GAS TREATMENT PROCESS

Abstract

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

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 an electrically connected pair of electrodes at least partially in contact with a working fluid in an electrochemical cell to generate a plurality of ions; and b) reacting at least a portion of the at least one contaminant gas with the plurality of ions in the working fluid to convert the portion of the at least one contaminant gas to one or more reaction products, thereby sequestering at least some of the contaminant gas from the first gas stream to produce a second gas stream, wherein the pH of the working fluid is substantially maintained at a predetermined set point for improving the efficiency of the conversion, and wherein the first gas stream comprises at least one contaminant gas selected from the group consisting of CO.sub.2, H.sub.2S, NH.sub.3, NOx, and a halogen.

2. The method according to claim 1, wherein the predetermined set point is maintained by adjusting at least one of: the flowrate of the first gas stream; the voltage; and current applied across the 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, and wherein the voltage across the electrodes is adjusted between a range of 0.5 V and 50.0 V.

4. The method according to claim 2, 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), Aluminium (Al), titanium (Ti), or an alloy thereof.

5. The method according to claim 3, wherein at least one of the electrodes comprises iron (Fe) and the and the first gas stream comprises at least one contaminant gas that is carbon dioxide (CO.sub.2), wherein the one or more reaction products formed comprises a slurry of iron (II) carbonate (FeCO.sub.3), which is separated from the working fluid; and wherein the second gas stream has less than 30 wt. % CO.sub.2.

6. (canceled)

7. The method according to claim 3, wherein at least one of the electrodes comprises iron (Fe) and the first gas stream comprises at least one contaminant gas that is hydrogen sulfide (H.sub.2S), wherein the one or more reaction products formed comprises a slurry of iron (II) sulfide (FeS), which is separated from the working fluid, and wherein the second gas stream has less than 1000 ppmv H.sub.2S.

8. (canceled)

9. The method according to claim 3, wherein the first gas stream comprises at least one contaminant gas that is ammonia (NH.sub.3), and wherein the one or more reaction products formed comprises aqueous ammonium (NH.sub.4.sup.+), at least some of which is removed from the working fluid by purging at least a portion thereof from the electrochemical cell, and wherein the second gas stream has less than 1000 ppmv NH.sub.3.

10. (canceled)

11. The method according to claim 2, wherein at least a portion of the second gas stream is recycled back into the first gas stream.

12. (canceled)

13. The method according to claim 4, wherein the first gas stream is biogas comprising at least one contaminant gas selected from the group consisting of CO.sub.2, H.sub.2S, NH.sub.3, NOx, and a halogen, and wherein the biogas is generated during anaerobic digestion of organic matter, wherein the organic matter is derived from the group consisting of sewage/wastewater, agricultural wastes, municipal wastes, manure, plant materials, green wastes and food wastes.

14. The method according to claim 4, wherein the working fluid comprises an electrolyte solution of an alkali metal or alkaline earth metal salt.

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

16. The method according to claim 15, wherein the electrolyte is an aqueous NaCl solution, and wherein the concentration of the NaCl solution is between about 0.1% to about 5%.

17. The method according to claim 11, further comprising, prior to step a) or prior to step b), the step of: a1) fluidly communicating at least a portion of the first gas stream having the at least one contaminant gas from a gas source to the electrochemical cell.

18. The method according to claim 17, wherein step b) comprises fluidly communicating at least a portion of the first gas stream having the at least one contaminant gas, with at least a portion of the working fluid in a gas-liquid reactor in circulating fluid communication with the electrochemical cell, wherein the plurality of electrochemically generated ions from the electrochemical cell and the at least one contaminant gas are brought into fluidic contact in the gas-liquid reactor.

19. The method according to claim 18, wherein the gas-liquid reactor is selected from a gas scrubber and a bubbling column, wherein the plurality of electrochemically generated ions and at least a portion of the first gas stream are brought into fluidic contact in a counter-current arrangement.

20-21. (canceled)

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 is 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 to one or more reaction products, thereby sequestering at least some of the contaminant gas from the first gas stream thereby to produce a second gas stream, wherein the pH of the working fluid is substantially maintained at a predetermined set point for improving the efficiency of the conversion.

23. (canceled)

24. A process for treating sewage or wastewater, comprising the step of: treating sewage, wastewater, and/or a sludge derived therefrom, with iron (II) carbonate (FeCO.sub.3), or iron (II) carbonate (FeCO.sub.3) when produced by the method according to claim 5, to: (i) react with one or more contaminants therein to facilitate at least the partial removal of the contaminant(s) therefrom; and/or (ii) enhance the settling and/or dewatering performance of the sludge.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0125] The invention will now be described, by way of example with reference to the accompanying drawings, in which:

[0126] FIG. 1 is a process flow schematic of an exemplary embodiment of the present invention utilising a single-stage reaction vessel;

[0127] FIG. 2a is a process flow schematic of a preferred embodiment of the present invention, comprising a two-stage reaction scheme across an electrochemical cell and a bubbling column. FIG. 2b shows a similar two-stage reaction scheme with a scrubbing column replacing the bubbling column of FIG. 2a;

[0128] FIG. 3 is a simplified process flow diagram of one embodiment of the present invention wherein a gas feed including a contaminant gas and N.sub.2 as a proxy for CH.sub.4 was electrochemically treated to produce an enriched gas stream;

[0129] FIG. 4 is a graphical representation of test results obtained from electrochemically purifying a feed gas including N.sub.2 and CO.sub.2 at pH 7.5;

[0130] FIG. 5 is a graphical representation of test results obtained from electrochemically purifying a feed gas including N.sub.2 and CO.sub.2 at pH 8.0;

[0131] FIG. 6 is a graphical representation of test results obtained from electrochemically purifying a feed gas including N.sub.2 and CO.sub.2 at pH 8.5;

[0132] FIG. 7 is a graphical representation of test results obtained from electrochemically purifying a feed gas including N.sub.2 and CO.sub.2 at pH 9.0;

[0133] FIG. 8 is a graphical representation of test results obtained from electrochemically purifying a feed gas including N.sub.2 and CO.sub.2 at various pH levels;

[0134] FIGS. 9a to 9f are a graphical representation of test results obtained from electrochemically purifying a feed gas similar to raw biogas at pH 8.5. FIGS. 9g to 9i are a graphical comparison of results obtained from electrochemically purifying feed gases predominantly comprising CH.sub.4 and N.sub.2, showing N.sub.2 can be used as a proxy for CH.sub.4;

[0135] FIG. 10 is a graphical representation of test results obtained from electrochemically purifying a feed gas including N.sub.2 and CO.sub.2 at various feed gas flow levels;

[0136] FIG. 11 is a graphical representation of the XRD crystallography profile of the FeCO.sub.3 particles electrochemically produced from purifying a feed gas at pH 8.5;

[0137] FIG. 12 is a graphical representation of particle size distribution and settleability of various FeCO.sub.3 particles electrochemically produced at pH 8.5 and stored for varying amounts of time;

