WO2014116186A1

WO2014116186A1 - A process for mitigating sulfate impact on and enhancing methane production in anaerobic systems

Info

Publication number: WO2014116186A1
Application number: PCT/SG2014/000033
Authority: WO
Inventors: Zhenyu Wang; Yan Zhou; Wenhai HUANG; Wun Jern Ng
Original assignee: Nanyang Technological University
Priority date: 2013-01-25
Filing date: 2014-01-27
Publication date: 2014-07-31
Also published as: CN105307985A

Abstract

Disclosed herein is an anaerobic two-phase process for the treatment of organic waste comprising the steps of: introducing a feed stream comprising organic waste into a first phase reactor; generating an effluent from the first phase reactor; and providing the effluent from the first phase reactor to a second phase reactor, wherein, during at least the start-up period of the process, the first phase reactor has a sludge retention time (SRT) of from 10 days to 25 days and the pH is maintained at a pH of from 4.5 to 6.0. Such a "pretreatment" reactor which decreases sulfate and sulfide concentration fed to the methanogenic reactor reduces the adverse impact of sulfate on methane production in the downstream methanogenic reactor.

Description

A Process for Mitigating Sulfate Impact on and Enhancing Methane Production in Anaerobic Systems

Field of Invention

This invention relates to an improved method of treatment to enhance the degradation of organic waste and biogas generation in an anaerobic digestion system where there are problems with sulfates.

Background

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge. Current state-of-the-art anaerobic systems include digesters, two-phase systems, upflow anaerobic sludge blanket systems (UASBs), expanded granular sludge bed systems (EGSBs), filters, baffled tanks, sequencing batch reactors (SBRs), and, of late, membrane bioreactors (MBRs). In these systems, the organic materials in the feed stream are degraded by way of hydrolysis, acidogenesis, and then methanogenesis.

A 2-phase anaerobic digestion system and/or process has been used to separate the acidogenesis reaction from the methanogenesis reaction. This approach allows for optimization of operating conditions for acidogenesis and methanogenesis separately, and thus optimizes the conditions to sustain specific microbes involved in the different stages of the conversion of material to biogas. These two stages take place simultaneously in two different reactors and provide the optimal conditions for acidogens and methanogens, thus enhancing the performance of the overall system, which includes organic degradation, and more specifically, methane production and carbon dioxide fixation.

Acidogenesis results in the production of volatile fatty acids (VFAs) and some hydrogen (H₂) from the organic materials. The VFAs are consumed by methanogens resulting in carbon dioxide and methane (CH₄), which may then be recovered. However, a variety of inhibitory substances like ammonia, sulfate, light metal ions, and heavy metals can adversely impact methane production. Sulfate, a common constituent of many industrial wastewaters, is reduced to sulfide by sulfate reducing bacteria (SRB). However, SRB also competes for organic substrates with the methanogens and SRB can replace the methanogens in the anaerobic reactor. However, it is generally agreed ' that SRB cannot effectively compete l against the fermentative microorganisms involved in monomer degradation and that competition for organic matter does not occur in the hydrolysis and acidogenesis stage. In addition, secondary inhibition results from the toxicity of sulfide to various bacteria groups. There are existing methods for removing sulfate in wastewater. These methods are largely physiochemical techniques involving wastewater dilution and stripping, chemical reactions such as coagulation, oxidation-reduction, and precipitation. These methods introduce additional chemicals to the wastewater stream or increase water volume. Some proposed biological conversions (partial oxidation to elemental sulfur) or adaptation of the methanogen to free H₂S, particularly in reactors with fixed biomass to increase the tolerance of methanogen to sulfide. This would typically require long-periods for acclimation.

Therefore, there remains a need for an improved method to reduce the sulfate and/or sulfide content of organic matter undergoing treatment.

Summary of Invention

The method described herein does not involve additional chemicals but biologically converts sulfate into H₂S and thereafter precipitates this as metal sulfides. This removes sulfate and sulfide from wastewater before feeding into a methanogenic reactor and consequently mitigates SRB competition with methanogens to improve CH₄ production.

