METHOD FOR TREATING WASTEWATER OR SLUDGE

Abstract

A method for treating wastewater or sludge comprises the steps of adding the wastewater or sludge to a reactor and mixing the wastewater or sludge with a stream to thereby decrease a ratio of alkalinity to ammonium in the reactor, the reactor containing ammonium oxidising bacteria that oxidise ammonium to produce nitrite and decrease pH.

Claims

1. A method for treating wastewater or sludge comprising the steps of adding the wastewater or sludge to a reactor and mixing the wastewater or sludge with a stream to thereby decrease a ratio of alkalinity to ammonium in the reactor, the reactor containing ammonium oxidising bacteria that oxidise ammonium to produce nitrite and decrease pH.

2. A method as claimed in claim 1 wherein the stream that decreases the ratio of alkalinity to ammonium is a stream containing ammonium that comprises a liquor or a suspension or a sludge.

3. A method as claimed in claim 1 wherein the stream comprises liquor from an anaerobic digestor or liquor from an anaerobic digester that receives sludge from a thermal hydrolysis pre-treatment or a suspension from an anaerobic digester or a suspension from an anaerobic digester that receives sludge from a thermal hydrolysis pre-treatment or a sludge from an anaerobic digester or a sludge from an anaerobic digester that receives sludge from a thermal hydrolysis pre-treatment.

4. A method as claimed in claim 1 wherein the reactor is operated such that the pH of solution or liquor in the reactor is less than 5.5, or less than 5.0, or from 4 to 5, or from 4.5 to 5.

5. A method as claimed in claim 1 wherein the method comprises providing a sludge or wastewater, splitting the sludge or wastewater into a first stream and a second stream, providing the first stream to the reactor and mixing the first stream in the reactor with the stream to thereby decrease the ratio of alkalinity in to ammonium in the reactor, wherein ammonium oxidising bacteria oxidise ammonium to produce nitrite and lower pH, removing a treated stream from the reactor and feeding the treated stream and the second stream to an anammox reactor.

6. A method as claimed in claim 1 wherein the ratio of alkalinity to ammonium in the reactor is less than 2, calculated on a molar basis, or the ratio of alkalinity to ammonium in the reactor is less than 1.9, or less than 1.8, or less than 1.7, or less than 1.6, or less than 1.5, or less than 1.4, or less than 1.3, or less than 1.2, or less than 1.1, or less than 1, or less than 0.9, or less than 0.8, or less than 0.7, or less than 0.6, or less than 0.5, calculated on a molar basis.

7. A method as claimed in claim 1 comprising the steps of feeding a wastewater or sludge to the reactor, the reactor containing ammonium oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB), the AOB oxidising ammonium to reduce pH in the reactor to between 5.5 and 6.0, continuing to operate the reactor at a pH of between 5.5 and 6.0 until a population of acid resistant AOB is selected, and continuing to operate the reactor such that the acid resistant AOB oxidise ammonia and the pH within the reactor is lowered to below 5, or to between 4 to 5, or to between 4.5 to 5.

8. A method as claimed in claim 1 wherein the wastewater or sludge that is fed to the reactor comprises a primary sludge or activated sludge or a high rate activated sludge or aerobically digested sludge or anaerobically digested sludge.

9. A method as claimed in claim 1 comprising analysing one or both of alkalinity and ammonium in a feed material supplied to the reactor, analysing the stream fed to the reactor to determine the amount of components that reduce the ratio of alkalinity to ammonium in the stream and controlling addition of the stream to achieve the desired alkalinity to ammonium ratio.

10. A method as claimed in claim 1 wherein hydraulic retention time in the reactor and solid retention time in the reactor are controlled to promote the growth of AOB and the hydraulic retention time is greater than 1 hours, or greater than 2 hours, or greater than 3 hours, or greater than 4 hours, or greater than 5 hours, or greater than 10 hours, or greater than 12 hours, or greater than 1 day, or greater than 1.5 days, or greater than 2 days, or about 6 hours, and the solids retention time is between 1 day and 100 days, or between 10 days and 30 days.

11. A method as claimed in claim 1 wherein the reactor comprises an aerobic reactor.

12. A method for treating wastewater or sludge comprising the steps of adding the wastewater or sludge to a reactor and mixing the wastewater or sludge with an ammonium-containing liquor or suspension or stream to thereby decrease a ratio of alkalinity to ammonium in the reactor, the reactor containing ammonium oxidising bacteria that oxidise ammonium to produce nitrite and decrease pH, wherein the pH in the reactor is less than 5.5.

13. A method for treating wastewater or sludge comprising the steps of adding the wastewater or sludge to a reactor and mixing the wastewater or sludge with a liquor, suspension or sludge from an anaerobic digester or a liquor, suspension or sludge from an anaerobic digester fed with a sludge treated by a thermal hydrolysis pre-treatment to thereby decrease a ratio of alkalinity to ammonium in the reactor, the reactor containing ammonium oxidising bacteria that oxidise ammonium to produce nitrite and decrease pH, wherein the pH in the reactor is less than 5.5.