[0138] FIGS. 13a to 13d are a graphical representation of the effects to sulfide concentrations in wastewater when FeCO.sub.3 slurries of varying age are dosed in varying concentrations. FIGS. 13e and 13f are graphical representations of the effects on wastewater pH when FeCO.sub.3 slurries of varying age are under- and over-dosed, respectively;

[0139] FIG. 14 is a graphical representation of the effects on wastewater pH from dosing a FeCO.sub.3 slurry to wastewater containing sulfide in a sewer-like condition;

[0140] FIG. 15 is a graphical representation of the effects from dosing a FeCO.sub.3 slurry to aerated sludge containing phosphate in a simulated biological wastewater treatment condition;

[0141] FIG. 16 is a graphical representation of the effects from dosing a FeCO.sub.3 slurry to sludge containing sulfide in a simulated anaerobic digestor;

[0142] FIGS. 17a and 17b are graphical representations of the effects from dosing a FeCO.sub.3 slurry to sewer-like conditions and the flow-on effect on aerated biological treatment, respectively, in a simulated wastewater treatment plant;

[0143] FIGS. 18a and 18b are a graphical comparison of the conversions of nitrogenous compounds in an aerated sludge receiving wastewater without FeCO.sub.3 dosing (FIG. 18a) and with FeCO.sub.3 dosing (FIG. 8b);

[0144] FIG. 19 is a graphical representation of the flow-on effects to a simulated anaerobic digestor from dosing a FeCO.sub.3 slurry in a simulated sewer;

[0145] FIG. 20a is a graphical comparison of the settleability, measured as the sludge volume index (SVI), of the wastewater sludge with and without FeCO.sub.3 dosing, and FIG. 20b is a graphical comparison of the dewaterability, measured as the suction resistance force (SRF), of the anaerobically digested sludge, with and without FeCO.sub.3;

[0146] FIG. 21 is a graphical representation of the effects to a simulated anaerobic digestor from dosing a FeCO.sub.3 slurry directly thereto;

[0147] FIG. 22 is a process flow diagram of one embodiment of the present invention showing a wastewater treatment plant including a cell for purifying raw biogas from an anerobic digestor and dosing FeCO.sub.3 slurry produced therefrom in the sewer network leading to said plant; and

[0148] FIG. 23 is a process flow diagram of another embodiment of the wastewater treatment plant wherein the FeCO.sub.3 electrochemically produced from purifying anaerobically digested biogas is directly dosed throughout several unit operations of the plant.

DETAILED DESCRIPTION OF THE INVENTION

[0149] The skilled addressee will understand that the invention comprises the embodiments and features disclosed herein as well as all combinations and/or permutations of the disclosed embodiments and features.

Example 1Single-Stage Purification

[0150] Referring initially to one exemplary embodiment of the invention shown in FIG. 1, there is provided a system for purifying a gas stream comprising biogas from an anaerobic digestor, by removing at least one contaminant gas, CO.sub.2, H.sub.2S and NH.sub.3, by precipitating/ionising said contaminant gas with electrochemically produced iron (Fe) ions.

[0151] The electrochemical cell 1 produces an electrical potential between the anode 2 and the cathode 3 using an electrical supply 4. The cell 1 produces Fe.sup.2+ ions at the anode 2 by sacrificially oxidising the iron-comprising anode, and H.sub.2 and OH at the cathode 3 by splitting water comprising the electrolyte 5. Biogas is compressed by a compressor 6 and introduced into the cell bubblers 7 to forming fine, or micro- or nano-scale bubbles 8. The up-travelling gas bubbles 8 contact with the electrolyte 5 containing Fe.sup.2+ and OH.sup., where contaminant gases, predominated by CO.sub.2, in the gas bubbles diffuse into the electrolyte 5, forming ions including bicarbonate, carbonate, sulfide and ammonium in alkali pH conditions.

[0152] The dissolved ions in the electrolyte 5 precipitate with Fe.sup.2+ to form insoluble iron salts including FeCO.sub.3, which settle to the raked base 9 of the cell 1 in the form of a slurry and is subsequently removed from the base 9. The slurry is thickened in a liquid-solids separator 10, with the thickened slurry transported for dosing operations on-site or stored in a suitable vessel for further use. The electrolyte 5 that is removed from the slurry in the liquid-solids separator is recirculated back into the electrolyte replenishment feed 11 for the cell 1.

[0153] The remaining CH.sub.4, now substantially comprising the biogas bubbles 6, has a low solubility in the electrolyte 5, and thus remains entrained in the bubbles traveling upwardly to enter the headspace 12 of the cell 1 along with the H.sub.2 gas formed at the cathode 3. The substantially enriched biogas top-product is removed from the headspace 8 to be pumped through or stored in suitable vessels.

[0154] In optional embodiments where the enriched biogas in headspace 12 still contains CO.sub.2 above the desired level, at least a portion of the enriched biogas is recirculated back to the gas bubbler 7 in a gas recycle stream 13 for further purification. The electrolyte 5 of the cell 1 can be similarly recirculated in an optional embodiment comprising an electrolyte recycling stream 14.

[0155] In this optional embodiment, both recycling streams join with their respective biogas and electrolyte feed streams to moderate the incoming concentrations of each feed and improve efficiency of the unit's steady state operations. Both or either of the streams can be opened or closed in this optional embodiment, their operations responsive to manual user intervention or via electronic control inputs from a controller such as a PID controller.

[0156] In another optional embodiment, the cell 1 includes an electrolyte purge stream 15 to purge a portion of the recirculating electrolyte separated from the slurry in the liquid-solid separator 10. The purge stream 15 is substantially adapted to remove at least a portion of the electrolyte containing non-sedimentary precipitates and/or soluble matters of other gas contaminants from the biogas feed stream, including sulfide such as FeS and ammonium salts formed in the cell 1. These precipitates tend to be smaller and less sedimentary, resulting in difficulties removing them from the electrolyte 5. When opened by manual inputs or electronic controller inputs, the purge stream removes at least a portion of the electrolyte 5, with it removing dissolved salts and the smaller non-sedimentary precipitates from further circulation in the system.

Example 2Two Stage Purification

[0157] In certain embodiments of the present invention, there is provided a two-stage process for: 1) electrochemically producing ions; and 2) purifying the contaminated gas feed using said ions. These two stages are performed in separate reaction vessels using a variety of methods to contact the gas feed and the dissolved electrochemically produced ions. The contacting methods include, but are not limited to, gas bubbling and/or mist-based gas scrubbing.

Example 2.1

[0158] Referring to FIG. 2A, the system 100 comprises an electrochemical cell 101 comprising a sacrificial iron-containing anode 102 and a cathode 103 in electrical communication with an electrical supply 104 providing an electrical potential to the electrolyte 105. The cell 101 produces metallic ions such as Fe.sup.2+ at the sacrificial anode 102, while the electrolyte is reduced at the cathode 103 to form H.sub.2 gas and OH.sup. ions. The electrochemically produced ions remain in the electrolyte 105, while the H.sub.2 gas bubbles through the electrolyte to the headspace 106 of the cell 101 to be subsequently pumped out of the cell for storage or to be reused.