In a first aspect of the invention, there is disclosed an anaerobic two-phase process for the treatment of organic waste comprising the steps of:

introducing a feed stream comprising organic waste into a first phase reactor;

generating an effluent from the first phase reactor; and

providing the effluent from the first phase reactor to a second phase reactor, wherein, during at least the start-up period of the process, the first phase reactor has a Sludge retention time (SRT) of from 10 days to 25 days and the pH is maintained at a pH of from 4.5 to 6.

In embodiments of the invention, the SRT may be from 1 1 days to 24 days during at least the start-up period (e.g. from 14 days to 20 days, such as about 15 days, e.g. 20 days). Following the start-up period, the SRT may be from 10 days to 15 days in embodiments of the invention.

In embodiments of the invention, the pH may be from about 5.0 to 6.0 (such as 5.5). In embodiments of the invention, the first phase reactor may be supplied with an inert gas, such as argon or, more particularly, nitrogen. In still further embodiments of the current invention, the process may further comprise establishing a population of sulfate reducing bacteria in the first phase reactor to convert sulfate into hydrogen sulfide. Such embodiments may further comprise reacting the hydrogen sulfide with a metal or a metal ion native to the wastewater to generate a metal sulfide precipitate. Alternatively or additionally part or all the hydrogen sulfide is removed from the first reactor in gaseous form, for example by passage of the hydrogen sulfide through a chemical or biological scrubber.

In yet further embodiments of the invention, when the hydraulic retention time is fixed, the organic loading rate (OLR) of the first phase reactor is from about 0.5 to about 2 g COD/g MLSS.L.d (e.g. the OLR of the first phase reactor is from about 1 to about 1.5 g COD/g MLSS.L.d).

In still further embodiments of the invention, the organic waste may have an initial COD/SO₄ ^2" ratio of from about 10:1 to about 2:1 (e.g. from about 6:1 to about 2:1 , such as from about 5:1 to about 3:1 ).

In yet further embodiments of the invention, the organic waste may comprise biodegradeable solids, such as a sludge, food waste, oily waste, solid waste with a high organic content or any combination thereof (e.g. the sludge may be a waste sludge or a secondary sludge).

In still further embodiments of the invention, the first phase reactor is primarily an acidogenic reactor and/or the second phase reactor is primarily a methanogenic reactor or is a single stage anaerobic reactor. In yet further embodiments of the invention, the first phase reactor may be operated with a hydraulic retention time of from about 1.25 days to about 5 days. In further embodiments, the second reactor is operated at a pH of from about 6.5 to about 7.9, such as from about 6.8 to about 7.4 (e.g. at a pH of about 7.1 ). In still further embodiments of the invention, the first and second reactors may further comprise pH, oxidation reduction potential (ORP) and temperature controllers. In a second aspect of the current invention, there is disclosed a waste treatment plant using the process of the first aspect. In an embodiment of this aspect, the waste treatment plant is a wastewater treatment plant. In a third aspect of the current invention, there is disclosed a two-phase reactor system for the treatment of biodegradeable waste, comprising a first phase reactor and a second phase reactor that is downstream of the first phase reactor and is in fluid connection therewith, wherein the first and second phase reactors are adapted to use the process of the first aspect of the invention.

In an embodiment of this aspect, the first phase reactor may primarily be an acidogenic reactor and/or the second phase reactor may primarily be a methanogenic reactor or a single stage anaerobic reactor. In still further embodiments of the invention, the first and second reactors may further comprise pH, oxidation reduction potential (ORP) and temperature controllers.

Figures

The invention will now be described in further detail below, with the aid of the following figures.

Fig. 1 The pathway of sulfate reduction H₂S is described in schematic form.

Fig. 2 H₂S content in acidogenic (A-) and methanoganic (M-) reactors under normal operating parameters.

Fig. 3 H₂S content in acidogenic (A-) and methanoganic (M-) reactors under operating parameters of current invention.

