Sulfate reducing processes at extreme salinity and temperature. extending its application window

The characteristics of various sulfate-rich wastewaters, such as temperature, pH and salinity, are determined by the (industrial) process from which they originate, and can be far from the physiological optima of the sulfur cycle microorganisms. The main goal of the research described in this thesis was to investigate and develop high rate sulfate reducing wastewater treatment processes for the treatment of inorganic sulfate-rich wastewaters under extreme conditions, i.e. high temperature and high salinity. In this thesis, several simple organic bulk chemicals were tested as electron donor, viz. lower alcohols (methanol and ethanol) and volatile fatty acids (formate, acetate and propionate).With respect to the start-up of anaerobic sludge bed (UASB) reactors at high salinity or high temperature, the results obtained in this investigation indicate that the appearance of a targeted metabolic property (sulfate reduction at high salinity or at high temperature) is independent of the strategy for biomass acclimation (direct exposure vs. stepwise exposure).The stepwise adaptation of thermophilicsulfidogenic methanol degrading biomass to a highosmolarity environment, both at 55C or at 70C, likely does not occur in UASB reactors, as probably no methanol halotolerant thermophilic sulfate reducing bacteria (SRB) were present in the thermophilicinoculumsludge used in the investigations described in this thesis. Exposing the sludge directly to a very high salinity (50 g NaCl.L -1 ) stimulated the growth of amesophilic(30C) propionate- and ethanol-utilizinghalotolerantSRB population, which supported high rate sulfate reduction (up to 3.6 g SO 42- .L -1 .day -1 ) in a UASB reactor. The start-up ofthermophilic(55 to 65C) and extremethermophilicC or higher) anaerobic bioreactors inoculated withmesophilicsludgesat the targeted temperature proceeded fast and stable, as it provoked the rapid selection of (extreme)thermophiles. Therefore, the key for the successful treatment of high salinity or hot wastewaters is to invest enough time for the growth of the targeted microorganism in the biomass.The results of this investigation show that the competition between SRB, methane producingarchaeaandacetogenicbacteria for substrate is highly dependent of the type of substrate and operational conditions imposed to the bioreactor. This thesis describes a situation where the production of acetate and methane was completely suppressed in methanol-fed sulfate reducing UASB reactors operated at 70C. As a result, for the first time a fully sulfate reducing granular sludge has been cultivated in a methanol-fedthermophilicsulfate reducing reactor (with sulfate reduction rates as high as 14.4 g SO 42- .L -1 .day -1 ), provided that an operational temperature of 70C is kept. The production of methane can be easily suppressed inthermophilicmethanol fed reactors, either by running the reactor at temperatures equal or higher than 65C or by exposing 55C operated reactors to a short (2 days) temperature (65 - 70C) shock.Methanogenesiscan also be easily suppressed inmesophilicpropionate- and ethanol-fed reactors, provided high salinity conditions prevail (e.g. above 50 mS.cm -1 ). It seems, however, that the production of acetate, with the exception of methanol-fed reactors operated at 70C, is unavoidable both inthermophilicandmesophilicreactors.This thesis also describes the use of specialized microorganisms, the halophilicDesulfobacterhalotolerans , in bioreactors for the treatment of saline sulfate-rich wastewaters. Very high specific sulfate reduction rates (up to 6.6 g SO 42- .gVSS -1 .day -1 ) can be obtained in completely mixed tank reactors where the biomass grows in suspension and can be efficiently retained by membranes which are submerged in the reactor system. This investigation showed that anaerobic membrane bioreactors can be operated over extended periods of time at a fixed flux, if this flux is substantially below the nominal critical flux determined experimentally (18-21 L.m -2 .h -1 ). Chemical cleaning of the membranes will be required only at about 106 days, as long a low constant flux is imposed (4.7 L.m -2 .h .1 ) and intermittentbackflush(e.g. 1 minute each 10 minutes) is adopted as operational strategy

High rate sulfate reduction in a submerged anaerobic membrane bioreactor (SAMBaR) at high salinity

