mg / L-310mg / L, the phosphate of 4mg P / L-7mg P / L, the iron of 0.1mg Fe / L-0.3mg Fe / L, the sulfate of 10mg S / L-20mg S / L, the sulfide of 5mg S / L-10mg S / L and the sulfite and thiosulfate of negligible level. The wastewater has the pH of 7.1-7.4 and contains the oxygen of non-detectable level.

[0227] For the simulated sewer tests, a predetermined amount of FeCO slurry was added to each bottle to achieve a pre-designed initial iron concentration. To ensure that there was no gas headspace during the experimental period, two syringes filled with filtered raw oxygen-free wastewater were connected to the reactor to replenish the reactor after sampling.

[0228] All experiments of simulated biological wastewater treatment system were carried out using activated sludge collected from local WWTP (Brisbane, Australia). The mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) concentrations of the collected activated sludge were 13.2 ± 0.1 g / L and 10.6 ± 0.1 g / L, respectively. In the experiments simulating anaerobic digestion, the inoculated digested sludge used was derived from a laboratory anaerobic digestion reactor, wherein the concentrations of MLSS and MLVSS were 20.6 ± 0.1 g / L and 16.3 ± 0.1 g / L, respectively. FeCO slurry obtained via biogas purification at pH 8.5 was used.

[0229] The above batches used both freshly produced FeCO3 (i.e., where the testing was performed within one day of FeCO3 production) as well as FeCO3 slurries stored in further sealed slurry containers in a temperature controlled (22°C ± 1°C) laboratory for 1 week, 2 weeks and 4 weeks to determine the effect of FeCO3 storage on sulfide removal performance.

[0230] Example 8.1 - Removal of sulfide from sewers

[0231] For sulfide removal batch in each sewer, 290mL of the wastewater prepared above was filtered using a disposable microfiltration unit (0.45 μm), and then transferred to a 300mL sealed container. Before the experiment, the container was exposed to pure nitrogen for 30min to further remove dissolved oxygen. Then 5mL of sulfide stock solution (about 1.5g S / L of Na2S·9H2O) was added to the bottle to increase sulfide concentration to about 25mg S / L, followed by addition of 1M HCl to obtain pH to 7.2, simulating domestic wastewater.

[0232] Two levels of initial Fe levels, i.e. 30 mg Fe / L and 90 mg Fe / L, were used in the above-mentioned batches separately. According to the reaction stoichiometry, the initial Fe concentration of 30 mg / L was not enough to remove the sulfide (about 25 mg S / L) initially present in the wastewater, and therefore the ratio between the removed sulfide and the added Fe was determined. In contrast, the initial Fe of 90 mg / L was excessive, and therefore the lowest achievable sulfide concentration was identified.

[0233] Each sulfide removal test lasts for 3h, during which the reactor is mixed at 300rpm by a magnetic stirrer. Before FeCO3 dosing, and in the first hour after dosing, liquid samples are obtained every 15min, and then every 30min, for measuring the concentration of dissolved sulfide. The pH in the reactor is monitored with a portable pH meter and manually recorded at the same time intervals. Additional samples are obtained at the end of each test for measuring total iron concentration.

[0234] Example 8.1 - Results

[0235] Fig.13a shows the results of sulfide removal from sewer-like conditions using a freshly produced FeCO slurry at pH 8.5. By dosing the FeCO slurry at a concentration of 32.2 ± 1.9 mg Fe / L, the dissolved sulfide concentration decreased from an initial 20.8 ± 0.3 mg S / L to 4.6 ± 0.3 mg S / L within 0.5 h and remained at this level for the remainder of the 3 h experiment. The ratio between sulfide removal and dosing Fe was 0.51 ± 0.04 g S / g Fe. This is close to the theoretical stoichiometric ratio of 0.57 mg S / mg Fe (HS - +Fe2 + →FeS+H + ). At the same time, the pH of the wastewater increased from the initial 7.18±0.02 to 7.48±0.03 within 3 h, which was caused by the release of carbonate from FeCO3. The increase in pH can promote the degradation of sulfide and Fe 2+ precipitation and can provide more alkalinity for the downstream nitrification process. This is in contrast to dosing of FeCl2 or FeCl3, which are known to lower the pH of wastewater because their hydrolysis process will release some protons.

