Biological Sulfate Reduction Process Research Articles

PH-dependent biological sulfidogenic processes for metal-laden wastewater treatment: Sulfate reduction or sulfur reduction?

Both biological sulfate reduction process and sulfur reduction process are attractive technologies for metal-laden wastewater treatment. However, the acidity stress of metal-laden wastewater could affect the sulfidogenic performance and the microbial community, weaken the stability, efficiency and cost-effectiveness of the biological sulfidogenic processes (BSP). In this study, long-term lab-scale trials were conducted with a sulfate-reducing bioreactor and a sulfur-reducing bioreactor to evaluate the effects of acidity on sulfidogenic activities and the microbial community of the BSP. In the 300-day trial, the sulfate-reducing bacteria (SRB)-driven BSP was stable in terms of sulfidogenic performance and microbial community with the decline of pH, while the sulfur-reducing bacteria (S0RB)-driven BSP achieved high-rate and low-cost sulfide production under neutral conditions but unstable under acidic conditions. With the decline of pH, the sulfide production rate (SPR) of the SRB-driven BSP stably increased from 30 to 83 mg S/L-h; while it decreased from 56 to 37 mg S/L-h in the S0RB-driven BSP with high fluctuation. The results of estimation were consistent with the thermodynamical calculations, in which the sulfur reduction process showed a better performance at pH 5–7, while the sulfate reduction process might gain more energy when pH<5. The stable sulfidogenic performance and microbial community diversity of the SRB-driven BSP could be attributed to the alkalinity produced in sulfate reduction to buffer the acidic stress. In comparison, the microbial community in the S0RB-driven BSP was significantly re-shaped by acidity stress, and the predominant sulfidogenic bacterium changed from Desulfovibrio at neutral condition, to Desulfurella at pH≤5.4. The stability of the microbial community significantly affected the SPR and the operational cost. Nevertheless, the organic consumption for sulfide production of the S0RB-driven BSP was still less than the SRB-driven BSP even in acidic conditions. Collectively, the S0RB-driven BSP was recommended under neutral or mild acid conditions, while the SRB-driven BSP was more suitable under fluctuating pH conditions, especially at low pH. Overall, this study presented the long-term performance of SRB- and S0RB-driven BSP under varying pH conditions, and provided guidance to determine the suitable BSP and operational cost for different metal-laden wastewater.

Metagenomic insights into aniline effects on microbial community and biological sulfate reduction pathways during anaerobic treatment of high-sulfate wastewater

For comprehensive insights into the change of sulfate reduction pathway responding to the toxic stress and the shift of microbial community and performance of sulfate reduction, we built a laboratory-scale expanded granular sludge bed reactor (EGSB) treating high-sulfate wastewater with elevated aniline concentrations from 0 to 480 mg/L. High-throughput sequencing and metagenomic approaches were applied to decipher the molecular mechanisms of sulfate reduction under aniline stress through taxonomic and functional profiles. The increasing aniline in the anaerobic system induced the accumulation of volatile fatty acids (VFA), further turned the bioreactor into acidification, which was the principal reason for the deterioration of system performance and finally resulted in the accumulation of toxic free sulfide. Moreover, aniline triggered the change of bacterial community and genes relating to sulfate reduction pathways. The increase of aniline from 0 to 320 mg/L enriched total sulfate-reducing bacteria (SRB), and the most abundant genus was Desulfomicrobium, accounting for 66.85–91.25% of total SRB. The assimilatory sulfate reduction pathway was obviously inhibited when aniline was over 160 mg/L, while genes associated with dissimilatory sulfate reduction pathways all exhibited an upward tendency with the increasing aniline content. The enrichment of aniline-resistant SRB (e.g. Desulfomicrobium) carrying genes associated with the dissimilatory sulfate reduction pathway also confirmed the underlying mechanism that sulfate reduction turned into dissimilation under high aniline condition. Taken together, these results comprehensively provided solid evidence for the effects of aniline on the biological sulfate reduction processes treating high-sulfate wastewater and the underlying molecular mechanisms which may highlight the important roles of SRB and related sulfate reduction genes during treatment.

