A kinetic study on anaerobic reduction of sulphate, Part I: Effect of sulphate concentration

Harnessing Fermentation May Enhance the Performance of Biological Sulfate-Reducing Bioreactors

Biological sulfate reduction (BSR) represents a promising strategy for bioremediation of sulfate-rich waste streams, yet the impact of metabolic interactions on performance is largely unexplored. Here, genome-resolved metagenomics was used to characterize 17 microbial communities in reactors treating synthetic sulfate-contaminated solutions. Reactors were supplemented with lactate or acetate and a small amount of fermentable substrate. Of the 163 genomes representing all the abundant bacteria, 130 encode 321 NiFe and FeFe hydrogenases and all genomes of the 22 sulfate-reducing microorganisms (SRM) encode genes for H2 uptake. We observed lactate oxidation solely in the first packed bed reactor zone, with propionate and acetate oxidation in the middle and predominantly acetate oxidation in the effluent zone. The energetics of these reactions are very different, yet sulfate reduction kinetics were unaffected by the type of electron donor available. We hypothesize that the comparable rates, despite the typically slow growth of SRM on acetate, are a result of the consumption of H2 generated by fermentation. This is supported by the sustained performance of a predominantly acetate-supplemented stirred tank reactor dominated by diverse fermentative bacteria encoding FeFe hydrogenase genes and SRM capable of acetate and hydrogen consumption and CO2 assimilation. Thus, addition of fermentable substrates to stimulate syntrophic relationships may improve the performance of BSR reactors supplemented with inexpensive acetate.

Integrated Kinetic Modelling and Microbial Profiling Provide Insights Into Biological Sulfate-Reducing Reactor Design and Operation

Biological sulfate reduction (BSR) is an attractive approach for the bioremediation of sulfate-rich wastewater streams. Many sulfate-reducing microorganisms (SRM), which facilitate this process, have been well-studied in pure culture. However, the role of individual members of microbial communities within BSR bioreactors remains understudied. In this study we investigated the performance of two up-flow anaerobic packed bed reactors (UAPBRs) supplemented primarily with acetate and with lactate, respectively, during a hydraulic retention time (HRT) study set up to remediate sulfate-rich synthetic wastewater over the course of 1,000 + days. Plug-flow hydrodynamics led to a continuum of changing volumetric sulfate reduction rates (VSRRs), available electron donors, degrees of biomass retention and compositions of microbial communities throughout these reactors. Microbial communities throughout the successive zones of the reactors were resolved using 16S rRNA gene amplicon sequencing which allowed the association of features of performance with discrete microorganisms. The acetate UAPBR achieved a maximum VSRR of 23.2 mg.L⁻¹. h⁻¹ at a one-day HRT and a maximum sulfate conversion of the 1 g/L sulfate of 96% at a four-day HRT. The sulfate reduction reactions in this reactor could be described with a reaction order of 2.9, an important observation for optimisation and future scale-up. The lactate UAPBR achieved a 96% sulfate conversion at one-day HRT, corresponding with a VSRR of 40.1 mg.L⁻¹. h⁻¹. Lactate was supplied in this reactor at relatively low concentrations necessitating the subsequent use of propionate and acetate, by-products of lactate fermentation with acetate also a by-product of incomplete lactate oxidation, to achieve competitive performance. The consumption of these electron donors could be associated with specific SRM localised within biofilms of discrete zones. The sulfate reduction rates in the lactate UAPBR could be modelled as first-order reactions, indicating effective rates were conferred by these propionate- and acetate-oxidising SRM. Our results demonstrate how acetate, a low-cost substrate, can be used effectively despite low associated SRM growth rates, and that lactate, a more expensive substrate, can be used sparingly to achieve high VSRR and sulfate conversions. We further identified the preferred environment of additional microorganisms to inform how these microorganisms could be enriched or diminished in BSR reactors.

Sulfate-reducing bioreactors subjected to high sulfate loading rate or acidity: variations in microbial consortia

Sulfate-reducing bioreactors are used in e.g. the mining industry to remove sulfate and harmful metals from process waters. These bioreactors are expected to be run for extended periods of time and may experience variations in the influent quality, such as increasing sulfate loading rate and decrease in pH, while being expected to function optimally. In this study we followed the sulfate removal rate and variation in microbial communities over a period of up to 333 days in three different up-flow anaerobic sludge blanket (UASB) bioreactors being submitted to increasing sulfate loading rate or decreasing pH. Sodium lactate was used as the sole carbon source and electron donor. All three bioreactors contained highly diverse microbial communities containing archaea, fungi and bacteria. Sulfurospirillum and Desulfovibrio were the most prominent bacterial genera detected in the bioreactors receiving the highest sulfate loading rates, and the greatest relative abundance of methanogenic archaea and the fungal genus Cadophora coincided with the highest sulfate reduction rates. In contrast, Sulfuricurvum was dominant in the bioreactor receiving influent with alternating pH, but its relative abundance receded in response to low pH of the influent. All bioreactors showed excellent sulfate removal even under extreme conditions in addition to unique responses in the microbial communities under changing operational conditions. This shows that a high diversity in the microbial consortia in the bioreactors could make the sulfate removal process less sensitive to changing operational conditions, such as variations in influent sulfate loading rate and pH.