Description of Invention

It has been discovered that SRB competition and impact on the functioning of methanogens can be mitigated by adjusting the operating parameters of the anaerobic system used. For a 2-phase system, it was surprisingly found that adjusting the operating parameters in accordance with the disclosed invention results in a high H₂S output in the acidogenic reactor and a lower H₂S in the methanogenic reactor, thereby reducing the impact of SRB competition in the methanogenic reactor. This surprising result suggests that the acidogenic reactor contains further useful processes other than acidogenesis.

By way of example, the process may be run and set up according to the following protocol. a. A two-phase anaerobic process is set-up. The acidogenic reactor is equipped with pH, oxidation-reduction potential (ORP) and temperature controllers. pH is controlled at 5.0-5.5 and temperature is maintained at 40+1 °C. The ORP is about -400 mV. ;

b. Nitrogen or argon is piped into the acidogenic reactor to regulate the microbial population during start up;

c. The sequencing batch reactor system is operated with a hydraulic retention time (HRT) of 1.25-5 days, a sludge retention time (SRT) of 10-25 days, and an organic loading rate (OLR) of 0.5-2 g COD/g MLSS.L.d; and d. Biogas is recycled in the acidogenic reactor to enhance the contact between the microbes and biogas, to enhance sulfate reduction, and to enhance sulfide precipitation. H₂S content in the acidogenic biogas can increase from less than 10 ppm to several hundred ppm while H₂S in the methanogenic biogas shall decrease.

The acidogenic reactor so operated allowed selection of a microbial consortium which has higher than typical numbers of acidogenic SRB. It is speculated that the operating protocol described herein for the acidogenic reactor results in a microbial consortium which included hydrogen-utilizing SRB (HSRB), acetic acid SRB (ASRB) and fatty acid utilizing SRB (FSRB) microbial groups.

During conversion of S0₄ ^" to H₂S, the group (ASRB) using lactate, pyruvate, alcohols and fatty acids, and the group (HSRB) using H₂ as the electron donor, are positioned towards the end of the sulfate reducing pathway. The HSRB act as a H₂-consumer and contribute to a low hydrogen partial pressure. Our data shows the sulfate reducing pathway as being from H₂ to H₂S. In general, 1 molar sulfate shall require 4 molar H₂ to produce 1 molar HS-(-151 .9 KJ/M). The reduction of sulfate is mediated by an electron transport system composed of dehydrogenases, electron carriers, and a series of reductases.

The pathway of sulfate reduction H₂S is described in Figure 1.

Overall, there is disclosed a 2-phase anaerobic organic waste (e.g. wastewater) treatment strategy, whereby sulfate is optimally reduced in the acidogenic reactor, while biogas production is also improved/enhanced in the downstream methanogenic reactor. Sulfate, which can be a constituent of organic waste (e.g. wastewater), is typically converted to sulfides by sulfate reducing bacteria (SRB), which also competes for organic substrates with the methanogens in the downstream methanogenic reactor, thereby affecting the biogas (methane) production. - To reduce this SRB competition with methanogens, sulfate and sulfide (i.e. H₂S) concentration from the acidogenic reactor that is fed to the methanogenic reactor has to be optimally decreased, while not affecting the acetic acid production in the acidogenic reactor that is also required for methane production in the methanogenic reactor.

This process involves tuning critical parameters, e.g. controlling the pH in the acidogenic reactor strictly between pH 4.5-pH 6 (i.e. at pH 5-pH 6, preferably at pH 5.5) and maintaining an inert atmosphere in the acidogenic reactor or, optionally, maintaining a temperature of 30Ό-50°Ο. Other critical parameters include the sludge retention time, which should be maintained during the startup period at from 10 to 25 days (e.g. from 1 1 days to 24 days, such as from 14 days to 20 days, such as from 15 days to 18 days, e.g. 20 days). Sulfate is biologically converted into H₂S, and thereafter it is precipitated and removed as metal sulfides. As such, both sulfate and sulfide are removed from the wastewater before feeding into the downstream methanogenic reactor and consequently mitigating the SRB competition with methanogens, so as to improve/enhance methane production. Thus, the process described herein may be generally described as an anaerobic two-phase process for the treatment of organic waste comprising the steps of:

introducing a feed stream comprising organic waste into a first phase reactor;

generating an effluent from the first phase reactor; and

providing the effluent from the first phase reactor to a second phase reactor, wherein, during at least the start-up period of the process, the first phase reactor has a sludge retention time (SRT) of from 10 days to 25 days and the pH is maintained at a pH of from 4.5 to 6.0.