[0236] In order to identify the minimum concentration of dissolved sulfide achievable by dosing the FeCO 3 slurry obtained, another sulfide removal batch test was conducted via over dosing of the FeCO 3 slurry. Fig.13 a shows that by quantitatively supplying FeCO3 with a concentration of 88.4 mg Fe / L into wastewater containing 20.1 mg S / L sulfide, the dissolved sulfide concentration dropped sharply to 3.2 mg S / L within 0.5 h, and then dropped to 0.08±0.00 mg S / L within 3 h, and the efficiency of sulfide removal reached 99.5%.

[0237] These results show that by using electrochemically generated FeCO3, sulfide removal to very low levels (<0.1 mg / L) can be achieved under overdosing conditions, which is comparable to the sulfide removal performance of FeCl2 and FeCl3 used in the art. Fig.14 As shown in Fig. e, using the same test time period, the pH of the wastewater increased from 7.17 to 7.40 and then to 7.53. One of the reasons for achieving low sulfide levels is the increase in pH, which favors the precipitation of iron (II) sulfide, as well as sulfide precipitation.

[0238] like Fig.13 b to Fig.13 As shown in d, the FeCO slurries obtained after storage for 1, 2 and 4 weeks showed similar sulfide removal performance as the freshly purified batches ( Fig.13a). The results showed that after 1, 2 and 4 weeks of storage, the FeCO3 slurry could still remove most of the sulfide within one hour, and the ratio between sulfide removal and the quantitatively supplied Fe was 0.46±0.02g S / g Fe, 0.51±0.03g S / g Fe and 0.48±0.02g S / g Fe, respectively. It was also shown that the FeCO3 pellets stored for 4 weeks could still effectively reduce the sulfide concentration to less than 0.1mg S / L, while also increasing the wastewater pH from about 7.2 to about 7.5 to facilitate the reduction of sulfide (still in Figure 2). Fig.14 (see in ).

[0239] Overall, regardless of the storage period, FeCO3 particles produced from electrochemical biogas purification are effective in removing sulfides and simultaneously providing alkalinity for the downstream nitrification process.

[0240] Example 8.2 - Phosphate Removal

[0241] For each phosphate removal test, 100mL of activated sludge was mixed with 400mL of filtered wastewater, which was identical to the wastewater used in the sulfide removal test. Then 1.5mL of phosphate stock solution (5gP / L of KH PO ) was added to the sludge and wastewater mixture to increase phosphate concentration to about 20mg P / , after which the phosphate-containing wastewater was divided into test batches. About 0.8mL and about 3.5mL of FeCO slurry (about 10g Fe / L) was quantitatively supplied to each batch to obtain two levels of initial Fe concentration, namely about 16mg and about 70mg Fe / L, respectively, and a control batch without any quantitatively supplied iron.

[0242] Each test run 6h, during this period, by programmable logic controller (PLC) via the on / off control of air flow the DO concentration of reactor is controlled at 2.0mg O2 / L-3.0mg O2 / L.By magnetic stirrer with 300rpm mixed reactor.In the program similar to sulfide removal, before FeCO3 quantitative supply, obtain liquid sample, and obtain liquid sample every 0.5h in the first two hours, and then obtain per hour, for measuring phosphate concentration.The pH of reactor is monitored with portable pH meter, and is manually recorded at the same time interval.At the end of each test, also obtain other sample for measuring total iron concentration.

[0243] Example 8.2 - Results

[0244] As in Fig.15As seen in Figure 3, after adding 24.3±2.1 mg Fe / L of FeCO3, the measured dissolved phosphate concentration decreased from 19.2±0.1 mg P / L to 9.7±0.3 mg P / L within 1 hour, and then decreased to 6.7±0.3 mg P / L within 6 hours. The results indicate that the reduction in phosphate concentration is caused by the dosing of FeCO3. The ratio between phosphate removal and dosing of Fe was 0.56 g P / g Fe±0.02 g P / g Fe, while the pH values ​​of the control reactor and the experimental reactor were 7.43±0.02 and 7.86±0.02, respectively.