Influents with high sulfate loading rate or low pH were successfully treated.

Microbial consortia showed versatility and adaption to changing operation.

Unknown interactions among microbial groups makes interpretation challenging.

Biological Desulfurization of Tannery Effluent Using Hybrid Linear Flow Channel Reactors

The tanning process generates a saline effluent with high residual organics, sulfate and sulfide concentrations. The transition from a linear to circular economy requires reimagining of waste streams as potential resources. The organics in tannery effluent have the potential to be converted to renewable energy in the form of biogas if inhibitors to anaerobic digestion are removed. Hybrid linear flow channel reactors inoculated with culture-enriched halophilic sulfate reducing bacteria from saline environments were evaluated as a novel pretreatment step prior to anaerobic digestion for the concurrent removal of sulfur species and resource recovery (elemental sulfur and biogas). During continuous operation of a 4-day hydraulic retention time, the reactors were capable of near-complete sulfide oxidation (>97%) and a sulfate reduction efficiency of 60–80% with the formation of a floating sulfur biofilm containing elemental sulfur. Batch anaerobic digestion tests showed no activity on untreated tannery effluent, while the pretreated effluent yielded 130 mL methane per gram COD consumed.

Comparative analysis of brewing wastewater and lactate as carbon sources for microbial community treating acid mine drainage in anaerobic MBBR systems

This study investigated the effect of carbon sources (n = 2) on the performance of a microbial community in an anaerobic moving-bed biofilm reactor (MBBR) treating acid mine drainage (AMD). The 1.5 L anaerobic MBBR was operated across a range of hydraulic retention times - HRT's (3-18 days), using different substrates, i.e. brewing wastewater and lactate as sole carbon sources and electron donors. Maximum sulphate reduction and chemical oxygen demand (COD) consumption rate was 21.94 and 24.28 mg SO42- L^-1 h^-1, and 0.473 and 0.697 mg COD L^-1 d^-1 for brewing wastewater and lactate supplemented bioreactors, respectively, at an HRT of 3 days. The maximum COD/SO42- ratio was found to be 2.564 in the bioreactor supplemented with brewing wastewater at an HRT of 15 days. The metal removal above 70% in the system supplemented with brewing wastewater followed the order; Be²⁺ > Fe²⁺ > Sr²⁺ > Pb²⁺ > Mg²⁺ > Cu²⁺ > Zn²⁺ > Li¹⁺ > Ca²⁺ in comparison to the system supplemented with lactate, Be²⁺ > Fe²⁺ > Sr²⁺ > Mg²⁺ > Cu²⁺ > Li¹⁺ > Zn²⁺ > Pb²⁺ after an HRT of 18 days. Complete removal of beryllium (II) was observed irrespective of the carbon source used. The results clearly showed that brewing wastewater can be deployed as a nutritional supplement in environmental remediation of AMD.

Simultaneous removal of nitrate and sulfate using an up-flow three-dimensional biofilm electrode reactor: Performance and microbial response

Nitrate removal from low carbon water is a problem in the water treatment, especially in the presence of high sulfate. In this work, an up-flow three-dimensional biofilm electrode reactor (3D-BER) was established to remove nitrate and sulfate from low organic carbon water. Results indicated that sulfate negatively affected nitrate removal. Moreover, high electric current and short hydraulic retention time deteriorated the performance of nitrate and sulfate removal. When the influent of SO₄^2- was 150 mg/L, the removal efficiency of NO₃^--N and SO₄^2- was 88.49 ± 4.5% and 29.35 ± 5.5%, respectively. The high-throughput sequencing revealed that denitrifying bacteria dominated in the lower part of the reactor while sulfate reducing bacteria dominated in the upper part of the reactor. It was speculated that oxidation products of sulfide could serve as supplementary electron donors to enhance nitrate removal in the 3D-BER.

Stratification of microbial communities throughout a biological sulphate reducing up-flow anaerobic packed bed reactor, revealed through 16S metagenomics

Biological sulphate reduction (BSR) is a promising low-cost treatment of acid rock drainage effluents. In this paper, the system performance and microbial ecology of a lactate supplemented BSR up-flow anaerobic packed bed reactor (UAPBR) are evaluated across reactor height and compared to a continuous stirred tank reactor (CSTR). The biomass concentrations of planktonic and biofilm communities were quantified and subsequently characterised by 16S rRNA gene amplicon sequencing. The defined microbial communities were shown to correlate with differing availability of lactate, volatile fatty acids produced from lactate degradation and sulphate concentration. The UAPBR was able to achieve near complete sulphate conversion at a 4-day hydraulic residence time (HRT) at a sulphate feed concentration of 10.41 mM (1 g/L). The high volumetric sulphate reduction rate of 0.184 mM/L.h achieved in the first third of the reactor was attributed to OTUs present in the planktonic and biofilm communities. While the scavenging of sulphate within the final third of the UAPBR was attributed to an acetate oxidising genus of SRB which was not detected in the lactate-fed CSTR. The detailed analyses of the microbial communities throughout the UAPBR and CSTR contribute to the growing understanding of the impact of the microbial communities of BSR reactors on system performance.