14. A method for treating a sludge from an anaerobic sludge digester, the method comprising feeding the sludge from the anaerobic sludge digester to a reactor having a population comprising ammonium oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB), wherein the AOB oxidise ammonium to form nitrite and to lower pH to between 5.5 and 6.5, or to between 5.5 and 6, continuing to operate the reactor for a period of time until an acid resistant AOB population is selected and is formed, whereby the acid resistant AOB population oxidises ammonia to thereby lower the pH in the reactor to less than 5.5 and continuing to operate the reactor at a pH of less than 5.5 and nitrite and free nitrous acid are formed in the reactor, whereby the pH of less than 5.5 is attained by in-situ generation of hydrogen ions/protons without requiring addition of external acid and wherein an external source of nitrite is not used.

15. A method for treating a sludge from an aerobic sludge digester or for treating wastewater, the method comprising feeding the sludge from the aerobic sludge digester or the wastewater to a reactor having a population comprising ammonium oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB), wherein the AOB oxidise ammonium to form nitrite and to lower pH to between 5.5 and 6.5, or to between 5.5 and 6, continuing to operate the reactor for a period of time until an acid resistant AOB population is selected and is formed, whereby the acid resistant AOB population oxidises ammonia to thereby lower the pH in the reactor to less than 5.5 and continuing to operate the reactor at a pH of less than 5.5 and nitrite and free nitrous acid are formed in the reactor, whereby the pH of less than 5.5 is attained by in-situ generation of hydrogen ions/protons without requiring addition of external acid and wherein an external source of nitrite is not used.

16. A method for treating a sludge from an anaerobic sludge digester or for treating wastewater, the method comprising feeding the sludge from the anaerobic sludge digester to a reactor having a population comprising ammonium oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB), wherein the AOB oxidise ammonium to form nitrite and to lower pH to between 5.5 and 6.5, or to between 5.5 and 6, continuing to operate the reactor for a period of time until an acid resistant AOB population is selected and is formed, whereby the acid resistant AOB population oxidises ammonia to thereby lower the pH in the reactor to less than 5.5 and continuing to operate the reactor at a pH of less than 5.5 and nitrite and free nitrous acid are formed in the reactor, whereby the pH of less than 5.5 is attained by in-situ generation of hydrogen ions/protons without requiring addition of external acid and wherein an external source of nitrite is not used.

17. A method for treating sewage or wastewater or sewage sludge comprising feeding sewage or wastewater or sewage sludge having a ratio of alkalinity to ammonium of less than 2.0, calculated on a molar basis, or less than 1.9, or less than 1.8, or less than 1.7, or less than 1.6, or less than 1.5, or less than 1.4, or less than 1.3, or less than 1.2, or less than 1.1, or less than 1, or less than 0.9, or less than 0.8, or less than 0.7, or less than 0.6, or less than 0.5, calculated on a molar basis, to a reactor containing ammonium oxidizing bacteria (AOB), wherein the AOB oxidise ammonia to reduce or maintain pH below 5.5, or below 5.0, or from 4 to 5, or from 4.5 to 5, or less than or equal to 3, or from 1 to 3, or from 1.5 to 3, or from 2 to 3.

18. A method as claimed in claim 17 wherein the ratio of alkalinity to ammonium in the reactor is less than 2.0, calculated on a molar basis, or less than 1.9, or less than 1.8, or less than 1.7, or less than 1.6, or less than 1.5, or less than 1.4, or less than 1.3, or less than 1.2, or less than 1.1, or less than 1, or less than 0.9, or less than 0.8, or less than 0.7, or less than 0.6, or less than 0.5, calculated on a molar basis.

19. A method for reducing metals in a sludge comprising the steps of selectively promoting growth of acid resistant ammonium oxidising bacteria by feeding sewage or wastewater or sewage sludge or a liquor having a ratio of alkalinity to ammonium of less than 2.0, calculated on a molar basis, or less than 1.9, or less than 1.8, or less than 1.7, or less than 1.6, or less than 1.5, or less than 1.4, or less than 1.3, or less than 1.2, or less than 1.1, or less than 1, or less than 0.9, or less than 0.8, or less than 0.7, or less than 0.6, or less than 0.5, calculated on a molar basis, to a reactor containing ammonium oxidizing bacteria (AOB), until a population of AOB that can generate a pH of less than 3 is obtained and supplying sludge to a reactor containing the population of AOB whereby the pH in the reactor is maintained at less than or equal to 3 and metals in the sludge are at least partly dissolved and at least partly removed from the sludge.