[0245] By overdosing an electrochemically produced FeCO 3 slurry with a concentration of 92.3±3.4 mg Fe / L into an activated sludge reactor containing 21.4±0.3 mg P / L phosphate, the minimum achievable phosphate concentration was identified to be 1.41±0.08 mg P / L.

[0246] Example 8.3 - Removal of Sulfide from Anaerobic Digester

[0247] For each test of sulfide removal from an anaerobic digester, 50 mL of inoculated digested sludge was added to a 100 mL sealed bottle and then deoxygenated with pure nitrogen for 10 min. 1.0 mL of sulfide stock solution (about 1.5 g S / L of Na2S·9H2O) was added to the bottle to increase the sulfide concentration to about 30 mg S / L, followed by the addition of some 1 M HCl to achieve a pH of 7.5, which is typical for anaerobic digestion.

[0248] About 0.15 mL and 0.45 mL of FeCO 3 slurry (approximately 10 g Fe / L) were then dosed into the sludge batches to increase the initial iron concentration to approximately 30 mg Fe / L and 90 mg Fe / L, respectively, while a control batch remained undosed.

[0249] Each test lasts for 3h, during which the reactor is mixed at 300rpm by a magnetic stirrer. Before FeCO3 dosing, and after dosing, sludge samples were obtained every 15min in the first hour, and then every 30min, for measuring dissolved sulfide concentration. The pH in the reactor was monitored with a portable pH meter and manually recorded at the same time intervals. Additional samples were obtained at the end of each test for measuring total iron concentration.

[0250] Example 8.3 - Results

[0251] like Fig.16As shown, the addition of 32.2 ± 1.9 mg Fe / L of FeCO3 slurry for half an hour resulted in a decrease in the dissolved sulfide concentration from an initial 26.9 ± 0.5 mg S / L to 11.6 ± 1.4 mg S / L, and maintained at this concentration for the remaining 2.5 hours. The successful removal of sulfide from the sludge resulted in a ratio of sulfide removal to the dosed Fe of 0.41 ± 0.03 gS / g Fe. By over dosing FeCO3 at a concentration of 95.5 ± 3.8 mg Fe / L, a minimum achievable dissolved sulfide concentration of 1.41 ± 0.08 mg S / L was observed.

[0252] Conclusion - Example 8

[0253] Overall, the tests showed that electrochemically generated FeCO3 slurry containing FeCO3 particles can support municipal wastewater management with comparable efficiency to the addition of commonly used iron salts containing soluble iron ions, particularly in controlling sulfide in sewers and anaerobic digesters, and phosphate in biological wastewater treatment systems. The in-situ generation of FeCO3 slurry by the electrochemical biogas purification process allows any such integrated WWTP system to avoid the otherwise required expensive and hazardous transportation of corrosive concentrated iron salt solutions.

[0254] Example 9 - FeCO3 Flow Effect

[0255] An urban wastewater system is an integrated system comprising a sewer network followed by an aerated biological wastewater treatment unit and an anaerobic waste sludge digestion unit. In this example, the effect of dosing FeCO 3 slurry to the sewer network on the performance of the downstream biological wastewater treatment unit and the waste sludge digestion unit was investigated through a series of batch experiments.

[0256] This example utilized a procedure similar to that of the sewer sulfide removal test of Example 8.1, except that only one-day-old FeCO slurry was used under under-dosing conditions in order to avoid residual iron effects on downstream performance of the wastewater treatment system. The initial sulfide and Fe concentrations in this test were approximately 18 mg S / L and 20 mg Fe / L, respectively.

[0257] After the sulfide removal test of Example 8.1, all the remaining 300 mL of FeCO slurry-modified sewage was mixed with 300 mL of activated sludge, which was prepared by mixing 150 mL of original activated sludge with original wastewater at a ratio of 1: 1 (v / v). Then 3.0 mL of phosphate stock solution (5 g P / L of KH2PO4) was added to the bottle to increase the phosphate concentration to about 25 mg P / L. Each test lasted 6 h, during which the same mixing and aeration conditions and sampling procedures as those described in Example 8.2 for the phosphate removal test were applied.