Carbohydrate based polymeric materials as slow release electron donors for sulphate removal from wastewater

Many industrial sulphate rich wastewaters are deficient in electron donors to achieve complete sulphate removal. Therefore, pure and expensive chemicals are supplied externally. In this study, carbohydrate based polymers (CBP) as potato (2 and 5 mm³), filter paper (2 and 5 mm²) and crab shell (2 and 4 mm Ø) were tested as slow release electron donors (SRED) for biological sulphate reduction at 30 °C and initial pH of 7.0. Using the CBP as SRED, sulphate reduction was carried out at different rates: filter paper 0.065-0.050 > potato 0.022-0.034 > crab shell 0.006-0.009 mg SO₄^2-.mg VSS^-1d^-1. These were also affected by the hydrolysis-fermentation rates: potato 0.087-0.070 > filter paper 0.039-0.047 > crab shell 0.011-0.028 mg COD_S.mg VSS^-1d^-1, respectively. Additionally, the sulphate removal efficiencies using filter paper (cellulose, > 98%), potato (starch, > 82%) and crab shell (chitin, > 32%) were achieved only when using CBP as SRED and in the absence of other easily available electron donors. This study showed that the natural characteristics of the CBP limited the hydrolysis-fermentation step and, therefore, the sulphate reduction rates.

Mathematical modeling of simultaneous carbon-nitrogen-sulfur removal from industrial wastewater

A mathematical model of carbon, nitrogen and sulfur removal (C-N-S) from industrial wastewater was constructed considering the interactions of sulfate-reducing bacteria (SRB), sulfide-oxidizing bacteria (SOB), nitrate-reducing bacteria (NRB), facultative bacteria (FB), and methane producing archaea (MPA). For the kinetic network, the bioconversion of C-N by heterotrophic denitrifiers (NO₃^-→NO₂^-→N₂), and that of C-S by SRB (SO₄^2-→S^2-) and SOB (S^2-→S⁰) was proposed and calibrated based on batch experimental data. The model closely predicted the profiles of nitrate, nitrite, sulfate, sulfide, lactate, acetate, methane and oxygen under both anaerobic and micro-aerobic conditions. The best-fit kinetic parameters had small 95% confidence regions with mean values approximately at the center. The model was further validated using independent data sets generated under different operating conditions. This work was the first successful mathematical modeling of simultaneous C-N-S removal from industrial wastewater and more importantly, the proposed model was proven feasible to simulate other relevant processes, such as sulfate-reducing, sulfide-oxidizing process (SR-SO) and denitrifying sulfide removal (DSR) process. The model developed is expected to enhance our ability to predict the treatment of carbon-nitrogen-sulfur contaminated industrial wastewater.

Modelling Methane Production and Sulfate Reduction in Anaerobic Granular Sludge Reactor with Ethanol as Electron Donor

In this work, a mathematical model based on growth kinetics of microorganisms and substrates transportation through biofilms was developed to describe methane production and sulfate reduction with ethanol being a key electron donor. The model was calibrated and validated using experimental data from two case studies conducted in granule-based Upflow Anaerobic Sludge Blanket reactors. The results suggest that the developed model could satisfactorily describe methane and sulfide productions as well as ethanol and sulfate removals in both systems. The modeling results reveal a stratified distribution of methanogenic archaea, sulfate-reducing bacteria and fermentative bacteria in the anaerobic granular sludge and the relative abundances of these microorganisms vary with substrate concentrations. It also indicates sulfate-reducing bacteria can successfully outcompete fermentative bacteria for ethanol utilization when COD/SO₄²⁻ ratio reaches 0.5. Model simulation suggests that an optimal granule diameter for the maximum methane production efficiency can be achieved while the sulfate reduction efficiency is not significantly affected by variation in granule size. It also indicates that the methane production and sulfate reduction can be affected by ethanol and sulfate loading rates, and the microbial community development stage in the reactor, which provided comprehensive insights into the system for its practical operation.

Understanding the performance of sulfate reducing bacteria based packed bed reactor by growth kinetics study and microbial profiling

A novel marine waste extract (MWE) as alternative nitrogen source was explored for the growth of sulfate reducing bacteria (SRB). Variation of sulfate and nitrogen (MWE) showed that SRB growth follows an uncompetitive inhibition model. The maximum specific growth rates (μmax) of 0.085 and 0.124 h(-1) and inhibition constants (Ki) of 56 and 4.6 g/L were observed under optimized sulfate and MWE concentrations, respectively. The kinetic data shows that MWE improves the microbial growth by 27%. The packed bed bioreactor (PBR) under optimized sulfate and MWE regime showed sulfate removal efficiency of 62-66% and metals removal efficiency of 66-75% on using mine wastewater. The microbial community analysis using DGGE showed dominance of SRB (87-89%). The study indicated the optimum dosing of sulfate and cheap organic nitrogen to promote the growth of SRB over other bacteria.