An important feature of the disclosed method is the. longer than usual SRT used in the start- up phase of the anaerobic process in the first phase reactor. The SRT in this start-up phase may be from 1 1 days to 24 days, such as 14 days to 20 days, or about 20 days.

As noted above, the pH of the first phase reactor must also be carefully controlled in combination with the SRT. While the pH may be maintained at a level of from 4.5 to 6.0, the pH may preferably be from about 5.0 to 6.0. In particular examples, the pH should be maintained at about 5.5. While not wishing to be bound by theory, it is believed that the SRBs existed in the first reactor may be strict anaerobes. Given this, the process may further supply the first phase reactor is supplied with an inert gas, such as argon or, more preferably, nitrogen. It will be appreciated that the first phase reactor will establish microbial populations that will generate a biogas with hydrogen production. In particular, the process described employs conditions that establish a population of sulfate reducing bacteria in the first phase reactor to convert sulfate into hydrogen sulfide. Thus, the biogas produced by the first phase reactor will comprise hydrogen sulfide.

In order to improve sulfate reduction, the process may also recycle the biogas produced in the first phase reactor to enhance contact between microbes and biogas, improve sulfate reduction, and sulfide precipitation. The organic waste used in the disclosed method may comprise trace levels of metals and metal ions which may react with the hydrogen sulfide to generate a metal sulfide precipitate. While some of the hydrogen sulfide produced by the current invention will be trapped in this manner, it is to be expected that most (i.e.≥95%) of the hydrogen sulfide produced will be removed from the first phase reactor in gaseous form. Preferably, the hydrogen sulfide may be removed by a chemical or biological scrubber that selectively removes the same from the biogas, thereby allowing the remaining biogas to be recycled into the first phase reactor.

Increasing the presence of SRBs in the acidogenic reactor may be acheived where the hydraulic retention time of the process is fixed and the organic loading rate (OLR) of the first phase reactor is from about 0.5 to about 2 g COD/g MLSS.L.d (e.g. from about 1 to about 1.5 g COD/g MLSS.L.d).

Additionally or alternatively, the presence of SRBs in the acidogenic reactor, a feedstock with a high sulfate concentration may be used. For example, the initial COD/SO₄ ^2" ratio of the organic waste may be from about 10:1 to about 2:1 (e.g. from about 6:1 to about 2:1 , such as from about 5:1 to about 3:1 ).

The organic waste used in the process described above will generally comprise biodegradeable solids. Preferably, the organic waste is a sludge, food waste, oily waste, solid waste with a high organic content or any combination thereof. For example, organic waste may be a waste sludge or a secondary sludge. Preferably, the first phase reactor is primarily an acidogenic reactor, while the second phase reactor is primarily a methanogenic reactor or is a single stage anaerobic reactor.

It is preferred that the first phase reactor is operated with a hydraulic retention time of from about 1.25 days to about 5 days and that the second reactor is operated at a pH of from about 6.5 to about 7.9 (e.g. at a pH of from about 6.8 to about 7.4, such as at a pH of about 7.1 )

It will be appreciated that the first and second reactors may further make use of pH, oxidation reduction potential (ORP) and temperature controllers.

A waste treatment plant (e.g. a wastewater treatment plant) may use the process described above. As will be apparent, a two-phase reactor system for the treatment of biodegradeable waste, comprising a first phase reactor and a second phase reactor that is downstream of the first phase reactor .and is in fluid connection therewith, wherein the first and second phase reactors may be adapted to use the process as described herein. Preferably, the first phase reactor is primarily an acidogenic reactor and the second phase reactor is primarily a methanogenic reactor or is a single stage anaerobic reactor. Further, the first and second phase reactors may further comprise pH, oxidation reduction potential (ORP) and temperature controllers.