[0258] The effect of dosing FeCO slurry in sewer on anaerobic digestion performance was also studied in three steps, namely, sulfide removal in sewer, phosphate removal during aerated wastewater treatment, and sulfide control in anaerobic digestion. The first two steps are similar to the steps of the method tested in Example 8, however the initial sulfide, Fe and phosphate concentrations in this test were much higher than those of Example 8.2 and Example 8.3, using concentrations of approximately 200 mg S / L, 300 mg Fe / L and 300 mg P / L, respectively, in order to simulate the inevitable accumulation of Fe dosing in sewer in the sludge of a simulated WWTP.

[0259] To simulate an aerated clarifier located within a WWTP and downstream of sulfide removal in the sewer, the mixed liquor from sulfide and phosphate removal in the sewer was concentrated by centrifuge at 2500 rpm for 3 min. With the supernatant discarded, the concentrated sludge (about 15 g VS / L) was used as feed for the biochemical methane potential (BMP) test to measure the biogas output from the anaerobic digester.

[0260] In the third step, the influence of the sulfide control in anaerobic digestion of FeCO3 quantitatively supplied in the sewer is evaluated by the BMP test, thus measuring the sulfide concentration in the biogas and liquid phase BMP reaction container. In this test, about 20mL of the activated sludge (from step II) of thickening is mixed with the digested sludge of about 40mL inoculation, and then transferred to a 100mL sealed container. The feed of the digested sludge of about 20mL wastewater and about 40mL inoculation is also used to establish a blank test. After deoxygenation with nitrogen, 1.0mL of sulfate stock solution (Na2SO4 of about 1.5g S / L) is added to the container to increase the sulfate concentration to about 25mg S / L, and 1M HCl is subsequently added to adjust the pH of the bottle to about 7.5, which is typical of an anaerobic sludge digester. Once quantitatively supplied to the initial conditions of the BMP test, all containers are cultivated in a temperature-controlled (37°C ± 1°C) incubator for anaerobic digestion of sludge.

[0261] The biogas released from the containers was measured over 30 days, at which time almost no further biogas release was detected. Gas samples were taken every two days for the first 10 days and every five days towards the end to measure the N2, CH4 and CO2 composition of the biogas. The volume of biogas in each sealed container was also measured at the same time intervals. Gas samples and liquid samples were taken every 5 days for measurement of sulfide / sulfur species and concentrations. Additional sludge samples were also taken at the end of each test to measure the dewaterability of the sludge.

[0262] The effect of FeCO3 dosing on the settling properties of activated sludge and the dewatering capacity of anaerobic digested sludge was measured by comparison with sludge without FeCO3 dosing (i.e., control). Both SVI and SRF, respectively, are indicators of the settling properties and dewatering capacity of sludge, measured using methods known in the art, or are standards in check water and wastewater. In this regard, SRF was analyzed by using a polycoupled measuring device known in the art.

[0263] Example 9 - Results

[0264] As shown in Figure 17a, by dosing a FeCO slurry with a concentration of 26.3 ± 0.7 mg Fe / L, the dissolved sulfide concentration in the simulated sewer system dropped from 16.9 ± 0.2 mg S / L to 4.4 ± 0.3 mg S / L. Although the magnitude was small, the phosphate concentration also slowly decreased from 5.3 ± 0.1 mg P / L to 5.0 ± 0.1 mg P / L during this period.

[0265] In the following aerated biological wastewater treatment process, as shown in Figure 17b, the dissolved sulfate concentration increased from 5.9±0.3 mg S / L to 17.1±0.4 mg S / L within 6 hours, while the phosphate concentration decreased from 19.8±0.5 mg P / L to 4.7±0.3 mg P / L during the same time period.

[0266] The decrease in sulfide concentrations indicates that FeS particles are formed under anaerobic sewage conditions ( Fig.17A ). Increase in sulfate concentration during aerated biological wastewater treatment ( Fig. 17B ) indicates the oxidation of FeS formed under sewage conditions. Sulfides are oxidized to harmless dissolved sulfates, while the released iron ions are precipitated in the sludge together with phosphates as insoluble iron-phosphate-hydroxide complexes, resulting in the removal of dissolved phosphates. By comparison, the concentration profile of nitrogenous compounds in the vessels of the simulated bioreactor remained similar to that of the control, as shown in Fig.18 a and Fig.18This means that the dosing of FeCO3 slurry does not affect the nitrification process.