Application of horizontal-flow anaerobic immobilized biomass reactor for bioremediation of acid mine drainage

The production of low-pH effluent with sulfate and metals is one of the biggest environmental concerns in the mining industry. The biological process for sulfate reduction has the potential to become a low-cost solution that enables the recovery of interesting compounds. The present study analyzed such a process in a horizontal-flow anaerobic immobilized biomass (HAIB) reactor, employing ethanol as the carbon and energy source. Results showed that a maximal efficiency in the removal of sulfate and ethanol could only be obtained by reducing the applied sulfate load (225.1 ± 38 g m(-3) d(-1)). This strategy led to over 75% of chemical oxygen demand (COD) and sulfate removal. Among the COD/SO4(2-) studied ratios, 0.67 showed the most promising performance. The effluent's pH has naturally remained between 6.8 and 7.0 and the complete oxidation of the organic matter has been observed. Corrections of the influent pH or effluent recirculation did not show any significant effect on the COD and sulfate removal efficiency. Species closely related to strains of Clostridium sp. and species of Acidaminobacter hydrogenomorfans and Fusibacter paucivorans that can be related to the process of sulfate reduction were found in the HAIB reactors when the initial pH was 5 and the COD/SO4(2-) ratio increased to 1.0.

Kinetics of sulfate reduction and sulfide precipitation rates in sediments of a bar-built estuary (Pescadero, California)

The bar-built Pescadero Estuary in Northern California is a major fish rearing habitat, though recently threatened by near-annual fish kill events, which occur when the estuary transitions from closed to open state. The direct and indirect effects of hydrogen sulfide are suspected to play a role in these mortalities, but the spatial variability of hydrogen sulfide production and its link to fish kills remains poorly understood. Using flow-through reactors containing intact littoral sediment slices, we measured potential sulfate reduction rates, kinetic parameters of microbial sulfate reduction (Rmax, the maximum sulfate reduction rate, and Km, the half-saturation constant for sulfate), potential sulfide precipitation rates, and potential hydrogen sulfide export rates to water at four sites in the closed and open states. At all sites, the Michaelis-Menten kinetic rate equation adequately describes the utilization of sulfate by the complex resident microbial communities. We estimate that 94-96% of hydrogen sulfide produced through sulfate reduction precipitates in the sediment and that only 4-6% is exported to water, suggesting that elevated sulfide concentrations in water, which would affect fish through toxicity and oxygen consumption, cannot be responsible for fish deaths. However, the indirect effects of sulfide precipitates, which chemically deplete, contaminate, and acidify the water column during sediment re-suspension and re-oxidation in the transition from closed to open state, can be implicated in fish mortalities at Pescadero Estuary.

A novel biological sulfate reduction method using hydrogenogenic carboxydotrophic mesophilic bacteria

Sulfate reduction by carbon monoxide (CO) utilizing anaerobic biomass from a large scale upflow anaerobic sludge blanket reactor was studied. Anaerobic mixed microbial consortia from five different sources were initially examined for their biological CO conversion potential. Among the different biomass, the biomass from an upflow anaerobic sludge blanket reactor treating domestic wastewater, located in Kavoor, Karnataka, India, showed a maximum CO conversion efficiency. The effect of three main culture parameters, i.e. inoculum volume, initial CO concentration and temperature on simultaneous CO conversion and sulfate reduction was assessed employing the Taguchi experimental design technique. A maximum CO conversion of 85.62% and a maximum sulfate reduction of 50.65% were achieved. Furthermore, the experimental data was fitted to substrate inhibition models reported in the literature. Among the different models, Monods and Haldane kinetic models were found most suitable to describe the kinetics of biomass growth and CO removal by the anaerobic biomass.

Sulfate reduction at low pH to remediate acid mine drainage

Industrial activities and the natural oxidation of metallic sulfide-ores produce sulfate-rich waters with low pH and high heavy metals content, generally termed acid mine drainage (AMD). This is of great environmental concern as some heavy metals are highly toxic. Within a number of possibilities, biological treatment applying sulfate-reducing bacteria (SRB) is an attractive option to treat AMD and to recover metals. The process produces alkalinity, neutralizing the AMD simultaneously. The sulfide that is produced reacts with the metal in solution and precipitates them as metal sulfides. Here, important factors for biotechnological application of SRB such as the inocula, the pH of the process, the substrates and the reactor design are discussed. Microbial communities of sulfidogenic reactors treating AMD which comprise fermentative-, acetogenic- and SRB as well as methanogenic archaea are reviewed.

Sulfate-reduction, sulfide-oxidation and elemental sulfur bioreduction process: modeling and experimental validation

This study describes the sulfate-reducing (SR) and sulfide-oxidizing (SO) process using Monod-type model with best-fit model parameters both being reported and estimated. The molar ratio of oxygen to sulfide (ROS) significantly affects the kinetics of the SR+SO process. The S(0) is produced by SO step but is later consumed by sulfur-reducing bacteria to lead to "rebound" in sulfide concentration. The model correlated well all experimental data in the present SR+SO tests and the validity of this approach was confirmed by independent sulfur bioreduction tests in four denitrifying sulfide removal (DSR) systems. Modeling results confirm that the ratio of oxygen to sulfide is a key factor for controlling S(0) formation and its bioreduction. Overlooking S(0) bioreduction step would overestimate the yield of S(0).