[0159] In this embodiment, the cell 101 is in fluid communication with a bubbling column 107 filled with electrolyte 105 and comprising a bubbler 108 located at the bottom of said column, adapted to deliver a feed of contaminated biogas, compressed by a compressor 109, in the form of fine, or micro- or nano-scale bubbles 110 into said electrolyte 105.

[0160] The biogas bubbles 110 produced by the bubbler 108 is brought into contact with the electrolyte 105 containing metallic ions and OH.sup., where CO.sub.2 and other contaminant gases comprising thereof diffuse into the electrolyte 105, forming ions including carbonate and bicarbonate, sulfide, and ammonium ions in alkali pH conditions. The biogas bubbles 110, now substantially stripped of contaminant gases bubble to the column headspace 111 of the bubbling column 107, from which the enriched biogas (labelled in FIG. 2A as purified gas) top product is pumped out for immediate use elsewhere or stored in a vessel for future use.

[0161] The electrolyte 105 comprising metallic ions produced in the cell 101 by sacrificially oxidising the iron-comprising anode 102 is pumped into the bubbling column 107, where the dissolved contaminant gas ions, in particular the carbonates, in the electrolyte 105 precipitate with the metallic ions to form metallic carbonate precipitates, which settle to the raked base 112 of the bubbling column 107 in the form of a slurry. In a similar manner to Example 1, this slurry is subsequently removed from the base 112 for thickening in a liquid-solids separator 113, with the thickened slurry transported for dosing operations on-site or stored in a suitable vessel for further use. The electrolyte 106 that is removed from the slurry in the liquid-solids separator 113 is recirculated back into the electrolyte replenishment feed 114 for the cell 101. Similarly, electrolyte 105 that not comprising the slurry formed at the raked base 112 is also recirculated back into the cell 101 for further electrolysis.

[0162] Similar to the single-tank embodiment of Example 1, optional embodiments of the system 100 illustrated in FIG. 2A can also comprise a variety of recycling streams, as well as purge streams to remove non-sedimenting reaction products. In this regard, optional embodiments can include a gas recycling stream 115 where the enriched biogas in the column headspace 111 still contains CO.sub.2 above the desired level in order to enable at least a portion of the enriched biogas to be recirculated back to the gas bubbler 108. As per the single cell-only embodiment of Example 1, the various recirculating streams of this embodiment are controlled to optimise the reaction efficiency during steady state operation and manage the material balance across the system 100.

[0163] In another optional embodiment, the system 100 includes an electrolyte purge stream 116 to purge a portion of the recirculating electrolyte separated from the slurry in the liquid-solid separator 113. Similar to that of Example 1, the purge stream 116 is substantially adapted to remove at least a portion of the electrolyte 105 containing non-sedimentary materials of other gas contaminants from the biogas feed stream, including sulfide such as FeS and ammonium salts formed in the bubbling column 107.

Example 2.2

[0164] Referring to FIG. 2B, there is another embodiment of the gas purification system provided, wherein the gas-liquid contacting is made in a spraying scrubber.

[0165] In this embodiment, the system 100a comprises a scrubbing column 107a replacing the bubbling column 107 of system 100, wherein the compressed biogas feed passes through a gas dispersion outlet 108a to disperse uniformly across the scrubbing column 107a. The electrolyte 105a in the electrochemical cell 101a, containing metallic ions such as Fe.sup.2+ and OH.sup., is pumped to the top of the scrubbing column 107a where it enters the column through a spray nozzle 117a, forming droplets.

[0166] The up-travelling biogas and the down-travelling liquid droplets of electrolyte 105a achieve counter-current contact, through which contaminant gases such as CO.sub.2 is absorbed into the droplets, converted to ions such as bicarbonate and carbonate in the presence of OH.sup. ions therein. In this case, insoluble precipitates of FeCO.sub.3 from the carbonate and Fe.sub.2 within the droplets and collect at the raked base of the scrubbing column 107a. In particular, the metallic carbonate particles such as FeCO.sub.3 precipitates settle to form a slurry, which is subsequently removed as bottoms product while the supernatant of the slurry is recycled to the electrochemical cell 101a to replenish the electrolyte 105a.

[0167] The CH.sub.4 comprising the biogas feed has a low solubility in the droplets, and thus travels to the top of the scrubbing column 107a for collection. The headspace 111a may still contain some CO.sub.2.

[0168] As per the embodiment illustrated in Example 2.1, optional embodiments of the system 100a illustrated in FIG. 2B can also comprise a gas recycling stream 115a, as well as a purge stream 116a to remove non-sedimenting reaction products from the recirculating electrolyte 105a.

Example 3CO.SUB.2 .Removal

[0169] This example relates to an integrated approach for removing a portion of one contaminant gas from a first gas stream using electrochemically generated ions to react with and sequester said contaminant gas under a set pH level. In particular, the example relates to removing CO.sub.2 from a feed gas analogous to raw biogas.

[0170] As illustrated in FIG. 3, the CO.sub.2 removal was conducted in a modified glass bottle with a total volume of 325 mL in a fume hood in a temperature-controlled (221 C.) laboratory. The reactor was sealed to ensure gas-tightness and mixed by a magnetic stirrer at a speed of 300 rpm. Two iron plates (Mild steel, Harding steel), served as anode and cathode, respectively, were placed in parallel and fixed to the lid of the bottle, with an interelectrode gap of 1.0 cm. The dimensions of iron plates were 15 cm1.4 cm0.3 cm. Each iron plate was submerged at a depth of 3.5 cm in the electrolyte, achieving a submerged surface area of 11.9 cm.sup.2. Iron oxidation was achieved by controlling the current of electrochemical cell via a bench power supply (72-2685, TENMA, China). pH in the reactor was monitored with a portable pH meter (miniCHEM, Labtek). The reactor has sampling ports for gas, liquid, and solid sampling, as illustrated in FIG. 3.

[0171] In each test, 200 ml of 2 g/L of NaCl solution was prepared using tap water as the electrolyte. The electrolyte was sparged with simulated biogas (with N.sub.2 used as a proxy of CH.sub.4 in most tests to reduce laboratory risks; N.sub.2 and CH.sub.4 have similarly low solubility) for about 30 min at a flow rate of 0.1 L/min, to remove the dissolved oxygen (DO) therein before the test was conducted.

[0172] The simulated biogas was transported from the gas cylinder to the reactor via a gas flow controller (EL-FLOW, Bronkhorst). The air diffuser for delivering air into the electrolyte was a needle with 0.5 mm in diameter in reactor. The feed gas was a combination of 60% N.sub.2 and 40% CO.sub.2 used to mimic the composition of raw biogas which typically comprises 60% CH.sub.4 and 40% CO.sub.2. As the electrolytic process does not react with neither CH.sub.4, nor N.sub.2, the results obtained are analogous to feeding raw biogas. The flowrate of the feed gas was controlled by a gas flow controller (Bronkhorst, Netherlands), and the upgraded gas was collected with a 5 L gas bag that was connected to the outlet of reactor.