Heavy metal precipitation from sulfide produced from anaerobic sulfidogenic reactor

A sulfidogenic reactor was established employing biological sulfate reduction process for treating industrial wastewater, Acid Mine Drainage (AMD) and leachate produced from E-waste landfill sites containing heavy metals. Heavy metal sulfide precipitation was carried out by using sulfide rich effluent from sulfidogenic reactor as well as through chemical precipitation using Na2S. A methanogenic Continuously Stirred Tank Reactor (CSTR) operating on synthetic jaggery wastewater was converted into sulfidogenic reactor by gradually increasing SO4-- concentration in the influent. The process steps involved (i) Establishing and operating methanogenic reactor (ii) Converting of methanogenic reactor into sulfidogenic reactor and (iii) Utilizing sulfide rich effluent for heavy metal precipitation. CSTR methanogenic reactor was first established and operated at loading rate of 0.5 kg COD/m3d. COD removal of 80% was achieved. Volatile fatty acid (VFA) in the reactor was 1900–1950 mg/l with biogas production of 1–1.2 l/d. Subsequently, methanogenic reactor was converted into a sulfidogenic reactor by gradually increasing COD: SO4--from 20:1 to 5:1. Sulfate removal efficiency of 80% demonstrated that methanogenic population in the reactor was successfully replaced by Sulfate Reducing Bacteria (SRB) population. Sulfate in the influent was converted into sulfide under anaerobic conditions prevailing in sulfidogenic reactor and sulfide rich effluent from the reactor was then used to precipitate Copper from wastewater. Copper removal efficiency upto 60% was achieved by biogenic sulfide precipitation as compared to 90% by chemical precipitation.

The influence of biologically produced sulfide-containing solutions on nickel and cobalt precipitation reactions and particle settling properties

Wastewater from mining and metallurgical activities provides a potential profit source for recovering leftover but valuable metals such as nickel (Ni) and cobalt (Co). For this, sulfide precipitation is an attractive option, although the cost of providing chemical sulfide and the poor settling of metal sulfides discourage its application. As an alternative, sulfide-containing solution (i.e. biogenic sulfide) can be produced on site by sulfate reducing bacteria that can utilize sulfate present in the wastewater, thus reducing reagent and transportation costs. Additionally, the various components in biogenic ‘sulfide’, i.e., bacterial cells, cell by-products and exopolymer may enhance the settling properties of Ni and Co sulfide, thus improving metal recovery. In this project, the effect of using chemical sulfide and four biogenic sulfide-containing solutions for Ni and Co precipitation and settling was evaluated. The biogenic sulfide-containing solutions were obtained from four bioreactors fed with volatile fatty acids (VFA), leachate from fermentation of municipal solid waste, ethanol and lactate, respectively. The results showed that VFA-fed biogenic sulfide enhances the Ni precipitation reaction as compared with chemical sulfide, whereas Co was effectively precipitated from solution (> 98%) regardless of the sulfide source and metal concentration. Settling extent of both metal sulfides reached up to 80–90% in lactate-fed biogenic sulfide, while <20% of the precipitates settled by reaction with chemical sulfide. Phosphate was the main component affecting particle settling whereas acetate and sulfate exhibited a smaller influence. This research highlights the importance of the different components from the biogenic sulfide-containing solutions in the precipitation and settling of Ni and Co precipitates, demonstrating enhanced prospects of metal recovery through biological sulfate reduction process.

Inhibition of anaerobic biological sulfate reduction process by copper precipitates

The single-stage biological sulfate reduction process for treatment of heavy metal laden wastewater is a promising treatment method, but the formation of metal precipitates has been suggested to be inhibitory to the activity of sulfate reducers. The present study examined the impact of copper (Cu) precipitates on anaerobic biological sulfate reduction in semi continuous stirred tank reactors (SCSTRs) at 35 ± 2 °C. The results show that Cu precipitates significantly affected the sulfate reduction process. At an HRT of 50 days, steady-state sulfate reduction was approximately 55% at influent Cu concentrations of 0 (control) and 200 mg/L, which reduced to approximately 39% at influent Cu concentration of 400 mg/L. Microbial population (measured as volatile suspended solids) and rate of sulfate reduction were also affected, with reductions observed even at influent Cu of 200 mg/L. Copper precipitates also affected the microbial community diversity distribution.