Azo dye decolorization assisted by chemical and biogenic sulfide

The effectiveness of chemical and biogenic sulfide in decolorizing three sulfonated azo dyes and the robustness of a sulfate-reducing process for simultaneous decolorization and sulfate removal were evaluated. The results demonstrated that decolorization of azo dyes assisted by chemical sulfide and anthraquinone-2,6-disulfonate (AQDS) was effective. In the absence of AQDS, biogenic sulfide was more efficient than chemical sulfide for decolorizing the azo dyes. The performance of sulfate-reducing bacteria in attached-growth sequencing batch reactors suggested the absence of competition between the studied azo dyes and the sulfate-reducing process for the reducing equivalents. Additionally, the presence of chemical reduction by-products had an almost negligible effect on the sulfate removal rate, which was nearly constant (94%) after azo dye injection.

Bioremediation of mine water

Caused by the oxidative dissolution of sulfide minerals, mine waters are often acidic and contaminated with high concentrations of sulfates, metals, and metalloids. Because the so-called acid mine drainage (AMD) affects the environment or poses severe problems for later use, treatment of these waters is required. Therefore, various remediation strategies have been developed to remove soluble metals and sulfates through immobilization using physical, chemical, and biological approaches. Conventionally, iron and sulfate-the main pollutants in mine waters-are removed by addition of neutralization reagents and subsequent chemical iron oxidation and sulfate mineral precipitation. Biological treatment strategies take advantage of the ability of microorganisms that occur in mine waters to metabolize iron and sulfate. As a rule, these can be grouped into oxidative and reductive processes, reflecting the redox state of mobilized iron (reduced form) and sulfur (oxidized form) in AMD. Changing the redox states of iron and sulfur results in iron and sulfur compounds with low solubility, thus leading to their precipitation and removal. Various techniques have been developed to enhance the efficacy of these microbial processes, as outlined in this review.

The design of long-term effective uranium bioremediation strategy using a community metabolic model

Acetate amendment at uranium contaminated sites in Rifle, CO. leads to an initial bloom of Geobacter accompanied by the removal of U(VI) from the groundwater, followed by an increase of sulfate-reducing bacteria (SRBs) which are poor reducers of U(VI). One of the challenges associated with bioremediation is the decay in Geobacter abundance, which has been attributed to the depletion of bio-accessible Fe(III), motivating the investigation of simultaneous amendments of acetate and Fe(III) as an alternative bioremediation strategy. In order to understand the community metabolism of Geobacter and SRBs during artificial substrate amendment, we have created a genome-scale dynamic community model of Geobacter and SRBs using the previously described Dynamic Multi-species Metabolic Modeling framework. Optimization techniques are used to determine the optimal acetate and Fe(III) addition profile. Field-scale simulation of acetate addition accurately predicted the in situ data. The simulations suggest that batch amendment of Fe(III) along with continuous acetate addition is insufficient to promote long-term bioremediation, while continuous amendment of Fe(III) along with continuous acetate addition is sufficient to promote long-term bioremediation. By computationally minimizing the acetate and Fe(III) addition rates as well as the difference between the predicted and target uranium concentration, we showed that it is possible to maintain the uranium concentration below the environmental safety standard while minimizing the cost of chemical additions. These simulations show that simultaneous addition of acetate and Fe(III) has the potential to be an effective uranium bioremediation strategy. They also show that computational modeling of microbial community is an important tool to design effective strategies for practical applications in environmental biotechnology.

Effect of culture conditions on the competitive interaction between lactate oxidizers and fermenters in a biological sulfate reduction system

Kinetic constants (μ(max) and K(s)) describing the predominance of lactate oxidation and fermentation were determined in chemostat cultures. The kinetics of sulfate reduction and lactate utilization were determined from 0.5 to 5d residence times at feed sulfate concentrations of 1.0-10.0 g l(-1). The kinetics of lactate fermentation in the absence of sulfate were investigated at residence times of 0.5-5d. The lactate oxidizers (LO) were characterized by a μ(max) of 0.2h(-1) and K(s) value of 0.6 g l(-1) compared with a μ(max) of 0.3h(-1) and K(s) of 3.3 g l(-1) for the lactate fermenters (LF). Using mathematical models, it was shown that LO competed more effectively for lactate at low lactate concentrations (≤5 g l(-1)) and high sulfide concentrations (0.5 g l(-1)). Lactate fermenters outcompeted the oxidizers under conditions of excess lactate (>5 g l(-1)) and low sulfide (0.014-0.088 g l(-1)).

Temperature and feed strategy effects on sulfate and organic matter removal in an AnSBB