20. A method as claimed in claim 19 wherein the sludge is separated from other reactor contents.

21. A method as claimed in claim 19 wherein the population of acid resistant AOB is obtained by taking a sludge from a wastewater treatment plant, such as a sewage sludge or an activated sludge, and adding it to a reactor, feeding a stream of sewage or wastewater or sewage sludge or liquor having a ratio of alkalinity to ammonium of less than 2.0, calculated on a molar basis, or less than 1.9, or less than 1.8, or less than 1.7, or less than 1.6, or less than 1.5, or less than 1.4, or less than 1.3, or less than 1.2, or less than 1.1, or less than 1, or less than 0.9, or less than 0.8, or less than 0.7, or less than 0.6, or less than 0.5, to the reactor such that AOBs cause the pH to drop to around 6, and continuing to operate the reactor until the pH drops to below 3.

22. A method as claimed in claim 19 wherein the reactor is operated for 5-15 days, or around 10 days, to cause the pH to drop to around 6, and the reactor is continued to operate for between 40 and 70 days, or between 50 and 60 days and the pH fluctuates between 4-6, and continued operation results in the pH dropping to below 3.

23. A method as claimed in claim 22 wherein the population of acid resistant AOB that can withstand pH of 3 or less being generated and the population of acid resistant AOB is used to treat sewage or sludge, either by adding sewage or sludge to the reactor containing the population or by inoculating the population to another reactor.

24. A method as claimed in claim 19 wherein a sludge is fed to a first reactor operated at a pH of from 4 to 6 and the sludge is then fed to a second reactor operated at a pH of 3 or less, or the sludge is fed to a first reactor operated at a pH of 3 or less.

25. A method as claimed in claim 14 wherein effluent from the reactor is treated with external acid to at least partly solubilize metals or metal compounds in solids in the effluent.

26. A method for treating a sludge or for treating wastewater, the method comprising feeding a sludge to a reactor having a population comprising ammonium oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB), wherein the AOB oxidise ammonium to form nitrite and to lower pH to between 5.5 and 6.5, or to between 5.5 and 6, continuing to operate the reactor for a period of time until an acid resistant AOB population is selected and is formed, inoculating the acid resistant AOB population to a second reactor and feeding the sludge or wastewater to the second reactor and operating the second reactor at a pH of less than 5.5 and nitrite and free nitrous acid are formed in the reactor, whereby the pH of less than 5.5 is attained by in-situ generation of hydrogen ions/protons without requiring addition of external acid and wherein an external source of nitrite is not used.

27. A method as claimed in claim 14 wherein the reactor is an aerobic reactor.

28. A method as claimed in claim 1 wherein the reactor has a bacterial population that includes Candidatus Nitrosoglobus.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0081] Various embodiments of the invention will be described with reference to the following drawings, in which:

[0082] FIG. 1 shows a flow sheet of a method in accordance with one embodiment of the present invention;

[0083] FIG. 2 shows a flowsheet of another embodiment of the present invention;

[0084] FIG. 3 shows a flowsheet of another embodiment of the present invention;

[0085] FIG. 4 shows nitrogen profiles (including ammonium, nitrite, and nitrate) of (A) experimental reactor; (B) control reactor;

[0086] FIG. 5 shows (A) Comparison of pH between the experimental reactor and the control reactor; (B) comparison of in-situ free nitrous acid (FNA) concentration between the experimental reactor and the control reactor;

[0087] FIG. 6 shows (A) Volatile solids (VS) concentrations in the feed, control reactor, and the experimental reactor; (B) VS destruction efficiency of the experimental reactor and the control reactor;

[0088] FIG. 7 shows (A) Fecal coliform levels in the feed sludge, control reactor and the experimental reactor; (B) specific oxygen uptake rate (SOUR) of the feed sludge, sludge in control reactor and in the experimental reactor;

[0089] FIGS. 8A and 8B show Nitrogen compounds (NH.sub.4.sup.+, NO.sub.2.sup.−, NO.sub.3.sup.−, total nitrogen) in effluent and pH profiles in the reactor during the first 110-day operation;

[0090] FIG. 9 shows metal leaching from anaerobically-digested sludge. (A) N and pH profile during the seven-day batch; (B) solubilization efficiency of metals at the end of the batch;

[0091] FIG. 10 shows Physiological characterization of the enrichment. (A) optimum pH measured by oxygen uptake rate (OUR); (B) affinity constants of total ammonia at the optimal pH 5.5; (C) optimal temperature measured at pH 5.5 (***means p<0.001); (D) incubation growth test for estimating the maximum growth rate at the optimal pH 5.5; and

[0092] FIG. 11 shows a flowsheet of a process in accordance with the present invention that can reduce metal content in the sludge/effluent form the process.

DESCRIPTION OF EMBODIMENTS

[0093] In FIG. 1, an influent wastewater 10 is fed to a high rate activated sludge process (HRAS) or a chemically enhanced primary treatment (CEPT) 12. Some of the activated sludge from the high rate activated sludge process 12 is fed to sludge thickener 14. The sludge from sludge thickener 14 may then undergo an optional thermal hydrolysis pre-treatment step 15 and then be sent to an anaerobic digestion step 16. Following anaerobic digestion, the sludge is dewatered to obtain an anaerobic digestion liquor 20.