Example 3Experiment

[0173] FIG. 4a presents the profiles of the gas composition (i.e., H.sub.2, N.sub.2 and CO.sub.2) within the headspace of the batch testing container, pH, total iron concentration, and the current and voltage of the reactor, when the test is conducted at pH 7.5. Following sparging with the feed gas prior to time 0, the headspace comprised 60% N.sub.2 and 40% CO.sub.2, consistent with the known composition of the simulated biogas. As seen in FIGS. 4b and 4d, the pH at time 0 was 4.50.1, lower than the initial pH of the electrolyte (5.8), due to acidification caused by CO.sub.2 dissolution. In the preparatory phase, a current of 0.12 A was applied, leading to H.sub.2 formation, accompanied by a pH rise due to simultaneous hydroxide production at the cathode, reaching 7.30.1 in one hour, and 7.50.0 at the end of the preparatory phase. As indicated by the decrease of its concentration in the headspace, CO.sub.2 in the headspace back-diffused into the liquid phase, remaining in the liquid presumably as dissolved CO.sub.2bicarbonate and carbonate, the latter precipitating with Fe.sup.2+ produced at the anode.

[0174] The content of Na in the headspace progressively decreased in the preparatory phase due to the dilution caused by H.sub.2, which was produced at a rate higher than the CO.sub.2 removal rate. At the end of the preparatory phase, the total iron concentration reached 1.20.1 g/L, evidencing iron ion production at the anode.

[0175] During the experimental phase, the feed gas is supplied to the reactor at a rate of 5 mL/min from the beginning of the experimental phase. To maintain pH 7.5, the current of electrochemical cell is also elevated from 0.12 A to 0.270.01 A, alongside an increase in operational voltage from 2.440.01 V to 6.030.01 V.

[0176] After a transient period of two hours, the contents of H.sub.2, N.sub.2 and CO.sub.2 reached steady state, and remained stable in the last two hours.

Example 3Results

[0177] As summarised in Table 1 below, the CO.sub.2 content in the headspace once reaching steady state was 9.10.1%, markedly lower than the value of raw gas (i.e., 40%). Accordingly, the CO.sub.2 removal efficiency was calculated to be 76.60.2%.

TABLE-US-00001 TABLE 1 Summary of results of CO.sub.2 removal at pH 7.5 Inflow gas composition (%) N.sub.2 60.2 0.2 CO.sub.2 39.8 0.2 Outflow gas composition (%) N.sub.2 59.1 0.4 H.sub.2 32.5 0.4 CO.sub.2 9.1 0.1 CO.sub.2 removal efficiency (%) 76.6 0.2 Total iron concentration (g/L) 6.0 0.1 Coulombic efficiency (%) 93.2 0.8 Iron usage efficiency (%) 87.1 0.2 TIC/total iron (%) 84.1 1.8

[0178] As shown in FIG. 4c, the total iron concentration increased linearly across the duration of the experimental phase, reaching 6.00.1 g/L comprising 5.80.1 g/L and 0.20.1 g/L of Fe.sup.2+ and Fe.sup.3+, respectively.

[0179] The iron usage efficiency, which is calculated as the ratio between the total amount CO.sub.2 removed (in moles) and the total amount of iron produced (also in moles), is 87.10.2%, suggesting that the majority of the Fe.sup.2+ produced was used for CO.sub.2 removal. The iron consumption is supported by the TIC (in moles) to total iron (in moles) ratio in the solids collected from the batch reactor, which was 84.11.8%.

[0180] The results demonstrate the feasibility of achieving CO.sub.2 removal by using an iron-oxidising electrochemical cell. The results also showed part of produced ferrous ions are not combined with carbonate (approx. 15%)suggesting that some other iron compounds, such as ferrous hydroxide, may have been formed during the experimental period.

Example 4CO.SUB.2 .Removal at Differing pH

[0181] The efficacy of the CO.sub.2 removal is also evaluated at pH 7.5, 8.0, 8.5 and 9.0 in a series of batch tests. Initially, 200 ml of oxygen-free electrolyte is added into the reactor, leaving 125 mL as the headspace. In the preparatory phase, a current is supplied to the cell in the absence of a gas supply. pH in the reactor was progressively elevated to the pre-specified level (i.e., 7.5, 8.0, 8.5 or 9.0) due to the on-going production of ferrous ions and hydroxide (along with H.sub.2) in the cell. The gas-upgrade reaction is commenced when the pH set-point is reached, during which the contaminated feed gas is fed into the reactor at a rate of 5 mL/min. The current of experimental phase is 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 at a corresponding current and/or voltage, the pH of the reactor becomes substantially self-regulating as the total concentration of hydroxide ions are maintained due to the following reactions occurring concurrently:

##STR00001##

[0182] Gas samples are taken from the headspace of the reactor with a 100 L syringe hourly in the first 4 h, and then every half hour in the last 2 h. The liquid and solid samples are taken hourly for the analysis of iron concentration.

Example 4Results

[0183] Further to FIG. 4 showing the results at pH 7.5, the results from electrochemical feed gas purification at three more targeted pH levels-namely pH 8.0, 8.5 and 9.0 are illustrated in FIGS. 5, 6 and 7, respectively. Moreover, a comparison between the different pH levels is provided in FIG. 8, which compares the headspace gas composition (i.e., H.sub.2, N.sub.2 and CO.sub.2), CO.sub.2 removal efficiency, total iron concentration, coulombic efficiency, molar ratio of TIC to total iron, iron usage efficiency, the cell voltage and current. The gas compositions and test results are also summarised below in Table 2.

TABLE-US-00002 TABLE 2 Summary of results of CO.sub.2 removal at various pH levels Target pH 8.0 8.5 9.0 Inflow gas composition (%) N.sub.2 60.2 0.2 60.2 0.2 60.2 0.2 CO.sub.2 39.8 0.2 39.8 0.2 39.8 0.2 Outflow gas composition N.sub.2 60.0 0.5 59.5 0.2 60.4 0.6 (%) H.sub.2 34.7 0.2 34.6 0.2 36.1 0.4 CO.sub.2 6.6 0.5 5.9 0.2 5.0 0.0 CO.sub.2 removal efficiency (%) 83.3 1.2 85.1 0.4 87.4 0.1 Total iron concentration (g/L) 7.9 0.2 9.2 0.1 12.2 0.1 Coulombic efficiency (%) 91.4 0.3 95.8 1.9 92.8 1.3 Iron usage efficiency (%) 81.2 1.2 80.1 0.8 53.7 0.8 TIC/total iron (%) 82.9 2.3 82.1 2.0 61.3 1.9

[0184] Compared to the results at pH 7.5, lower CO.sub.2 composition in the headspace gases is achieved, decreasing from 76.60.2% at pH 7.5 to 6.60.5% at pH 8.0, 5.90.2% at pH 8.5 and 5.00.0% at pH 9.0. This led to a corresponding improvement in CO.sub.2 removal efficiency, as outlined above in Table 2.

[0185] By contrast, the iron usage efficiency decreased as pH increased, which was supported by the decrease in the TIC to total iron ratio in the generated iron compounds. These results support the view from Example 3 that an increasing fraction of ferrous ions combined with hydroxide rather than with carbonate as the pH is increased. In particular, a sharp decrease in the iron utilisation efficiency is observed between pH 8.5 to 9.0. Similarly, the cell voltage and current increased with the elevation of cell pH.