The objective of this work was to analyze the interaction effects between temperature, feed strategy and COD/[SO(4)(2-)] levels, maintaining the same ratio, on sulfate and organic matter removal efficiency from a synthetic wastewater. This work is thus a continuation of Archilha et al. (2010) who studied the effect of feed strategy at 30 °C using different COD/[SO(4)(2-)] ratios and levels. A 3.7-L anaerobic sequencing batch reactor with recirculation of the liquid phase and which contained immobilized biomass on polyurethane foam (AnSBBR) was used to treat 2.0 L synthetic wastewater in 8 h cycles. The temperatures of 15, 22.5 and 30 °C with two feed strategies were assessed: (a) batch and (b) batch followed by fed-batch. In strategy (a) the reactor was fed in 10 min with 2 L wastewater containing sulfate and carbon sources. In strategy (b) 1.2 L wastewater (containing only the sulfate source) was fed during the first 10 min of the cycle and the remaining 0.8 L (containing only the carbon source) in 240 min. Based on COD/[SO(4)(2-)] = 1 and on the organic matter (0.5 and 1.5 gCOD/L) and sulfate (0.5 and 1.5 gSO(4)(2-)/L) concentrations, the sulfate and organic matter loading rates applied were 1.5 and 4.5 g/L.d, i.e., same COD/[SO(4)(2-)] ratio (=1) but different levels (1.5/1.5 and 4.5/4.5 gCOD/gSO(4)(2-)). When reactor feed was 1.5 gCOD/L.d and 1.5 gSO(4)(2-)/L.d, gradual feeding (strategy b) showed to favor sulfate and organic matter removal in the investigated temperature range, indicating improved utilization of the electron donor for sulfate reduction. Sulfate removal efficiencies were 87.9; 86.3 and 84.4%, and organic matter removal efficiencies 95.2; 86.5 and 80.8% at operation temperatures of 30; 22.5 and 15 °C, respectively. On the other hand, when feeding was 4.5 gCOD/L.d and 4.5 gSO(4)(2-)/L.d, gradual feeding did not favor sulfate removal, indicating that gradual feeding of the electron donor did not improve sulfate reduction.

Sulfidogenic biotreatment of synthetic acid mine drainage and sulfide oxidation in anaerobic baffled reactor

The treatment of synthetic acid mine drainage (AMD) water (pH 3.0-6.5) containing sulfate (3.0-3.5 g L(-1)) and various metals (Co, Cu, Fe, Mn, Ni, and Zn) was studied in an ethanol-fed sulfate-reducing 4-compartment anaerobic baffled reactor (ABR) at 32°C. The reactor was operated for 160 days at different chemical oxygen demand (COD)/sulfate ratios, hydraulic retention times (HRT), pH, and metal concentrations to study the robustness of the process. The last compartment of the reactor was aerated at different rates to study the bio-oxidation of sulfide to elemental sulfur. The highest sulfate reduction efficiency (88%) was obtained with a feed sulfate concentration of 3.5 g L(-1), COD/sulfate mass ratio of 0.737, feed pH of 3.0 and HRT of 2 days without aeration in the 4th compartment. The corresponding COD removal efficiency was about 92%. The alkalinity produced in the sulfidogenic ethanol oxidation neutralized the acidic mine water from pH 3.0-4.5 to pH 7.0-8.0. Effluent soluble and total heavy metal concentrations were substantially reduced with removal efficiencies generally higher than 99%, except for Mn (25-77%). Limited aeration in the 4th compartment of ABR promoted incomplete oxidation of sulfide to elemental sulfur rather than complete oxidation to sulfate. Depending on the aeration rate and HRT, 32-74% of produced sulfide was oxidized to elemental sulfur. This study demonstrates that by optimizing operating conditions, sulfate reduction, metal removal, alkalinity generation, and excess sulfide oxidation can be achieved in a single ABR treating AMD.

Combined strategy for the precipitation of heavy metals and biodegradation of petroleum in industrial wastewaters

The precipitation of chromium(III), copper(II), manganese(II) and zinc(II) by biogenic hydrogen sulfide generated by sulfate-reducing bacteria, Desulfovibrio sp., and the degradation of total petroleum hydrocarbons (TPH) in the presence of heavy metal by Pseudomonas aeruginosa AT18 have been carried out. An anaerobic stirred tank reactor was used to generate hydrogen sulfide with Desulfovibrio sp. culture and the precipitation of more than 95% of each metal was achieved in 24 h (metal solutions contained: 60, 49, 50 and 80 mg L(-1) of chromium, copper, manganese and zinc sulfates). A stirred tank reactor with P. aeruginosa AT18, in the presence of the heavy metal solution and 2% (v/v) of petroleum, led to the degradation of 60% of the total petroleum hydrocarbons and the removal of Cr(III) 99%, Cu(II) 93%, Zn(II) 46% and Mn(II) 88% in the medium through biosorption phenomena. These results enabled the development of an integrated system in which the two processes were combined. The overall aim of the study was achieved, with 84% of TPH degraded and all of the metals completely removed. Work is currently underway aimed at improving this system (decrease in operation time, culture of P. aeruginosa in anaerobic conditions) in an effort to apply this process in the bioremediation of natural media contaminated with heavy metals and petroleum.

Influence of feed time and sulfate load on the organic and sulfate removal in an ASBR

The removal of sulfate and organic matter was assessed in an ASBR, which treated wastewater containing 500 mg CODL(-1) (3 g CODL(-1)d(-1)) in 8h-cycles at 30 degrees C. The wastewater was enriched with sulfate at [COD/SO(4)(2-)] ratios of 1.34, 0.67 and 0.34 (8.8,4.5 and 2.2 gSO(4)(2-)L(-1)d(-1)). For each COD/[SO(4)(2-)] ratio fill times used were: 10 min (batch), 3 and 6h (fed-batch), achieving sulfate reduction of 30%, 72% and 72% (COD/[SO(4)(2-)] of 1.34); 25%, 58% and 55% (COD/[SO(4)(2-)] of 0.67) and 23%, 37% and 27% (COD/[SO(4)(2-)] of 0.34), respectively, and organic matter removal of 87%, 68% and 80% (COD/[SO(4)(2-)] of 1.34); 78%, 75% and 69% (COD/[SO(4)(2-)] of 0.67) and 85%, 84% and 83% (COD/[SO(4)(2-)] of 0.34), respectively. The results showed that fed-batch operation improved sulfate reduction, whereas organic matter removals were similar for batch and fed-batch operation. In addition, increase in sulfate loading in the fed-batch operation improved organic matter removal.