Sulfate reduction in salt rich wastewaters (50 g NaCl L¿1 and 1 g MgCl2·6H2O L¿1; conductivity 60¿70 mS cm¿1) was investigated in a 6 L submerged anaerobic membrane bioreactor (SAMBaR) and inoculated solely with the halotolerant sulfate reducing bacterium Desulfobacter halotolerans. The SAMBaR was fed with acetate and ethanol at organic loading rates up to 14 g COD L¿1 day¿1 in excess of sulfate (COD/SO42¿ of 0.5) and operated at pH 7.2 ± 0.2 and a hydraulic retention time (HRT) from 8 to 36 h. A sulfate reduction rate up to 6.6 g SO42¿ L¿1 day¿1 was achieved in the SAMBaR operating at a flux of 17.1 L m¿2 h¿1, which resulted in a HRT of 9 h including the backflow of permeate used for backflushing. The fairly constant very high specific sulfate reduction rate of 5.5 g SO42¿ g VSS¿1 day¿1 showed that the performance of the SAMBaR was limited by the low amount of biomass (0.85 g VSS L¿1) present in the reactor at the end of the experiment. It was shown that sulfate reducing submerged anaerobic membrane bioreactors can be operated over extended periods of time without chemical cleaning of the membranes at a certain fixed flux if this flux is substantially below the nominal critical flux determined experimentally (18¿21 L m¿2 h¿1). Intermittent operation as well as backflush of the membranes were shown to slow the fouling in the membranes. Frequent backflush (e.g. 1 min each 10 min) is the suggested operational strategy to minimize fouling in anaerobic MBRs

Assessment of compatible solutes to overcome salinity stress in thermophilic (55 oC) methanol-fed sulfate reducing granular sludges

High NaCl concentrations (25 g.L-1) considerably decreased the methanol depletion rates for sludges harvested from two lab-scale sulfate reducing UASB reactors. In addition, 25 gNaCl.L-1 strongly affected the fate of methanol degradation, with clear increase in the acetate production at the expense of sulfide and methane production. The addition of different osmoprotectants, viz. glutamate, betaine, ectoine, choline, a mixture of compatible solutes and K+ and Mg2+, slightly increased methanol depletion rates for UASB reactors sludges. However, the acceleration in the methanol uptake rate favored the homoacetogenic bacteria, as the methanol breakdown was steered to the formation of acetate without increasing sulfate reduction and methane production rates. Thus, the compatible solutes used in this work were not effective as osmoprotectants to alleviate the acute NaCl toxicity on sulfate reducing granular sludges developed in methanol degrading thermophilic (55°C) UASB reactorsHigh NaCl concentrations (25 g(.)L(-1)) considerably decreased the methanol depletion rates for sludges harvested from two lab-scale sulfate reducing UASB reactors. In addition, 25 gNaCl.L-1 strongly affected the fate of methanol degradation, with clear increase in the acetate production at the expense of sulfide and,methane production. The addition of different osmoprotectants, viz. glutmate, betaine, ectoine, choline, a mixture of compatible solutes and K+ and Mg2+, slightly increased methanol depletion rates for UASB reactors sludges. However, the acceleration in the methanol uptake rate favored the homoacetogenic bacteria, as the methanol breakdown was steered to the formation of acetate without increasing sulfate reduction and methane production rates. Thus, the compatible solutes used in this work were not effective as osmoprotectants to alleviate the acute NaCl toxicity on sulfate reducing granular sludges developed in methanol degrading thermophilic (55degreesC) UASB reactors

Long-term adaptation of methanol-fed thermophilic (55°C) sulfate-reducing reactors to NaCl

A laboratory-scale upflow anaerobic sludge bed (UASB) reactor was operated during 273 days at increasing NaCl concentrations (0.5-12.5 g NaCl l(-1)) to assess whether the stepwise addition of the salt NaCl results in the acclimation of that sludge. The 6.5-1 thermophilic (55 degreesC), sulfidogenic [a chemical oxygen demand (COD) to SO42- ratio of 0.5] UASB reactor operated at an organic loading rate of 5 g COD l(-1) day(-1), a hydraulic retention time of 10 h and was fed with methanol as the sole electron donor. The results show that the adaptation of the thermophilic, sulfidogenic methanol-degrading biomass to a high osmolarity environment is unlikely to occur. Sulfide was the main mineralization product from methanol degradation, regardless of the NaCl concentration added to the influent. However, sulfide production in the reactor steadily decreased after the addition of 7.5 g NaCl l(-1), whereas acetate production was stimulated at that influent NaCl concentration. Batch tests performed with sludge harvested from the UASB reactor when operating at different influent salinities confirmed that acetate is the main metabolic product at NaCl concentrations higher than 12.5 g l(-1). The apparent order of NaCl toxicity towards the different trophic groups was found to be: sulfate-reducing bacteria > methane-producing archaea > acetogenic bacteria