[0094] The remainder of the mainstream wastewater 22 from HRAS or CEPT 12 is split into a first stream 24 and a second stream 26, with the first stream 24 and second stream 26, each comprising approximately half of the remainder of the mainstream wastewater. The second stream 26 contains ammonium and is fed to an anammox process 28. The first stream 24 is fed to an aerobic reactor 30. The AD liquor 20 is also fed to reactor 30. The AD liquor 20 is rich in ammonium yet short of alkalinity and as a result, feeding stream 20 to the reactor 30 results in the ratio of alkalinity to ammonium in reactor 30 decreasing. Ammonium oxidation will therefore be produced in reactor 30 (along with free nitrous acid, which selectively promotes growth of AOBs at the expense of NOBs) Ammonium oxidation will therefore be stopped at nitrite rather than being further oxidized to nitrate in reactor 30. Accordingly, liquor from the reactor 30 can be transferred via stream 32 to the anammox process 28. The streams 26 and 32 that are fed to the anammox process 28 provide the required ammonium and nitrite for the anammox bacteria to proliferate and reduce the bulk of the nitrogen compounds fed to the anammox process 28 to nitrogen gas (N.sub.2).

[0095] In another embodiment, liquor from an anaerobic digester that is fed with sludge that has not undergone a thermal hydrolysis pre-treatment may be fed to the reactor. In this embodiment, the THP step 16 in FIG. 1 may be omitted or by-passed. In another embodiment, the stream from the anaerobic digester can be sent to the reactor without de-watering.

[0096] FIG. 2 shows a flowsheet of another embodiment of the present invention. In FIG. 2, a domestic wastewater 40 is sent to an optional carbon recovery process 42. This carbon recovery process may be HRAS or CEPT. The sludge is sent to sludge thickening at 44 and the thickened sludge is then sent to an anaerobic digester 46. The anaerobic digester sludge or the anaerobic digester liquor 48 is sent to a reactor 60.

[0097] The liquor 52 from carbon recovery 42 is sent to a nitrite shunt process or partial nitritation/anammox process 54. The sludge or slurry 56 from process 54 is sent to optional sludge thickening 58 and then fed to reactor 60. Reactor 60 is equivalent to reactor 30 in FIG. 1. The liquor 62 from reactor 60, which contains nitrite and free nitrous acid, is then fed as stream 62 to process 54. Effluent 64 is removed from process 54.

[0098] FIG. 3 shows another embodiment of the present invention in which a waste activated sludge 70 is fed to an anaerobic digester 72. The digestate 74 is fed to reactor 76 which is operated in accordance with the present invention, in which the digestate 74 has a low ratio of alkalinity to ammonium. As a result, AOB bacteria that are acid resistant are selected for growth and pH drops to below 5. As a result, the AOB consume the digestate and the amount of sludge that needs to be disposed is reduced. Sludge disposal is shown at 78 in FIG. 3.

[0099] In the process shown in FIG. 3, reactor 76 is operated at a pH of from 4.5 to 5 without requiring the addition of any external acid or nitrite to obtain that pH. Although the AOB population in reactor 76 that has been selected in the acidic conditions that form in reactor 76 could continue to oxidise ammonia and cause the pH to be lowered to below 4.5, addition of the digestate 74 maintains the pH in the range of 4.5 to 5. This is advantageous in that free nitrous acid is unstable at pH below 4.5 and this could result in the unwanted formation of nitrate. Reactors 30 and 60 in FIGS. 1 and 2 are also operated in a similar manner.

Example 1

[0100] An MBR (membrane bioreactor) was established for the verification of the concept shown in FIG. 1. The MBR had a working volume of 1.8 L. The MBR was fed with the mixture of sidestream THP-AD liquor and mainstream HRAS (high rate activated sludge) effluent in the ratio of 1:50. HRT (hydraulic residence time) was 1.5 d. No wastage was performed and thus SRT (solids residence time) was not calculated. HRAS effluent in mainstream was split into two streams. The alkalinity deficiency in the feed to the MBR was intentionally intensified through mixing THP-AD liquor with the half mainstream fed to the MBR. The reactor was fed with the effluent of a pilot-scale HRAS unit in Luggage point (Brisbane, Australia). THP-AD liquor was collected from the centrate after THP-AD processes in Oxley Creek WRP (Brisbane, Australia). The characteristics of HRAS effluent and THP-AD liquor were listed in Table 1.