[0186] Overall, pH 8.5 appears to be a favourable condition with a relatively high CO.sub.2 removal efficiency (i.e., 85.10.4%) and Columbic Efficiency (i.e., 95.81.9%), and satisfactory iron usage efficiency (i.e., 80.10.8%).

Example 5NH.SUB.3 .and H.SUB.2.S Removal in Methane-Containing Biogas

[0187] Another test was carried out using the gas containing 60% CH.sub.4 and 40% CO.sub.2, and trace NH.sub.3 and H.sub.2S as the feed gas of the electrochemical cell. The feed gas content substantially mimics that of raw biogas obtainable from wastewater treatment processes. The operational procedure of this test is similar to that of above-described CO.sub.2 removal experiment conducted at pH 8.5, with the sole difference being the difference in feed gas.

Example 5Results

[0188] As depicted in FIG. 9a, CO.sub.2 in the raw gas is efficiently removed at a removal efficiency of 87.91.3%, while the content of CH.sub.4 hardly changes during electrochemical purification. Overlaying the results obtained from electrochemically purifying methane-containing containing feed gas with the equivalent test performed at pH 8.5 in Example 4, as done in FIGS. 9g, 9h and 9i, the differences across various performance factors including purification performance are insignificant.

[0189] The above results obtained from the methane-containing feed gas suggests that the results obtained in the batch electrochemical purification of the N.sub.2-containing feed gas is representative of corresponding purification of raw biogas from wastewater treatment processes. In addition, FIGS. 9e and 9f shows that the toxic NH.sub.3 and H.sub.2S in raw gas can also be efficiently removed by the electrochemical process performed at pH 8.5. The average concentrations of NH.sub.3 and H.sub.2S in the feeding gas were 267.526.1 ppmv and 884.163.5 ppmv, in contrast to that of 54.312.3 ppmv and 46.26.8 ppmv in the outlet gas. These represent the decreases of 78.710.2% in NH.sub.3 and 94.86.1% in H.sub.2S between the inlet and outlet gas.

Example 6Effect of Gas Flowrate

[0190] One of them aimed to evaluate the effect of gas flow rate on the cell performance. The test lasted for 9 h, comprising a 2 h preparatory phase with pH elevated to 8.5 in the absence of a gas supply, and a 7 h experimental phase, during which the gas flow rate was stepwise increased from 2 mL/min (3 h), 5 mL/min (2 h), and 10 mL/min (2 h). Gas samples were taken hourly in the first 4 h, and then every half hour in the following 5 h.

Example 6Results

[0191] The CO.sub.2 removal efficiency of the electrochemical system was investigated with different feed gas flow rates at pH 8.5. After a transmission period following each change in the feed gas flow rate, FIG. 10a shows that the CO.sub.2 concentration increases with the gas flow rates from 3.20.3% at 2 mL/min, to 6.20.1% at 5 mL/min, and then to 9.50.5% at 10 mL/L. In the corresponding FIG. 10b, the CO.sub.2 removal efficiency decreased significantly from 92.00.8% to 83.80.4% and finally to 74.00.3%.

[0192] The results suggest that the increase in the gas flow rate resulted in a reduced gas residence time, and thus the CO.sub.2 reaction time. The reduction in time for the gaseous CO.sub.2 to dissolve and react with the iron ions led to a decline in the CO.sub.2 removal efficiency. Accordingly, it is speculated that CO.sub.2 removal from biogas can be maximised with complementary reactor design and gas flowrates such that a satisfactory gas retention time is achieved.

Example 7Particle Analysis and Characterisation

[0193] In this example, particle analysis was performed on the FeCO.sub.3 precipitates formed from electrochemically purifying a CO.sub.2-containing feed gas at pH 8.5, as per the present invention. XRD analysis was performed on about 50 ml of the Fe-containing slurry, using an X-ray diffractometer (Bruker D8). Prior to XRD measurements, 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.

[0194] Furthermore, particle size measurements using 3 ml sample were performed on the freshly produced slurry and the slurry stored for 1, 2 and 4 weeks. Particle size was measured using dynamic light scattering (Zetasizer Nano ZS, Malvern Instruments).

Example 7Results

[0195] FIG. 11 represents the XRD pattern of the Fe-containing slurry produced at pH 8.5 condition. Three crystalline iron species in the Fe-containing slurry were identified, including siderite (FeCO.sub.3), Goethite (-FeO(OH)) and Hematite (Fe.sub.2O.sub.3). Siderite was formed by the precipitation reaction of carbonate and Fe.sup.2+. Goethite and hematite were also detected in the slurry, which is consistent with previous results, indicating that the slurry contained some Fe.sup.3+ ions produced during electrolysis within the cell.

[0196] FIG. 12 represents the particle size distribution and suspension performance of the FeCO.sub.3 precipitates (produced in pH 8.5) at various ages post-precipitation. The particles were stable with nearly identical particle size distribution profiles (FIG. 12a) and D.sub.10, D.sub.50 and D.sub.90 values (FIG. 12b), representing 10, 50 and 90 percentiles of the particle sizes. These results indicate long term storage of the products would not lead to significant particle aggregation or breakup. Results of settleability performance test confirmed that the FeCO.sub.3 slurry substantially do not settle under the typical turbulent conditions in sewers (FIG. 12c), regardless of their age. These results suggest that the FeCO.sub.3 precipitates remain substantially suspended in sewage after in-sewer dosing, making in-sewer dosing upstream of a wastewater treatment plant feasible and effective.

Example 8Sulfide/Phosphate Removal

[0197] In this example, the suitability of FeCO.sub.3 produced in biogas upgrading for supporting urban wastewater management was investigated across several batches. The FeCO.sub.3 slurry collected from the reactors after upgrading the feed gas and/or biogas is directly dosed in batches simulating a sewer, an aerated biological treatment process as well as an anaerobic digestor to ascertain its effect on sulfide and phosphate concentrations.

[0198] Sewer conditions are established by using wastewater collected from a local domestic wastewater pumping station (Brisbane, Australia). The wastewater initially contains a total COD at 400-600 mg/L, soluble COD at 220-310 mg/L, phosphate at 4-7 mg P/L, iron at 0.1-0.3 mg Fe/L, sulfate at 10-20 mg S/L, sulfide at 5-10 mg S/L, and negligible levels of sulfite and thiosulfate. The wastewater has a pH of 7.1-7.4 and contains an undetectable level of oxygen.

[0199] For the sewer-mimicking tests, a pre-determined amount of the FeCO.sub.3 slurry is added to each bottle to achieve a pre-designed initial iron concentration. To guarantee there is no gas headspace during the experiment period, two syringes filled with filtered raw oxygen-free wastewater are connected to the reactor to replenish the reactor after sampling.