The invention shall have application where there is interest in wastewater treatment with energy recovery and the wastewater contains sulfate. The former is of growing importance given the growing awareness of the energy-environment nexus in industrial wastewater management. Currently, the operation of most state-of-the-art anaerobic systems would be upset by SRB competition with methano^'gehs. The process disclosed herein is a move to mitigate SRB competition with methanogens and, thus, enhances energy recovery. The invention can be used at new plants and as a retrofit addition to existing facilities.

Examples

Any source of organic waste with a COD/SO4^2" ratio ranging from 10:1 to 2:1 can be used. Normal Operation of Acidogenic in 2-Phase Reactor System

An acidogenic reactor was set up and operated with the following operating conditions: the pH was maintained within a range of 5.0-6.0;

at a temperature between 30 and 50°C; and

with a SRT at 1 -10 days. During normal operation, the H₂S content in the acidogenic reactor and methanogenic reactor is shown in Figure 2. As shown in Figure 2, the concentration of H₂S in the acidogenic reactor varied from at most about 200 ppm to less than about 10 ppm, in contrast, the concentration of H₂S in the methanogenic reactor was consistently over 2000 ppm , as the maximum detection limit for H₂S was 2000 ppm. This indicates that, under conventional operating conditions, the SRB can compete with methanogens in the methanogenic reactor when the acidogenic reactor is operated using conventional conditions.

Operation of 2-Phase Reactor to Mitigate Sulfate Content a. A two-phase anaerobic process is set-up. The acidogenic reactor is equipped with pH, ORP and temperature controllers. The pH is controlled at 4.5-5.5 and temperature is maintained at 40±1°C. The ORP is about -400 mV.

b. Nitrogen is piped into the acidogenic reactor to regulate the microbial population during start up;

c. The sequencing batch reactor system is operated with a HRT of 1 .25-5 days, an SRT of 15-25 days, and an OLR of 0.5-2 g COD/g MLSS.L.d.

d. Biogas is recycled in the acidogenic reactor to enhance contact between microbes and biogas, improve sulfate reduction, and sulfide precipitation. H₂S content in the acidogenic biogas can increase from less than 10 ppm to several hundred ppm while H₂S in the methanogenic biogas shall decrease.

Compared with conventional system, the new system performed better in terms of COD removal and methane production (50% relative increase in biogas volume). As shown in Figure 3, when the operation of the 2-phase reactor was changed to use the new operating parameters for the acidogenic reactor, the concentration of H₂S in the acidogenic reactor increased significantly (consistently over 200 ppm), while the concentration of H₂S decreased significantly in the methanogenic reactor (consistently under 800 ppm, compared to over 2000 ppm under conventional conditions). This indicates that the modified conditions used to operate the acidogenic reactor, increased sulfate reduction in the acidogenic reactor and mitigated the inhibition of the methanogens in the methanogen reactor. This suggests that operating the acidogenic reactor using these modified conditions allowed the selection of a microbial consortium that has higher than normal numbers of acidogenic SRB. The operating protocol designed for the acidogenic reactor is different to conventional protocols and it is speculated that this resulted in a microbial consortium which included hydrogen-utilizing SRB (HSRB), acetic acid SRB (ASRB) and fatty acid utilizing SRB (FSRB) microbial groups. Because the SRB compete better than the acetogens for substrates such as propionate and butyrate, the SRB manipulated hydrogen transfer in the overall metabolic processes. It is thought that the sulfate is reduced by H₂ which is simultaneously formed in the acidogenic reactors. This decreases the sulfate and sulfide concentration fed to the methanogenic reactor and. so reduces SRB competition with the methanogens for substrates. Thus, such a "pretreatment" reactor reduces adverse impact of sulfate on methane production in the downstream methanogenic reactor.