Competitive dynamics of anaerobes during long-term biological sulfate reduction process in a UASB reactor

To reveal the long-term competitive dynamics of anaerobes in anaerobic bioreactors with sulfate reduction, a comprehensive structured mathematical model was designed for an extension of the Anaerobic Digestion Model No. 1 (ADM1). Sulfate reduction bacteria (SRB) were categorized into acetogenic-likewise SRB (ASRB) and methanogenic-likewise SRB (MSRB). Experimental data from 329 days of continuous operation of a laboratory-scale upflow anaerobic sludge bed (UASB) reactor was used for model calibration and validation. Results show that the model has a good agreement with experimental data and that three stages including the MPA dominant, stalemate and SRB dominant stages were clearly appeared throughout the whole competition period. The model was capable of predicting the long-term dynamic competition of sulfidogens and methanogens for electrons. This could explain a long-term of over 200 days needed for the SRB out-competing the MPA, and support speculation that the SRB could finally out-compete both the AcB and the MPA.

Effect of nitrobenzene on the performance and bacterial community in an expanded granular sludge bed reactor treating high-sulfate organic wastewater

Nitrobenzene (NB) is frequently found in wastewaters containing sulfate and may affect biological sulfate reduction process, but information is limited on the responses of sulfate reduction efficiency and microbial community to the increased NB contents. In this study, a laboratory-scale expanded granular sludge bed reactor was operated continuously to treat high-sulfate organic wastewater with increased NB contents. Results successfully demonstrated that the presence of more than 50 mg/L NB depressed sulfate reduction and such inhibition was partly reversible. Bath experiments showed that the maximum specific desulfuration activity (SDA) decreased from 135.80 mg SO42‒/gVSS/d to 30.78 mg SO42‒/gVSS/d when the NB contents increased from none to 400 mg/L. High-throughput sequencing showed that NB also greatly affected bacterial community structure. Bacteroidetes dominated in the bioreactor. The abundance of Proteobacteria increased with NB addition while Firmicutes presented an opposite trend. Proteobacteria gradually replaced Firmicutes for the dominance in response to the increase of influent NB concentrations. The genus Desulfovibrio was the dominant sulfate-reducing bacteria (SRB) with absence or presence of NB, but was inhibited under high content of NB. The results provided better understanding for the biological sulfate reduction under NB stress.

Managing sulfate ions produced by sulfate radical-advanced oxidation process using sulfate-reducing bacteria for the subsequent biological treatment

Persulfate and sulfate radicals with strong standard oxidation potentials and relatively low cost be used in advanced oxidation processes. The object of this study was to evaluate the performance of a sulfate radical-based advanced oxidation process with surfactant-modified zeolite for treatment of the composting leachate followed by the effect of Surfactant-modified zeolite for the removal of sulfate from leachate in an ASBR biological process. The BOD/COD ratio under the optimal condition of oxidation process was 0.7712. The most contributed factor in this process was temperature. Analysis showed that persulfate causes an increase in sulfate ions during the oxidation process. Surfactant-modified zeolite had a significant effect on decrease of the final sulfate ions concentration produced by persulfate. Therefore, the combination of advanced oxidation process and surfactant-modified zeolite with biological sulfate reduction processes for simultaneous removal of COD and sulfate ions could be recommended. The effect of COD/sulfate ratio on the activity of sulfate-reducing bacteria in the ASBR reactor was discussed. The study showed that applying the process can clearly improve the removal efficiencies of COD and sulfate from the leachate. The optimum condition for ASBR reactor operation was obtained in COD/sulfate ratio of 1.34. These process provides significant efficiencies of 93% and 74% for sulfate ion and COD removal, respectively. On the other hands, the TOC removal efficiency increased from 61% to 76% in row leachate samples without biological treatment by addition of SMZ to SR-AOP.