[0200] All experiments mimicking biological wastewater treatment system are conducted using activated sludge collected from a local WWTP (Brisbane, Australia). The mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solid (MLVSS) concentrations of the collected activated sludge are 13.20.1 g/L and 10.60.1 g/L, respectively. In the experiments mimicking anaerobic digestion, the used inoculated digested sludge is sourced from a laboratorial anaerobic digestion reactor, with the MLSS and MLVSS concentrations of 20.60.1 g/L and 16.30.1 g/L, respectively. The FeCO.sub.3 slurry obtained via biogas purification at pH 8.5 is used.

[0201] The above batches use both freshly produced FeCO.sub.3, i.e., with the tests undertaken within one day following the FeCO.sub.3 production, and FeCO.sub.3 slurries stored in a further sealed serum container at a temperature-controlled (221 C.) laboratory for 1, 2 and 4 weeks to determine the impact of FeCO.sub.3 storage on the sulfide removal performance.

Example 8.1Sulfide Removal from Sewers

[0202] For each in-sewer sulfide removal batch, 290 mL of the wastewater prepared above is filtered using disposable Millipore filter units (0.45 m), and then transferred into a 300 mL sealed container. Prior to the experiment, the container is exposed to pure nitrogen gas for 30 min to further remove dissolved oxygen. A sulfide stock solution (Na.sub.2S.Math.9H.sub.2O of 1.5 g S/L) of 5 mL is then added to the bottle to increase the sulfide concentration to approximately 25 mg S/L, followed by the addition of 1M HCl to obtain a pH to 7.2, simulating domestic wastewater.

[0203] Two levels of initial Fe levels, namely 30 and 90 mg Fe/L, are used in the separate above-described batches. According to reaction stoichiometry, an initial Fe concentration of 30 mg/L is insufficient for removing the sulfide initially present in the wastewater (25 mgS/L), and hence the ratio between sulfide removed and Fe added is determined. In contrast, an initial Fe of 90 mg/L is in excess, and hence the lowest achievable sulfide concentration is identified.

[0204] Each sulfide removal test lasts for 3 h, during which the reactor is mixed by a magnetic stirrer at 300 rpm. Liquid samples are taken before FeCO.sub.3 dosing, and every 15 mins in the first hour after the dosing, and then every 30 mins, for the measurement of dissolved sulfide concentration. pH in the reactor is monitored with a portable pH meter and recorded manually with the same intervals. An additional sample is taken at the end of each test for the measurement of the total iron concentration.

Example 8.1Results

[0205] FIG. 13a shows the results of sulfide removal from conditions akin to sewers using the FeCO.sub.3 slurry freshly produced at pH 8.5. By dosing FeCO.sub.3 slurry at a concentration of 32.21.9 mg Fe/L, the dissolved sulfide concentration decreased from the initial 20.80.3 mg S/L to 4.60.3 mg S/L in 0.5 h and remained at this level for the remaining period of the 3 h experiment. The ratio between the sulfide removal and the dosed Fe was 0.510.04 g S/g Fe. This is close to the theoretical stoichiometric ratio of 0.57 mgS/mg Fe (HS.sup.+Fe.sup.2+.fwdarw.FeS+H.sup.+). Meanwhile, the pH of wastewater was raised from initial 7.180.02 to 7.480.03 within 3 h, caused by the release of carbonate from FeCO.sub.3. The increase of pH can promote sulfide and Fe.sup.2+ precipitation and can provide more alkalinity for downstream nitrification processes. This is in contrast with dosing FeCl.sub.2 or FeCl.sub.3, which is known to reduce the pH of sewage as their hydrolytic process will release some protons.

[0206] To identify the achievable minimum concentration of dissolved sulfide by dosing obtained FeCO.sub.3 slurry, another sulfide removal batch test was conducted via overdosing the FeCO.sub.3 slurry. FIG. 13a showed that, by dosing FeCO.sub.3 at a concentration of 88.4 mg Fe/L to the wastewater containing 20.1 mg S/L of sulfide, the dissolved sulfide concentration sharply reduced to 3.2 mg S/L within 0.5 h, and then to 0.080.00 mg S/L within 3 h, the efficiency of the sulfide removal reaching 99.5%.

[0207] These results suggest that by using electrochemically produced FeCO.sub.3, sulfide can be removed to very low levels (<0.1 mg/L) in overdosing conditions, which is comparable to the sulfide removal effects of FeCl.sub.2 and FeCl.sub.3 used in the art. As shown in FIG. 14e, using the same testing period, the pH of wastewater increased from 7.17 to 7.40, and then to 7.53. One of the reasons for the low sulfide levels achieved was the pH elevation along with sulfide precipitation, the former favouring ferrous (II-valent) sulfide precipitation.

[0208] As shown in FIGS. 13b to 13d, the obtained FeCO.sub.3 slurry stored for 1, 2 and 4 weeks showed similar sulfide removal performance to that of the freshly purified batch (FIG. 13a). Results showed that, after storage for 1, 2 and 4 weeks, FeCO.sub.3 slurry still can remove the majority of sulfide within one hour, and the ratio between the sulfide removal and the dosed Fe were 0.460.02 g S/g Fe, 0.510.03 g S/g Fe, and 0.480.02 g S/g Fe, respectively. It was also shown that FeCO.sub.3 particles stored for 4 weeks are still effective in reducing sulfide concentration to below 0.1 mg S/L, while also increasing the wastewater pH from 7.2 to 7.5 to aid with the reduction of sulfide (as seen in FIG. 14).

[0209] Overall, the FeCO.sub.3 particles produced from the electrochemical biogas purification were effective in removing sulfide, regardless of storage period, as well as simultaneously providing alkalinity for downstream nitrification process.

Example 8.2Phosphate Removal

[0210] For each phosphate removal test, activated sludge of 100 mL is mixed with the 400 mL filtered wastewater identical to that used in the sulfide removal tests. A phosphate stock solution (5 g P/L of KH.sub.2PO.sub.4) of 1.5 mL is then added to the sludge and wastewater mixture to increase the phosphate concentration to about 20 mg P/, after which the phosphate-containing wastewater is separated into test batches. Amounts of about 0.8 and 3.5 mL of FeCO.sub.3 slurry (10 g Fe/L) is dosed to each batch to obtain two levels of initial Fe concentrations, namely approx. 16 mg and approx. 70 mg Fe/L, respectively, as well as a control batch without any dosed iron.

[0211] Each test is run for 6 h, during which the DO concentration of reactor is controlled at 2.0-3.0 mg O.sub.2/L by a programmable logic controller (PLC) via on/off control of the air flow. The reactor is mixed by a magnetic stirrer at 300 rpm. In a similar procedure to the sulfide removal, liquid samples are taken before FeCO.sub.3 dosing, and every 0.5 h in the initial two hours, and then hourly, for the measurement of phosphate concentration. pH of the reactor was monitored with a portable pH meter and recorded manually with the same intervals. An additional sample was also taken at the end of each test for the measurement of the total iron concentration.