Claims

1. An anaerobic two-phase process for the treatment of organic waste comprising the steps of:

introducing a feed stream comprising organic waste into a first phase reactor;

generating an effluent from the first phase reactor; and

2. The process of Claim 1 , wherein the SRT is from 1 1 days to 24 days.

3. The process of Claim 2, wherein the SRT is from 14 days to 20 days.

4. The process of Claim 3, wherein the SRT is about 15 days.

5. The process of any one of the preceding claims, wherein the pH is from about 5.0 to 6.0.

6. The process of claim 5, wherein the pH is about 5.5.

7. The process of any one of the preceding claims, wherein the first phase reactor is supplied with an inert gas.

8. The process of Claim 6, wherein the inert gas is nitrogen or argon.

9. The process of any one of the preceding claims, wherein the process further comprises establishing a population of sulfate reducing bacteria in the first phase reactor to convert sulfate into hydrogen sulfide.

10. The process of Claim 9, wherein the process further comprises reacting part of the hydrogen sulfide with a metal or a metal ion native to the waste water to generate a metal sulfide precipitate.

11. The process of Claim 9, wherein part or all the hydrogen sulfide is removed from the first reactor in gaseous form.

12. The process of Claim 11 , wherein the hydrogen sulfide is removed by a chemical or biological scrubber.

13. The process of any one of the preceding claims, wherein, when the hydraulic retention time is fixed, the organic loading rate (OLR) of the first phase reactor is from about 0.5 to about 2 g COD/g MLSS.L.d.

14. The process of Claim 13, wherein the OLR of the first phase reactor is from about 1 to about 1.5 g COD/g MLSS.L.d.

15. The process of any one of the preceding claims, wherein the organic waste has an initial COD/S0₄ ^2" ratio of from about 10:1 to about 2:1.

16. The process of Claim 15, wherein the initial COD/S0₄ ^2" ratio of the organic waste is from about 6: 1 to about 2:1.

17. The process of Claim 16, wherein the initial COD/S0₄ ^2" ratio of the organic waste is from about 5:1 to about 3:1.

18. The process of any one of the preceding claims, wherein the organic waste comprises biodegradeable solids.

19. The process of Claim 18, wherein the organic waste is a sludge, food waste, oily waste, solid waste with a high organic content or any combination thereof.

20. The process of Claim 19, wherein the sludge is a waste sludge or a secondary sludge.

21. The process of any one of the preceding claims, wherein the first phase reactor is primarily an acidogenic reactor.

22. The process of any one of the preceding claims, wherein the second phase reactor is primarily a methanogenic reactor or is a single stage anaerobic reactor.

23. The process of any one of the preceding claims, wherein the first phase reactor is operated with a hydraulic retention time of from about 1.25 days to about 5 days.

24. The process of any one of the preceding claims, wherein the second reactor is operated at a pH of from about 6.5 to about 7.9.

25. The process of Claim 24, wherein the second reactor is operated at a pH of from about 6.8 to about 7.4.

26. The process of Claim 25, wherein the second reactor is operated at a pH of about 7.1.

27. The process of any one of the preceding claims, wherein the first and second reactors further comprise pH, oxidation reduction potential (ORP) and temperature controllers.

28. A waste treatment plant using the process of any one of Claims 1 to 27.

29. The waste treatment plant of Claim 28, wherein the waste treatment plant is a wastewater treatment plant.

30. A two-phase reactor system for the treatment of biodegradeable waste, comprising a first phase reactor and a second phase reactor that is downstream of the first phase reactor and is in fluid connection therewith, wherein the first and second phase reactors are adapted to use the process of Claims 1 to 27.

31. The reactor system of Claim 30, wherein the first phase reactor is primarily an acidogenic reactor.

32. The reactor system of Claim 30 or Claim 31 , wherein the second phase reactor is primarily a methanogenic reactor or is a single stage anaerobic reactor.

33. The reactor system of any one of Claims 30 to 32, wherein the first and second phase reactors further comprise pH, oxidation reduction potential (ORP) and temperature controllers.