Effect of feeding strategy and COD/sulfate ratio on the removal of sulfate in an AnSBBR with recirculation of the liquid phase

The objective of this work was to analyze the effect of the interaction between feeding strategy and COD/sulfate ratio on the removal efficiency of sulfate and organic matter from a synthetic wastewater. An anaerobic sequencing batch reactor with recirculation of the liquid phase and containing immobilized biomass on polyurethane foam (AnSBBR) was used. The AnSBBR with a total volume of 3.7 L, treated 2.0 L synthetic wastewater in 8-h cycles at 30+/-1 degrees C and was inoculated with anaerobic biomass from a UASB. Two feeding strategies were assessed: (a) batch and (b) batch followed by fed-batch. In strategy (a) the reactor was fed in 10 min with 2L wastewater containing sulfate and carbon sources. In strategy (b) 1.2 L wastewater (containing only the sulfate source) was fed during the first 10 min of the cycle and the remaining 0.8L (containing only the carbon source) in 240 min. The COD/sulfate ratios assessed were 1 and 3. Based on these values and on the concentrations of organic matter (0.5-11.25 gCOD/L) and sulfate (0.5 and 2.5 gSO(4)(2-)/L), the sulfate and organic matter loading rates applied equaled 1.5 and 4.5 gSO(4)(2-)/Ld for sulfate and 1.5, 4.5 and 13.5 gCOD/Ld for organic matter. After stabilization of the system time profiles were run of monitored parameters (COD, sulfate, sulfide and sulfite). In general, the reactor showed to be robust for use in the anaerobic treatment of wastewaters containing sulfate. Gradual feeding (strategy b) of the carbon source favored sulfate reduction, resulting in sulfate removal efficiencies of 84-98% and organic matter removal efficiencies of 48-95%. The best results were observed under COD/sulfate ratio equal to 1 (loading rates of 1.5 and 4.5 gSO(4)(2-)/Ld for sulfate, and 1.5 and 4.5 gCOD/Ld for organic matter). When COD/sulfate ratio was 3 (loading rates of 1.5 and 4.5 gSO(4)(2-)/Ld for sulfate, and 4.5 and 13.5 gCOD/Ld for organic matter) the effect of feed mode became less significant. These results show that the strategy batch followed by fed-batch is more advantageous for COD/sulfate ratios near the stoichiometric value (0.67) and higher organic matter and sulfate concentrations.

Experimental and mathematical modeling studies on Cr(VI) reduction by CRB, SRB and IRB, individually and in combination

Cr(VI) reduction studies were carried out with chromium reducing bacteria (CRB), sulphate reducing bacteria (SRB) and iron reducing bacteria (IRB), individually and in combination. Biokinetic parameters such as maximum specific growth rate (micro(max)), half saturation constant (K(s)), yield coefficient (Y(T)) and inhibition coefficient (K(i)) for individual cultures were evaluated. A mathematical model was proposed for simulating the chromium reduction, COD utilization and biomass growth, by individual cultures as well as by a combination of two or three different cultures, for different initial Cr(VI), SO(4)(2-) and Fe(III) concentrations. The biokinetic parameters evaluated from one set of experiments for individual cultures were utilized in all the validation studies. The performance of the mathematical model in terms of the dimensionless modified coefficient of efficiency (E) indicated that the proposed model simulates the system behavior very well.

Performance of sulfidogenic anaerobic baffled reactor (ABR) treating acidic and zinc-containing wastewater

The applicability of anaerobic baffled reactor (ABR) was investigated for the treatment of acidic (pH 4.5-7.0) wastewater containing sulfate (1000-2000 mg/L) and Zn (65-200mg/L) at 35 degrees C. The ABR consisted of four equal stages and lactate was supplemented (COD/SO(4)(2-)=0.67) as carbon and energy source for sulfate reducing bacteria (SRB). The robustness of the system was studied by decreasing pH and increasing Zn, COD, and sulfate loadings. Sulfate-reduction efficiency quickly increased during the start-up period and reached 80% within 45 days. Decreasing feed pH, increasing feed sulfate and Zn concentrations did not adversely affect system performance as sulfate reduction and COD removal efficiencies were within 62-90% and 80-95%, respectively. Although feed pH was steadily decreased from 7.0 to 4.5, effluent pH was always within 6.8-7.5. Over 99% Zn removal was attained throughout the study due to formation of Zn-sulfide precipitate.