[0267] The dissolved sulfide concentration and gaseous sulfide concentration of the BMP sealed container were much lower than those of the control. Fig.19 , which illustrates the concentration profile during the subsequent anaerobic digestion. The average dissolved sulfide concentration and gaseous sulfide concentration in the experimental reactor are 1.8 ± 0.2 mg S / L and 82.5 ± 23.6 ppmv, which contrasts with 29.1 ± 0.6 mg S / L and 1123.7 ± 134.5 ppmv in the control reactor. These represent a relative reduction of 94.2 ± 3.1% and 92.6 ± 8.3% between the dissolved sulfide concentration and gaseous sulfide concentration of the experimental and control vessels. These results suggest that the iron in the sludge containing the iron-phosphate complex is regenerated in the anaerobic digester, causing sulfide precipitation, thereby significantly reducing dissolved sulfide concentration and gaseous sulfide concentration.

[0268] Similar profiles of methane accumulation in the headspace of the experimental and control reactors were observed, indicating that dosing of FeCO 3 slurry did not affect the methane production performance.

[0269] also, Fig. 20 The results presented in also show that the sedimentation and dewatering ability of the sludge are significantly enhanced by the quantitative supply of FeCO3 slurry. The SVI and SRF of the sludge from the control reactor were 119.0±4.8mL / g and 2.3±0.2 (1013m / kg), respectively. In the sludge from the experimental container, these values ​​were 75.1±4.1mL / g and 1.4±0.1 (1013m / kg), respectively. This means that by quantitatively supplying FeCO3 slurry, the sedimentation and dewatering ability of the sludge were increased by 36.9±2.7% and 39.1±4.5%, respectively.

[0270] Finally, the sulfide removal and biogas production characteristics of the anaerobic digester under rationing in the sewer were investigated ( Fig.19 ) were compared with the results of direct FeCO3 dosing from the digester, which Fig.21 Overall, it is clear that Fig.21 Compared to direct dosing as seen in the anaerobic digester, upstream sewer dosing of FeCO3 has minimal to no adverse effect on biogas production. Furthermore, considering that FeCO3 is useful in suppressing both sulfide and phosphate concentrations upstream of the anaerobic digester, dosing in the sewer is clearly beneficial and preferred.

[0271] Example 10 - Application in wastewater treatment equipment

[0272] Taking into account the above-mentioned flow benefits of dosing FeCO3 into a process or vessel upstream of the anaerobic digester itself, both examples of the present invention are feasible and effective.

[0273] Fig. 22 The first example illustrated in shows the delivery of an electrochemically generated FeCO 3 slurry from a cell to a sewer network upstream of a wastewater treatment plant (WWTP). Based on the flow effects observed in Example 9, such injection of the slurry will provide the full benefits of sulfide and phosphate inhibition through the WWTP.

[0274] The further mass balance analysis of this embodiment shows that the minimum FeCO in the electrochemical cell produces enough to provide significant sulfide removal benefits. In doing so, theoretically achievable iron dosage and sulfide removal capabilities are calculated based on three different influent situations (that is, low-concentration wastewater, medium-concentration wastewater and high-concentration wastewater). The result shows that when the influent only contains 200mg / L of COD-which is enough to eliminate the sulfide concentration (<10mg / L) in typical sewage, the sulfide removal capability can reach 10.0mg S / L. In the case of the influent COD concentration of 500mg / L and 800mg / L, theoretical sulfide removal capabilities can reach 24mg S / L and 39mg S / L respectively.

[0275] As shown in Example 9, upstream dosing of iron salts can significantly reduce phosphate concentrations in the effluent of the biological treatment reactor, significantly reduce H2S concentrations in the biogas of the AD, and substantially improve the settling and dewatering capabilities of the sludge. Test results confirm that, in this regard, dosing of electrochemically generated FeCO3 slurry can produce and maintain these flow benefits to the downstream wastewater treatment process while also increasing the pH of the wastewater to facilitate nitrification of the sludge.

[0276] Considering that nitrification in biological wastewater treatment reactors and biogas production in anaerobic digesters are not adversely affected by sewer dosing of purified electrochemically produced FeCO3 from said biogas, the electrochemical system of the present invention provides an efficient and integrated system capable of providing synergistic benefits. By producing iron salts on-site, which reduces or eliminates the need to transport materials to the site, the electrochemical FeCO3 method is more cost-effective than existing methods for delivering iron ions to wastewater treatment equipment, while also being able to purify biogas from anaerobic digesters.