TABLE-US-00001 TABLE 1 Parameters HRAS THP-AD liquor NH.sub.4.sup.+—N (mgN/L) 44.71 ± 4.49   2440 ± 123 TCOD (mg COD/L)   287 ± 51  11700 ± 400 SCOD (mg COD/L)   95 ± 21   4500 ± 260 TSS (mg/L)   180 ± 160   8350 ± 1750 TKP (mg P/L)  8.23 ± 1.31   753 ± 47 TKN (mg N/L) 63.07 ± 12.23   2660 ± 160 Alkalinity (mg   334 ± 12   4775 ± 124 CaCO.sub.3/L) pH  7.44-7.89   7.6 ± 0.1

[0101] The ratio of alkalinity to ammonium was 5.36 mg CaCO.sub.3/mg N in the mixture feed, lower than the theoretical ratio of 7.14 for nitrification. The MBR was operated at long HRT and SRT. Therefore, pH could decrease to such a low value (4.8-5.2) due to the acidification of ammonium oxidation that NOB were successfully suppressed by the in-situ FNA (0.90-2.23 mgN/L). At steady state (shown in Table 2), NAR (nitrite accumulation ratio) could be stably maintained at more than 0.9. The effluent with nitrite could rejoin the other half HRAS effluent with ammonium for the following ANAMMOX process.

TABLE-US-00002 TABLE 2 Parameters MBR (50-140 d) Influent ammonium (mgN/L) 84.83 ± 3.61 Ratio of alkalinity/ammonium in influent  5.36 ± 0.29 (mg CaCO.sub.3/mgN) Effluent ammonium (mgN/L) 14.06 ± 4.77 Effluent nitrite (mgN/L) 63.74 ± 5.08 Effluent nitrate (mgN/L)  4.65 ± 2.27 Nitrite accumulation ratio in effluent (NO.sub.2.sup.−/  0.93 ± 0.04 NO.sub.x) Operating pH  4.8-5.2 In situ AOB activity (mgN/(L h))  1.08 ± 0.36 In situ NOB activity (mgN/(L h))  0.08 ± 0.03

[0102] In preferred embodiments of the present invention, the ratio of alkalinity to ammonium in the reactor was decreased by adding THP-AD liquor to the reactor. This resulted in the proliferation of AOBs at the expense of NOBs in the reactor, and the production of nitrite and free nitrous acid. Liquor from this reactor was used as part of a feed to an anammox reactor to achieve mainstream deammonification.

Example 2—Aerobic Digestion of Sludge

[0103] In this example, the experimental reactor was inoculated with an acid tolerant and FNA-resistant AOB. The reactor size was 750 cm.sup.3. The pH in the reactor during the experimental runs was around 5. This pH was maintained without having to add additional acid. FIGS. 4 to 7 show the results obtained. In this experiment, the control reactor was not inoculated in the same manner as the experimental reactor.

[0104] As can be seen from FIGS. 5A and 5B, the pH in the experiment reactor was significantly lower than the pH in the control reactor, with the pH in the experimental reactor averaging around 5. In the experiment reactor, the pH was consistently between 6 and 7. FIGS. 7A and 7B also showed lower faecal coliform readings in the experimental reactor and lower specific oxygen uptake rate in the experiment reactor.

Example 3—Leaching of Metals from Sludge

[0105] The centralized collection and treatment of wastewater generates large amounts of sludge, which needs to be safely disposed. Sludge treatment and disposal can incur considerable operational cost, up to 50% the total cost of a wastewater treatment plant (WWTP). The most economical and commonly-used method of sludge disposal is land application. Through applying the sludge to soil as a fertilizer, the N, P, other micronutrients, and organic matter can be released into the soil, hence improving its physical, chemical and biological properties. Despite the economically-effectiveness of land application, it is restricted from wider use due to the presence of heavy metals in the sludge. The heavy metals in sludge, once leached into the soil, can contaminate the underground water and accumulate along the food chain, ultimately causing metabolic disorder and chronic diseases in humans.

[0106] Therefore, to facilitate the use of metal-laden sludge on agriculture land, the sludge has to be detoxified prior to its land application, namely solubilizing the metal cations from the solid substrates. Metals in the sludge majorly exist in the form of sulfides, oxides, hydroxides, silicates, insoluble salts or linked with sludge organic matter. Generally, the metals can be solubilized from those insoluble complexes by acidifying the sludge to pH 2.0 to 3.0. This can be achieved through adding inorganic acids (H.sub.2SO.sub.4, HCl or HNO.sub.3), the so-called chemical leaching. However, chemical leaching incurs additional cost and causes secondary pollution associated with the external chemical dosage. A cost-effective and environmental-friendly alternate is called bioleaching, employing either the direct metabolism or the indirect product of metabolism of functional microorganisms to leach metals from sewage sludge. The two main functional groups, at present, are iron-oxidizing and sulfur-oxidizing microorganisms. Different energy sources, such as FeSO.sub.4, FeS.sub.2, and S.sub.0, need to be supplied for the involved microorganisms to function. Although bioleaching could save 80% chemical dosage compared to chemical leaching, the chemical cost and potential secondary pollution associated with the known methods mentioned above still cannot be neglected. For example, the residue sulfur dosed, if applied to land, may cause soil acidification.