Sulfidogenic volatile fatty acid degradation in a baffled reactor

The effect of staging the sludge bed on volatile fatty acid degradation by sulfidogenic reactors was evaluated in a baffled reactorThe effect of staging the sludge bed on volatile fatty acid degradation by sulfidogenic reactors was evaluated in a baffled reactor. In a 5.41 baffled reactor, containing three equal compartments, a volatile fatty acid (VFA) mixture (acetate: propionate:butyrate ratio 1:2:2 on COD basis; pH 8) was treated under mesophilic (30 degreesC) and sulfidogenic (COD:SO42- ratio: 0.5) conditions for 38 days. At a specific sludge loading rate of 0.50 g COD.gVSS(-1).d(-1), a COD and sulfate removal of 85% and 30%, respectively, was obtained. In the baffled reactor, staging of the sulfidogenic VFA degradation occurred. Propionate and butyrate were mainly degraded in the first compartment. Their degradation was incomplete, resulting in elevated acetate concentrations in compartment I. In the second and third compartment of the baffled reactor, a net degradation of acetate took place. Acetate was the sole substrate present in compartment III and residual acetate concentrations of about 200 mg/l were present in the effluent at a specific sludge loading of 0.50 g COD.gVSS(-1).d(-1). Sludges with different maximum specific VFA and acetate degrading activities developed in the first and second compartment. These maximal specific activities were almost equal for sludge present in compartment II and III

Thermophilic (55 - 65°C) and extreme thermophilic (70 - 80°C) sulfate reduction in methanol and formate-fed UASB reactors

The feasibility of thermophilic (55-65 degreesC) and extreme thermophilic (70-80 degreesC) sulfate-reducing processes was investigated in three lab-scale upflow anaerobic sludge bed (UASB) reactors fed with either methanol or formate as the sole substrates and inoculated with mesophilic granular sludge previously not exposed to high temperaturesThe feasibility of thermophilic (55-65 degreesC) and extreme thermophilic (70-80 degreesC) sulfate-reducing processes was investigated in three lab-scale upflow anaerobic sludge bed (UASB) reactors fed with either methanol or formate as the sole substrates and inoculated with mesophilic granular sludge previously not exposed to high temperatures. Full methanol and formate degradation at temperatures up to, respectively, 70 and 75 degreesC, were achieved when operating UASB reactors fed with sulfate rich (COD/SO42- = 0.5) synthetic wastewater. Methane-producing archaea (MPA) outcompeted sulfate-reducing bacteria (SRB) in the formate-fed UASB reactor at all temperatures tested (65-75 degreesC). In contrast, SRB outcompeted MPA in methanol-fed UASB reactors at temperatures equal to or exceeding 65 degreesC, whereas strong competition between SRB and MPA was observed in these reactors at 55 degreesC. A short-term (5 days) temperature increase from 55 to 65 degreesC was an effective strategy to suppress methanogenesis in methanol-fed sulfidogenic UASB reactors operated at 55 degreesC. Methanol was found to be a suitable electron donor for sulfate-reducing processes at a maximal temperature of 70 degreesC, with sulfide as the sole mineralization product of methanol degradation at that temperature

High-Rate Sulfate Reduction at High Salinity (up to 90 mS.cm-1) in Mesophilic UASB Reactors

Sulfate reduction in salt-rich wastewaters using unadapted granular sludge was investigated in 0.9 L UASB reactors (pH 7.0 ± 0.2; hydraulic retention time from 8-14 h) fed with acetate, propionate, or ethanol at organic loading rates up to 10 gCOD.L-1.day-1 and in excess sulfate (COD/SO of 0.5). High-rate sulfate reduction rates (up to 3.7 gSO42-.L-1.day-1) were achieved at salinities exceeding 50 gNaCl.L-1 and 1 gMgCl2.L-1. Sulfate reduction proceeded at a salinity of up to 70 gNaCl.L-1 and 1 gMgCl2.L-1 (corresponding to a conductivity of about 85-90 mS.cm-1), although at lower rates compared to a conductivity of 60-70 mS.cm-1. Ethanol as well as propionate were suitable substrates for sulfate reduction, with acetate and sulfide as the end products. The successful high-rate treatment was due to the proliferation of a halotolerant incomplete oxidizing SRB population present in the unadapted inoculum sludge. Bioaugmentation of this sludge with the acetate oxidizing halotolerant SRB Desulfobacter halotolerans was unsuccessful, as the strain washed out from the UASB reactor without colonizing the UASB granules. © 2004 Wiley Periodicals, Inc