[0277] exist Fig.23 In another aspect of the invention illustrated in , the FeCO 3 slurry is provided directly to various unit operations, including before the aerated bioreactor, before the secondary settler and / or the anaerobic digester, for mixing with activated sludge in each operation.

[0278] As shown in Example 8, direct dosing of iron salts into the sludge can significantly reduce the phosphate and sulfide concentrations in the wastewater / sludge of the corresponding unit process. Fig. 20 As a result of a, with regard to the dosing of FeCO3 slurry before and after the bioreactor, an improved settling property of the sludge was also achieved during the secondary settling / clarification.

[0279] Similarly, providing the FeCO3 slurry directly to the anaerobic digester, in addition to reducing the emission of H2S gas, also reduces the sulfide concentration of the digested sludge and improves its dewaterability for enhanced disposal during digestion. The above benefits are substantially the same as those of Examples 8.3 and Fig. 20 The benefits are consistent with those disclosed in b.

[0280] Those skilled in the art will appreciate that the invention described herein is susceptible to changes and modifications other than those specifically described. It should be understood that the invention includes all such changes and modifications that fall within the spirit and scope of the invention.

Claims

1. A method for removing a portion of at least one contaminant gas from a first gas stream, the method comprising the steps of: a) applying a voltage across a pair of electrically connected electrodes at least partially in contact with a working fluid in an electrochemical cell to generate a plurality of ions; as well as b) reacting at least a portion of the at least one pollutant gas with the plurality of ions in the working fluid to convert the portion of the at least one pollutant gas into one or more reaction products, thereby isolating at least some of the pollutant gas in the pollutant gas from the first gas stream to produce a second gas stream, The pH of the working fluid is substantially maintained at a predetermined set point to improve the efficiency of the conversion.

2. The method of claim 1, wherein the predetermined set point is maintained by adjusting at least one of: a flow rate of the first gas flow; a voltage; and a current applied across electrodes.

3. The method according to claim 1 or 2, wherein the predetermined set point falls within the range of pH 7.5 to pH 9.0, preferably pH 8.0 to pH 9.0, and even more preferably around pH 8.5, or wherein the voltage across the electrodes is adjusted between the range of 0.5 V and 50.0 V, preferably between 1.0 V and 10.0 V, and even more preferably between 2.0 V and 5.0 V.

4. The method according to any one of the preceding claims, wherein at least one of the electrodes comprises a metal selected from the group consisting of iron (Fe), magnesium (Mg), zinc (Zn), nickel (Ni), copper (Cu), aluminum (Al), titanium (Ti), or alloys thereof.

5. A method according to any one of the preceding claims, wherein at least one of the electrodes comprises iron (Fe) and the at least one pollutant gas is carbon dioxide (CO2), wherein the one or more reaction products formed comprise a slurry of iron (II) carbonate (FeCO3) separated from the working fluid.

6. The method of claim 5, wherein the second gas stream has less than 30 wt.%, 20 wt.%, 10 wt.%, 5 wt.% or 1 wt.% CO2.

7. A method according to any one of claims 1 to 3, wherein at least one of the electrodes comprises iron (Fe) and the at least one contaminant gas is hydrogen sulfide (H2S), wherein the one or more reaction products formed comprise a slurry of iron (II) sulfide (FeS) separated from the working fluid.

8. The method of claim 7, wherein the second gas stream has less than 1000 ppmv, 500 ppmv, 300 ppmv, 200 ppmv, 100 ppmv, or 10 ppmv H2S.

9. The method according to any one of claims 1 to 3, wherein the at least one pollutant gas is ammonia (NH3), and wherein the one or more reaction products formed comprise aqueous ammonium (NH4 + ), removing at least some of the aqueous ammonium from the working fluid by purging at least a portion of the working fluid from the electrochemical cell.

10. The method of claim 9, wherein the second gas stream has less than 1000 ppmv, 100 ppmv, 10 ppmv, or 1 ppmv NH3.

11. A method according to any one of the preceding claims, wherein at least a portion of the second gas stream is recycled back into the first gas stream.