[0107] This example is based on the hypothesis that indigenous acid-tolerant ammonia-oxidizers in wastewater treatment plant (WWTP) systems can be enriched and utilized to leach heavy metals from sludge, such as anaerobic digester sludge (ADS). The enrichment was conducted in a sequencing batch reactor (SBR), inoculated with wasted activated sludge from the local full-scale municipal WWTP. To create acidic pH in the reactor by ammonium oxidation, the SBR was fed with alkalinity-inadequate anaerobic digester (AD) centrate/liquor. Upon obtaining ammonia-oxidizing community capable of functioning at pH 2.0-3.0, the feasibility of metal-leaching from ADS using ammonium as the energy source was tested in a seven-day batch test. The dominant ammonia-oxidizers were phylogenetically and physiologically characterized, in order to gain insights into the microorganisms and support the process development.

[0108] Enrichment of indigenous acid-tolerant ammonia-oxidizers from WWTP

[0109] A two litre reactor was set up and initially inoculated with wasted activated sludge from the local full-scale municipal WWTP (Brisbane, Australia). The feed was collected from centrifuge supernatant of AD effluent in the same WWTP. The concentrations of ammonium and alkalinity in the AD centrate were 881.3±78.9 mg N/L and 32.1±2.3 mmol CaCO.sub.3/L, featured by a low alkalinity to ammonium molar ratio of 0.51 mol CaCO.sub.3/mol NH.sup.4+—N.

[0110] The reactor was operated in SBR mode. Starting with one litre of liquid, the reactor was fed with 100 mL AD centrate per day. Once hitting two litres, the influent, aeration and mixing would be manually stopped for one hour for the sludge to settle down. One litre of supernatant was then discharged by a peristaltic pump, after which the next 10-day cycle was initiated. Thereby, the hydraulic retention time (HRT) was 20 days, giving rise to an ammonium loading rate at 44 mg N/(L d). Compressed air was supplied by an air pump at 1 L/min through an air diffuser, with a dissolved oxygen concentration higher than 5 mg O.sub.2/L (measured by Optical DO sensor (inPro 6960i, METTLER TOLEDO) and Multi-parameter transmitter (M800, METTLER TOLEDO)). pH in the reactor was monitored but not controlled by a pH probe (general purpose pH probe, TPS) and a transmitter (mini CHEM, TPS). The average pH was calculated on a daily basis. The reactor was mixed by a magnetic stirrer at 250 rpm. The reactor was operated in an air-conditioned room with the temperature controlled at 22±1° C. No sludge was wasted during the whole reactor operation except for microbial sampling.

[0111] Ammonium, nitrite and nitrate concentrations in the effluent were monitored every 10 days. Microbial samples were taken also every 10 days and stored in a −80° C. freezer before community profiling.

Metal-Leaching from ADS Using Ammonium as the Energy Source

[0112] ADS was collected from anerobic digester in the local full-scale municipal WWTP (Brisbane, Australia). The ADS was characterised with 26.30±0.66 g/L total solids (TS) and 1130±52.44 mg NH.sup.4+—N/L in the filtered supernatant.

[0113] ADS was mixed with the enriched acid-tolerant culture in a volume ratio of 1:9 in a 500 mL Erlenmeyer flask. The mixture was agitated by magnetic stirrer at 250 rpm. Compressed air was constantly provided through an air diffuser, giving rising to a dissolved oxygen concentration above 5 mg/L (Optical DO sensor inPro 6960i, METTLER TOLEDO, Multi-parameter transmitter M800, METTLER TOLEDO). The batch test lasted for seven days, during which samples were taken every 24 h for determining pH (general purpose pH probe, TPS, mini CHEM, TPS), N concentrations, solubilized and total metal concentrations (as detailed in Section 2.6). The solubilization efficiency of each metal was calculated as

Solubilization efficiency=Cs/CT*100% [0114] Where Cs is the solubilized metal concentration; CT is the total metal concentration.

Results

[0115] Metal-Leaching from ADS Using Ammonium as Energy Source

[0116] In order to leach metals from ADS using ammonium as the energy source, the first step is to enrich acid-tolerant ammonia-oxidizers from wastewater treatment systems. The enrichment was performed in a lab-scale SBR. The SBR was inoculated with wasted activated sludge from the local WWTP and fed with alkalinity-inadequate AD supernatant. The average pH was calculated on a daily basis and effluent nitrogenous compounds (including ammonium, nitrite, and nitrate) were analysed regularly, as shown in FIG. 8.

[0117] The SBR was gradually acidified by ammonium oxidation with pH profiles presented in FIG. 8B, due to the inadequate alkalinity in the feed. Theoretically, with a CaCO.sub.3 alkalinity to ammonium molar ratio of 0.5, the alkalinity in AD supernatant would be exhausted once 50% of the influent ammonium was oxidized, which would give rise to pH drop to ˜6.0. Indeed, upon start-up, the pH gradually decreased to about 6.0 in the end of the first 10-day-cycle (FIG. 8-B). The pH stayed around 6.0 for a few days because the commonly dominant ammonium oxidizers in WWTPs are acid-sensitive, which usually cease to oxidize ammonium at pH below 6.0. In the second 10-day cycle, the pH in the reactor broke through 6.0 and fluctuated between 4.0-6.0 for about 50 days, likely resulting from the build-up of acid-tolerant ammonium oxidizers, which are capable of oxidizing ammonium (simultaneously generating protons to acidify the systems) below pH 6.0. The obstacle preventing pH from further decreasing was likely free nitrous acid (FNA) formed by the accumulated nitrite (FIG. 8-A) under the acidic pH, which was also reported to inhibit ammonium oxidation.