Competitive reaction kinetics of sulfate-reducing bacteria and methanogenic bacteria in anaerobic filters

A kinetic model for the anaerobic filter (AF) that takes into account the mass fractions of sulfate-reducing bacteria (SRB) (fSRB) and methanogenic bacteria (MB) (fMB) and an inhibiting effect of H2S on bacterial groups is proposed. When the acetate-fed AFs were maintained at the low organic loading rate of 2.5kg COD/m3d, variations of the influent COD/SO4(2-) ratio (0.5-3.0) does not materially affect the acetate removal efficiency (all varying between 98.1% and 99.7%). With an increase in influent COD/SO4(2-) ratio, both the biofilm thickness and the specific substrate utilization rate decreased slightly but f(SRB) decreased markedly. The estimated results of fSRB and fMB showed that SRB out-competed MB for bacterial growth if the influent COD/SO4(2-) ratio was maintained at less than 1.3, whereas MB out-competed SRB for bacterial growth if the influent COD/SO4(2-) ratio was maintained at greater than 2.0. The specific substrate utilization rate of SRB (0.19-0.24mg acetate/mg VSSd) was lower than that of MB (0.31-0.59mg acetate/mg VSSd). The estimated kinetic parameters disclosed that the affinity of acetate to MB was higher and unionized H2S imposed a greater inhibiting effect on MB. The model simulation results (acetate and sulfate removal) agreed well with the experimental results.

A study of the relative dominance of selected anaerobic sulfate-reducing bacteria in a continuous bioreactor by fluorescence in situ hybridization

The diversity and the community structure of sulfate-reducing bacteria (SRB) in an anaerobic continuous bioreactor used for treatment of a sulfate-containing wastewater were investigated by fluorescence in situ hybridization. Hybridization to the 16S rRNA probe EUB338 for the domain Bacteria was performed, followed by a nonsense probe NON338 as a control for nonspecific staining. Sulfate-reducing consortia were identified by using five nominally genus-specific probes (SRB129 for Desulfobacter, SRB221 for Desulfobacterium, SRB228 for Desulfotomaculum, SRB660 for Desulfobulbus, and SRB657 for Desulfonema) and four group-specific probes (SRB385 as a general SRB probe, SRB687 for Desulfovibrioaceae, SRB814 for Desulfococcus group, and SRB804 for Desulfobacteriaceae). The total prokaryotic population was determined by 4',6-diamidino-2-phenylindole staining. Hybridization analysis using these 16S rRNA-targeted oligonucleotide probes showed that, of those microbial groupings investigated, Desulfonema, Desulfobulbus, spp., and Desulfobacteriaceae group were the main sulfate-reducing bacteria in the bioreactor when operated at steady state at 35 degrees C, pH 7.8, and a 2.5-day residence time with feed stream containing 2.5 kg m-3 sulfate as terminal electron acceptor and 2.3 kg m-3 acetate as carbon source and electron donor.

Passive treatment of acid mine drainage in bioreactors using sulfate-reducing bacteria: critical review and research needs

Acid mine drainage (AMD), characterized by low pH and high concentrations of sulfate and heavy metals, is an important and widespread environmental problem related to the mining industry. Sulfate-reducing passive bioreactors have received much attention lately as promising biotechnologies for AMD treatment. They offer advantages such as high metal removal at low pH, stable sludge, very low operation costs, and minimal energy consumption. Sulfide precipitation is the desired mechanism of contaminant removal; however, many mechanisms including adsorption and precipitation of metal carbonates and hydroxides occur in passive bioreactors. The efficiency of sulfate-reducing passive bioreactors is sometimes limited because they rely on the activity of an anaerobic microflora [including sulfate-reducing bacteria (SRB)] which is controlled primarily by the reactive mixture composition. The most important mixture component is the organic carbon source. The performance of field bioreactors can also be limited by AMD load and metal toxicity. Several studies conducted to find the best mixture of natural organic substrates for SRB are reviewed. Moreover, critical parameters for design and long-term operation are discussed. Additional work needs to be done to properly assess the long-term efficiency of reactive mixtures and the metal removal mechanisms. Furthermore, metal speciation and ecotoxicological assessment of treated effluent from on-site passive bioreactors have yet to be performed.

Identification of population dynamics in sulfate-reducing consortia on exposure to sulfate

The microbial population structure and function of a mixed culture of sulfate-reducing bacteria (SRB) maintained in anaerobic continuous bioreactors were tracked before and after a major perturbation, which involved the addition of sulfate to the influent of a bioreactor when operated at steady state at 35 degrees C, pH 7.8 and a 2.5 day residence time with feed stream containing 10 and 15 kg m(-3) sulfate as terminal electron acceptor and 19.6 and 29.4 kg m(-3) ethanol as carbon source and electron donor, respectively. The population structure determined by fluorescence in situ hybridization (FISH), by using 16S rRNA-targeted oligonucleotide probes, was linked to the functional performance of the SRB in the reactor. Hybridization analysis using these 16S rRNA-targeted oligonucleotide probes revealed that a high concentration of sulfate was toxic for Desulfobacterium and Desulfobulbus. On the other hand, the Desulfococcus group was found to be the most dominant group of SRB in the feed stream containing 15 kg m(-3) sulfate as terminal electron acceptor and 29.4 kg m(-3) ethanol as carbon source and electron donor.