Example 8.2Results

[0212] As seen in FIG. 15, the measured dissolved phosphate concentration reduced from 19.20.1 mg P/L to 9.70.3 mg P/L within 1 hour and then decreased to 6.70.3 mg P/L within 6 hours after adding FeCO.sub.3 at a concentration of 24.32.1 mg Fe/L. The results suggest that the decrease in phosphate concentration was caused by dosing FeCO.sub.3. The ratio between the phosphate removal and the dosed Fe was 0.560.02 g P/g Fe, while the pH values of the control and experimental reactors were 7.430.02 and 7.860.02, respectively.

[0213] The achievable minimum concentration of phosphate was identified to be 1.410.08 mg P/L, demonstrated by overdosing the electrochemically produced FeCO.sub.3 slurry at a concentration of 92.33.4 mg Fe/L to the activated sludge reactor containing 21.40.3 mg P/L of phosphate.

Example 8.3Sulfide Removal from an Anaerobic Digestor

[0214] For each test removing sulfide from an anaerobic digestor, inoculated digested sludge of 50 mL is added into a 100 mL sealed bottle, then deoxygenated with pure nitrogen gas for 10 mins. A sulfide stock solution (1.5 g S/L of Na.sub.2S.Math.9H.sub.2O) of 1.0 mL is added to the bottle to increase the sulfide concentration to about 30 mg S/L, followed by the addition of some 1M HCl to achieve the pH to 7.5, typical of anaerobic digestion.

[0215] FeCO.sub.3 slurry (approx. 10 g Fe/L) of about 0.15 and 0.45 mL are then dosed into the sludge batches to increase the initial iron concentration to approximately 30 and 90 mg Fe/L, respectively, while one control batch is left un-dosed.

[0216] Each test lasts for 3 h, during which the reactor is mixed by a magnetic stirrer at 300 rpm. Sludge samples are taken before FeCO.sub.3 dosing, and every 15 mins in the first hour after the dosing, and then every 30 mins, for the measurement of dissolved sulfide concentration. pH in the reactor is monitored with a portable pH meter and recorded manually with the same intervals. An additional sample is taken at the end of each test for the measurement of the total iron concentration.

Example 8.3Results

[0217] As FIG. 16 shows, the addition of FeCO.sub.3 slurry at 32.21.9 mg Fe/L, resulted in a reduction in dissolved sulfide concentrations from the initial 26.90.5 mg S/L to 11.61.4 mg S/L by the half-hour mark, and maintained this concentration for the remaining 2.5 hours. The successful removal of sulfide from the sludge resulted in a ratio between the sulfide removal and the dosed Fe of 0.410.03 g S/g Fe. By overdosing FeCO.sub.3 at a concentration of 95.53.8 mg Fe/L, the achievable minimum dissolved sulfide concentration was observed to be 1.410.08 mg S/L.

ConclusionExample 8

[0218] Overall, the tests indicate that the electrochemically produced FeCO.sub.3 slurry containing FeCO.sub.3 particles can support the urban wastewater management at a comparable efficiency to adding commonly used iron salts containing soluble iron ions, specifically in terms of controlling sulfide in sewers and anaerobic digestors, as well as phosphates in biological wastewater treatment systems. The in-situ production of FeCO.sub.3 slurry through the electrochemical biogas purification process allows any such integrated WWTP system to avoid costly and hazardous transportation of corrosive concentrated iron salt solutions otherwise required.

Example 9FeCO.SUB.3 .Flow on Effects

[0219] Urban wastewater system is an integrated system, including a sewer network followed by an aerated biological wastewater treatment unit, and an anaerobic wasted sludge digestion unit. In this example, the impacts of dosing FeCO.sub.3 slurry to a sewer network on the performance of downstream biological wastewater treatment units and waste sludge digestion units were investigated by a series of batch experiments.

[0220] This example utilises procedures similar to that of the in-sewer sulfide removal tests of Example 8.1, other than only one-day-old FeCO.sub.3 slurry being used at under-dosing conditions in order to avoid the impacts of residual iron on the downstream performance of wastewater treatment system. The initial sulfide and Fe concentration in this test are approximately 18 mg S/L and 20 mg Fe/L, respectively.

[0221] After the sulfide removal test of Example 8.1, all of the remaining 300 mL FeCO.sub.3 slurry amended sewage was mixed with 300 mL of activated sludge which was prepared by mixing 150 mL raw activated sludge with the raw wastewater at a ratio of 1:1 (v/v). A phosphate stock solution (5 g P/L of KH.sub.2PO.sub.4) of 3.0 mL was then added to the bottle to increase the phosphate concentration to about 25 mg P/L. Each test lasted for 6 h, during which the same mixing and aeration conditions and the sampling procedure as that of the phosphate removal test described in Example 8.2 was applied.

[0222] The effect of in-sewer dosed FeCO.sub.3 slurry on the performance of anaerobic digestion is also investigated over three steps, sulfide removal in sewer, phosphate removal during aerated wastewater treatment, and sulfide control in anaerobic digestion. The first two steps are similar as that of the processes tested in Example 8, however the initial sulfide, Fe and phosphate concentrations in this test are much higher than that of Example 8.2 and 8.3, utilising concentrations of approximately 200 mg S/L, 300 mg Fe/L and 300 mg P/L, respectively in order to mimic the inevitable accumulation of in-sewer dosed Fe in the sludge of the simulated WWTPs.

[0223] To simulate an aerated clarifier located within a WWTP and located downstream from in-sewer sulfide removal, a mixed liquor from the in-sewer sulfide and phosphate removal is concentrated by a centrifuge at 2500 rpm for 3 mins. With the supernatant wasted, the concentrated sludge (15 g VS/L) is used as the feed for biochemical methane potential (BMP) tests to measure biogas output from an anaerobic digestor.

[0224] In the third step, the effect of in-sewer dosed FeCO.sub.3 on sulfide control in anaerobic digestion is evaluated by the BMP tests whereby the sulfide concentration in the biogas, as well as the liquid phase BMP reaction vessels is measured. In this test, about 20 mL of thickened activated sludge (from step two) is mixed with approx. 40 mL inoculated digested sludge, and then transferred into a 100 mL sealed container. A blank test is also set up using the feeding of 20 mL wastewater and 40 mL inoculated digested sludge. After deoxygenating with nitrogen gas, sulfate stock solution (Na.sub.2SO.sub.4 of 1.5 g S/L) of 1.0 mL is added into the containers to increase the sulfate concentration to about 25 mg S/L, followed by the addition of 1M HCl to adjust the pH of bottle to 7.5, typical of anaerobic sludge digesters. Once dosed to the initial conditions of the BMP test, all the containers are incubated in a temperature-controlled (371 C.) incubator for anaerobic digestion of the sludge.

[0225] Biogas released from the container is measured over 30 days, at which point almost no further biogas release was detected. A gas sample was taken every two days in the initial 10 days, and every five days to the end, for the measurement of the N.sub.2, CH.sub.4 and CO.sub.2 compositions in the biogas. The volume of biogas in each sealed container was also measured at the same intervals. Gas and liquid samples were taken every 5 days for the measurement of sulfide/sulfur species and concentration. An additional sludge sample was also taken at the end of each test for the measurement of the dewaterability of sludge.