[0118] The second pH drop happened since day 60 and finally stabilized around 2.5 until the end of the study. The second pH drop coincided with nitrite oxidation to nitrate (FIG. 8-A), which might release the suppression pressure of FNA. Nitrite nearly disappeared from the effluent when the pH hit below 4 while nitrate took over as the final product of nitrification. The oxidation from nitrite to nitrate was speculated to associate with chemical oxidation of nitrite, since no active nitrite-oxidizing bacteria (NOB) activity was detected. During the steady phase, loaded with 50 mg N/(L d), the ammonium oxidation rate was around 30 mg N/(L d). The reactor was kept running for another 500 days. During the extended phase, pH maintained at the 2.0-3.0, indicating the ammonia-oxidizers enriched in this study were capable of long-term growth at such a low pH level. Acid-tolerant ammonia-oxidizing community able to drive ammonium oxidization at pH 2.0-3.0, was successfully enriched from WWTP systems.

[0119] Upon obtaining the stable acid-tolerant ammonia-oxidizing community adapted to pH 2.0-3.0, the feasibility of metal-leaching from ADS using ammonium as the energy source was testified via batch tests. ADS was initially inoculated with the enrichment and kept aerated for seven days. The variations of ammonium and pH during the batch tests were presented in FIG. 9A. The solubilization efficiency of metals (including Al, B, Ba, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Zn) at the end of the batch test was calculated and displayed in FIG. 9B.

[0120] pH decrease concomitantly proceeded with the ammonium oxidation to nitrate (FIG. 9A). Driven by ammonium oxidation, the pH decreased to below 2.0 after 72 hours and finally reached 1.67 at the end of the batch test. More than 50% of Al, B, Ba, Ca, Co, Cu, K, Mg, Mn, Na, Ni, and Zn were leached into the aqueous phase with the intrinsic ammonium as energy source. In addition, 20.97±0.70% of Cr, 11.47±0.98% of Fe, 5.63±0.23% of Mo, and 25.54±2.18% of Pb were extracted into the aqueous phase. A chemical-free method of bioleaching from ADS was successfully testified using the indigenous acid-tolerant ammonia-oxidizers enriched from WWTP systems.

[0121] Phylogenetic analysis based on 16S full-length rRNA suggested the enriched ammonium oxidizer (Clone) is closely clustered with Candidatus Nitrosoglobus terrae, a gamma-proteobacterial AOB (γ-AOB) species recently isolated from acidic agriculture soil. The clone also has a relatively deep-branching association with Nitrosococcus genus (belongs to γ-AOB as well), forming a monophyletic lineage distinct form beta-proteobacterial AOB (β-AOB) genera including Nitrosomonas and Nitrosospira.

[0122] The enrichment (that is, the enriched microorganism population) could perform ammonium oxidation under a broad range of pH, as shown in FIG. 10A. The maximum ammonium consumption within 5 days was identified at pH 5.5, which was regarded as the optimum pH. Under pH 3.5 to 8.5, the ammonium consumption was greater than 60% of the maximum consumption, indicating a broad pH range for efficient ammonium oxidation. Even under pH 2.5, the ammonium consumption with 5 days is approximately 30% of the maximum value. Ammonium oxidation proceeded between 17-37° C. with an optimum temperature at 30° C. (FIG. 10B).

[0123] Kinetics parameters of the enriched ammonium oxidizers were estimated. The half saturation constant of total ammonia (NH.sub.3+NH.sup.4+) was estimated at 9.59±4.56 mg N/L at the optimal pH 5.5 (FIG. 10C) based on Michaelis-Menten equation. This suggests that the total ammonia concentration should be above 20 mg N/L to avoid substrate limitation. The maximum growth rate under optimal pH (i.e. 5.5) was estimated at 0.42±0.02 d.sup.−1 based on ammonium oxidation and nitrite production (FIG. 10C).

DISCUSSION

[0124] Detoxification of metal-laden sewage sludge is critical to the reuse of sewage sludge as fertilizer in agriculture land. Bioleaching prevails over chemical leaching because of about 80% chemical cost savings. However, the two prevailing bioleaching approaches, i.e. sulfur-based and iron-based processes, still require dosing sulfur and ferrous iron as the energy sources. The dosed chemicals would not only incur extra cost, but also remain in the bioleached sludge and cause soil acidification when applied to agriculture land.