12. The method according to any one of the preceding claims, wherein the first gas stream comprises at least one pollutant gas selected from the group consisting of CO2, H2S, NH3, NOx and halogens.

13. The method according to any one of the preceding claims, wherein the first gas stream is biogas, and wherein the biogas is produced during anaerobic digestion of organic matter, wherein the organic matter is derived from the group consisting of sewage / wastewater, agricultural waste, municipal waste, manure, plant material, green waste and food waste.

14. A method according to any one of the preceding claims, wherein the working fluid comprises an electrolyte solution of an alkali metal salt or an alkaline earth metal salt.

15. The method according to claim 14, wherein the alkali metal salt or alkaline earth metal salt is selected from the group consisting of alkali metal salts or alkaline earth metal salts consisting of sodium (Na), potassium (K), lithium (Li), magnesium (Mg), calcium (Ca) or mixtures thereof.

16. The method according to any one of the preceding claims, wherein the electrolyte is an aqueous NaCl solution, preferably the concentration of the NaCl solution is between about 0.1% to about 5%, preferably in the range of about 0.5% to about 3%, and even more preferably in the range of about 1% to about 2%.

17. The method according to any one of the preceding claims, further comprising the following steps before step a) or before step b): a1) fluidly communicating at least a portion of the first gas stream having the at least one pollutant gas from a gas source to the electrochemical cell.

18. The method according to any one of the preceding claims, wherein step b) comprises fluidly connecting at least a portion of the first gas stream having the at least one pollutant gas with at least a portion of the working fluid in a gas-liquid reactor, the gas-liquid reactor being in cyclic fluid communication with the electrochemical cell, Wherein a plurality of electrochemically generated ions from the electrochemical cell and the at least one pollutant gas are brought into fluid contact in the gas-liquid reactor.

19. The method of claim 18, wherein the gas-liquid reactor is selected from a gas scrubber and a bubble column, wherein the plurality of electrochemically generated ions are brought into fluid contact with at least a portion of the first gas stream in a countercurrent arrangement.

20. A second gas stream produced by a method according to any one of the preceding claims, wherein the second gas stream has a lower concentration of the at least one contaminant gas than the first gas stream.

21. Use of the second gas stream as claimed in claim 21, including as a transport fuel, as a fuel for injection into a natural gas grid to supplement the natural gas supply, for further purification and / or separation of its component gases.

22. A system for removing a portion of at least one contaminant gas from a first gas stream, the system comprising: an electrochemical cell comprising an electrically connected pair of electrodes at least partially in contact with a working fluid, wherein, when a voltage is applied across the electrodes, a plurality of ions are electrochemically generated in the working fluid for reacting with at least a portion of the at least one contaminant gas to convert the portion of the at least one contaminant gas into one or more reaction products, thereby isolating at least some of the contaminant gas in the contaminant gas from the first gas stream, thereby generating a second gas stream, The pH of the working fluid is substantially maintained at a predetermined set point to improve the efficiency of the conversion.

23. A method for treating sewage or wastewater, comprising the steps of: Obtaining sewage, wastewater and / or sludge derived therefrom; fluidly communicating at least a portion of the sewage, wastewater, and / or sludge derived therefrom to an anaerobic digester to produce a first gas stream comprising at least one pollutant gas; Fluidly connecting at least a portion of the first gas stream to The system of claim 22, to convert at least a portion of the at least one pollutant gas into one or more reaction products; and adding at least a portion of the one or more reaction products to one or more of: (a) the sewage and / or wastewater, (b) said sludge derived from said sewage and / or wastewater, or (c) the anaerobic digester, by: (i) reacting with one or more contaminants therein to facilitate at least partial removal of said contaminants therefrom; and / or (ii) enhancing the settling and / or dewatering performance of the sludge.

24. A method for treating sewage or wastewater, comprising the steps of: When produced by the process according to claim 5, sewage, wastewater and / or sludge derived therefrom are treated with iron (II) carbonate (FeCO3) or iron (II) carbonate (FeCO3) to: (i) reacting with one or more contaminants therein to facilitate at least partial removal of said contaminants therefrom; and / or (ii) enhancing the settling and / or dewatering performance of the sludge.

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