[0226] The effect of FeCO.sub.3 dosing on settleability of activated sludge and dewaterability of the anaerobically digested sludge was measured by comparison with sludges without FeCO.sub.3 dosing (i.e., the controls). Both SVI and SRF, indexes for sludge settleability and dewaterability respectively, were measured using method known in the art, or are standard in examining water and wastewater. In this regard, SRF was analysed by using a multi-couple measuring device known in the art.

Example 9Results

[0227] As shown in FIG. 17a, by dosing FeCO.sub.3 slurry at a concentration of 26.30.7 mg Fe/L, dissolved sulfide concentration in the simulated sewer system declined from 16.90.2 mg S/L to 4.40.3 mg S/L. Albeit small in magnitude, the phosphate concentration also slowly decreased from 5.30.1 mg P/L to 5.00.1 mg P/L during this phase.

[0228] In the following aerated biological wastewater treatment process, as shown in FIG. 17b, the dissolved sulfate concentration increased from 5.90.3 mg S/L to 17.10.4 mg S/L over 6 hours, while the phosphate concentration decreased from 19.80.5 mg P/L to 4.70.3 mg P/L during the same period.

[0229] The reduction in sulfide concentration suggest that the FeS particles formed in the anaerobic sewer conditions (FIG. 17A). The increase in sulfate concentration in the aerated biological wastewater treatment process (FIG. 17B) suggests oxidation of FeS formed in sewer conditions. The sulfide is oxidized to harmless dissolved sulfate, while the released iron ions precipitated with phosphate as insoluble iron-phosphate-hydroxide complexes in the sludge, leading to the removal of dissolved phosphate. By comparison, the concentration profiles of nitrogenous compounds in the bioreactor simulating containers remained similar to that of control, as seen in FIGS. 18a and 18b respectively. This implies that the dosed FeCO.sub.3 slurry did not impact the nitrification process.

[0230] The dissolved and gaseous sulfide concentrations of the BMP sealed containers are much lower than that of the control, as shown in FIG. 19 illustrating the concentration profiles during the subsequent anaerobic digestion. The average dissolved and gaseous sulfide concentrations in experimental reactors were 1.80.2 mg S/L and 82.523.6 ppmv, in contrast to 29.10.6 mg S/L and 1123.7134.5 ppmv in the control reactors. These represents relative decreases of 94.23.1% and 92.68.3% between the dissolved and gaseous sulfide concentrations of the experimental and control containers. These results imply that the iron in the iron-phosphate complexes containing sludge was regenerated in the anaerobic digester, leading to the precipitation of sulfide, thus significantly decreasing the dissolved and gaseous sulfide concentrations.

[0231] The similar profiles of methane accumulation in the headspace of experimental and control reactors are observed, suggesting that the dosed FeCO.sub.3 slurry did not impact the methane production performance.

[0232] Furthermore, results represented in FIG. 20 also show that the settleability and dewaterability of sludge were significantly enhanced by the dosed FeCO.sub.3 slurry. The SVI and SRF of the sludge from control reactor were 119.04.8 mL/g and 2.30.2 (1013 m/kg), respectively. These values were 75.14.1 mL/g and 1.40.1 (1013 m/kg), respectively, in the sludge from experimental container. This means the settleability and dewaterability of sludge were improved by 36.92.7% and 39.14.5%, respectively, by dosing FeCO.sub.3 slurry.

[0233] Finally, the sulfide removal and biogas production characteristics of the anaerobic digestor under in-sewer dosing (FIG. 19) is compared to the results from direct FeCO.sub.3 dosing of the digestor, the latter shown in FIG. 21. Overall, it is clear that there are minimal to substantially no adverse effects to biogas production from upstream in-sewer dosing of FeCO.sub.3 compared to the direct dosing seen in FIG. 21. Moreover, Considering the FeCO.sub.3 is useful in suppressing both sulfide and phosphate concentrations upstream from the anaerobic digestor, the in-sewer dosing is clearly beneficial and preferred.

Example 10Application in Wastewater Treatment Plants

[0234] Considering the above flow-on benefits of dosing FeCO.sub.3 to processes or vessel upstream from the anaerobic digestor itself, two examples of the present invention are feasible and effective.

[0235] The first, illustrated in FIG. 22 shows the electrochemically produced FeCO.sub.3 slurry being transported from the cell to the sewer network upstream from the of the wastewater treatment plant (WWTP). Such an injection of slurry would provide the full benefit of sulfide and phosphate suppression through the WWTP as per the flow-on effects observed in Example 9.

[0236] Further mass balance analysis of this embodiment suggests that minimal FeCO.sub.3 production at the electrochemical cell is sufficient in providing significant sulfide removal benefits. In doing so, the theoretically achievable iron dosage and sulfide removal capacity were calculated based on three different influent scenarios (i.e., low-, medium-, and high-strength wastewater). Results suggest that the sulfide removal capacity can reach 10.0 mg S/L when the influent only contains 200 mg/L of COD-enough to eliminate the sulfide concentration in the typical sewage (<10 mg/L). In the scenario of influent COD concentration of 500 and 800 mg/L, the theoretical sulfide removal capacity can reach 24 and 39 mg S/L, respectively.

[0237] As shown in Example 9, upstream dosing of iron salts can notably decrease the phosphate concentration in the effluent of biological treatment reactor, significantly decline the H2S concentration in biogas of AD, and substantially improve the settleability and dewaterability of sludge. The test results confirm that dosing the electrochemically produced FeCO.sub.3 slurry in this regard can generate and maintain these flow-on benefits to downstream wastewater treatment processes, while also increasing the pH of sewage to benefit the nitrification by the sludge.

[0238] Considering nitrification in the biological wastewater treatment reactor, and biogas production in the anaerobic digester are not adversely affected by the sewer dosing of electrochemically produced FeCO.sub.3 from the purification of said biogas, the electrochemical system of the present invention provides an efficient and integrated system that is able to provide synergised benefits. By producing the iron salts on-site, which reduces or eliminates the need to transport materials to the site, the electrochemical FeCO.sub.3 approach is more cost-effective than existing methods for delivering iron ions into wastewater treatment plants while also being able to purify biogas from anaerobic digestors.

[0239] In another aspect of the invention illustrated in FIG. 23, the FeCO.sub.3 slurry is provided directly to various unit operations, including prior to the aerated bioreactor, prior to the secondary settler and/or the anaerobic digestor for mixing with the activated sludge in each operation.

[0240] As shown in Example 8, direct dosing of iron salts to sludges can notably decrease the phosphate and sulfide concentrations in the wastewater/sludge of the respective unit process. With respect to the dosing of FeCO.sub.3 slurries before and after the bioreactor, improved settleability of the sludge is also achieved during secondary settling/clarification, as per the results of Example 8.2 and FIG. 20a.

[0241] Similarly, the direct provision of FeCO.sub.3 slurry to the anaerobic digestor, in addition to reducing the emission of H.sub.2S gas and during digestion, reduces the sulfide concentration of the digested sludge and improves its dewaterability for enhanced disposal. The above benefits are substantially in line with those disclosed in Example 8.3 and FIG. 20b.

[0242] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.