[0125] This study, for the first time, proposed ammonium-based bioleaching utilizing the intrinsic ammonium in sewage sludge as the energy source, therefore totally exempting the chemical dosage. In a seven-day batch test aiming for metal-leaching from ADS, the pH was driven down to 1.67 by ammonium oxidation. The ending pH is comparable to sulfur-based process and slightly lower than iron-based process. Concomitantly with the pH decrease, significant metal-leaching was observed in Al, B, Ba, Ca, Co, Cu, K, Mg, Mn, Na, Ni, and Zn, with a solubilization efficiency higher than 50%. However, only 20.97±0.70% of Cr, 11.47±0.98% of Fe, 5.63±0.23% of Mo, and 25.54±2.18% of Pb were extracted into the aqueous phase. The phenomenon had been identified in sulfur-based and iron-based processes as well. A possible explanation is that Cr, Fe, Mo, and Pb have a strong tendency to bound with organic matter, which cannot be readily solubilized in an acidic environment.

[0126] The concept of ammonium-based metal-leaching was firstly testified on ADS on account of the ample ammonium source (1 g/L) released during anaerobic digestion. However, it is noted that this concept is not only limited to ADS sludge sources, but is also applicable to other sludge sources, such as primary sludge, secondary sludge, and wasted sludge. For those sludge sources, where most of nitrogen in those sludge sources is organically bounded, a process called “simultaneous sewage sludge digestion with metal bioleaching” (SSDML), which involves the coupling of bioleaching process with sludge digestion process in a single reactor, can be applied. The nitrogen will be released during sludge digestion in the form of ammonium, which can be used as the energy sources for metal-leaching.

[0127] The ammonium-based metal leaching process was driven by an acid-tolerant AOB enriched from WWTP systems. It affiliated with Ca. Nitrosoglobus, a novel AOB genus recently isolated from acidic agriculture soil. Physiological characterization indicates the enriched AOB can survive at a broad range of pH from 2.5 to 8.5. Therefore, pre-acidification is not required for ammonium-based leaching process, which is commonly necessary for iron-based processes.

[0128] The ammonium affinity and maximum growth rate of the enriched AOB were also characterized in order to facilitate the development of the process for large-scale applications. The apparent affinity of total ammonia (NH.sub.3+NH.sup.4+) at the optimal pH 5.5 was estimated at 9.59±4.56 mg N/L. However, considering bioleaching usually happens at pH 2.0-3.0, the apparent affinity of total ammonia (NH.sub.3+NH.sup.4+) was also estimated at pH 2.5. The affinity to total ammonia at pH 2.5 was 67.45±20.30 mg N/L, much higher than that at pH 5.5. This suggests that the ammonium concentration needs to be maintained above 130 mg N/L during ammonium-based metal-leaching process in case of ammonium limitation. The maximum growth rate at pH 5.5 was estimated at 0.42±0.02 d.sup.−1. At pH 2.0-3.0, the maximum growth rate is one-third of that at pH 5.5 according to OUR verse pH relations (FIG. 10A), presuming the yield is the same at different pH.

[0129] FIG. 11 shows a flowsheet of a possible process in accordance with an embodiment of the present invention in which metals can be leached from sludge. The flowsheet shown in FIG. 11 is similar to that shown in FIG. 3, but with two alternative additional steps shown. For convenience, the process steps of FIG. 11 that are in common with the process steps of FIG. 3 are denoted by similar reference numerals and need not be described further. It will be understood that reactor 76 (in FIG. 3 and FIG. 11) is operated at a pH of from 4.5 to 5 without requiring the addition of any external acid or nitrite to obtain that pH. Reactor 76 is an aerobic digester. Solids reduction also occurs in reactor 76.

[0130] In the first embodiment shown in FIG. 11, effluent from reactor 76 is sent to further reactor 80. Further reactor 80 is also an aerobic digester. In this reactor, the pH is further reduced to less than 3 through the protons generated by the selectively grown acid resistant AOBs, such as Ca. Nitrosoglobus. As a result of the low pH in reactor 80, metals in the sludge are leached into solution. The liquid and solids from reactor 80 may be separated from each other, with the solids comprising solids having reduced metals content that may be suitable for use as a fertiliser or for disposal on fields. The liquid phase may be sent to a metals recovery process to remove metals therefrom, using any technology.

[0131] In the second embodiment shown in FIG. 11, effluent from reactor 76 is sent to a vessel 82 when it is mixed with an external source of acid, such as sulphuric acid (although any suitable acid can be used). This reduces the pH to below 3 and causes dissolution of metals from the solids. The majority of the alkalinity in the AD liquor has been consumed in reactor 76. As a result, the amount of external acid that is required to obtain the desired pH level is dramatically reduced, which greatly improves the economics of this alternative embodiment. Again, the solids and liquids from vessel 82 may be separated from each other and the solids disposed of and the liquids treated for metal removal, as described above.

[0132] In the present specification and claims (if any), the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0133] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0134] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.