Multiple sulfur isotope constraints on microbial sulfate reduction below an Archean seafloor hydrothermal system.

Microbial sulfate reduction is among the most ubiquitous metabolic processes on earth. The oldest evidence of microbial sulfate reduction appears in the ca. 3.5Ga Dresser Formation in the North Pole area of Pilbara Craton in Western Australia. That evidence was found through analysis of quadruple sulfur isotopes of sulfate and sulfide minerals deposited on the seafloor. However, the activity of microbial sulfate reduction below the Archean seafloor remains poorly understood. Here, we report the quadruple sulfur isotopic compositions of sulfide minerals within hydrothermally altered seafloor basalt and less altered basaltic komatiite collected from the North Pole Dome area. The Δ33 S values of the sulfide minerals were nonzero negative, suggesting that sulfate reduction occurred below the Archean seafloor. To constrain the substrate sulfate sources and sulfate reduction processes, we constructed a numerical model. Comparing the modeled and observed sulfur isotopes, we show that the substrate sulfate comprises seawater sulfate with a negative Δ33 S anomaly and 34 S-enriched sulfate with no anomalous Δ33 S. The latter component probably represents sulfate produced by local hydrothermal processes. The maximum sulfur isotopic fractionation between the putative substrate sulfate and the observed sulfide minerals within the altered basalt and basaltic komatiite is 35‰, which is consistent with a microbial origin. Alternatively, thermochemical sulfate reduction may also produce sulfide. However, considering the hydrothermal temperature inferred from the metamorphic grade of the altered basalt, the sulfur isotopic fractionation produced by inorganic sulfate reduction is probably below 20‰. Collectively, larger fractionations imply the involvement of biological sulfate reduction processes, both in the hydrothermal system below the seafloor and in less altered subsurface settings.

Inhibitory Effects of Silver Nanoparticles on Removal of Organic Pollutants and Sulfate in an Anaerobic Biological Wastewater Treatment Process.

The increasing use of silver nanoparticles (AgNPs) in commercial products and industrial processes raises issues regarding the toxicity of sludge biomass in biological wastewater treatment plants, due to potential antimicrobial properties. This study investigated the effects of AgNPs on removal of organic pollutants and sulfate in an anaerobic biological sulfate reduction process. At AgNPs concentrations of up to 10 mg/L, no significant inhibition of sulfate and COD removal was observed. However, at higher concentrations (50-200 mg/L) sulfate and COD removal efficiencies were significantly decreased to 51.8% and 33.6%, respectively. Sulfate and COD reduction followed first-order kinetics at AgNPs concentrations of up to 10 mg/L and second-order kinetics at AgNPs concentrations of 50-200 mg/L. Lactate dehydrogenase release profiles showed increases in cytotoxicity at AgNPs concentrations greater than 50 mg/L suggesting cell membrane disruption. Analysis of extracellular polymeric substances (EPS) from sulfidogenic sludge biomass and of Fourier transform infrared (FT-IR) spectra showed a decrease in concentrations of carbohydrates, proteins, humic substances, and lipids in the presence of AgNPs. Moreover, the interaction of AgNPs with sludge biomass and the damage caused to cell walls were confirmed through scanning electron microscopy with energy dispersive X-ray spectroscopy.

Effect of ZnO nanoparticles on biodegradation and biotransformation of co-substrate and sulphonated azo dye in anaerobic biological sulfate reduction processes

In recent years, rapid increase of nanomaterial applications exposed the wastewater treatment plants to the new challenges. Potential effects of nanoparticles (NPs) on biological sulfate reduction and dyes biodegradation in biological wastewater treatment systems still need to be investigated. This study explored the effects of zinc oxide nanoparticles (ZnO-NPs) on sulfate reducing bacteria (SRB) culture treating textile wastewater. Batch reactor studies were performed by exposing the mixed SRB culture to different initial ZnO-NPs concentrations. Increasing ZnO-NPs concentrations from 0 to 200 mg l−1 inhibited the reactor performance by decreasing COD, sulfate, and color removal efficiencies from 54.4%, 78.1%, and 95.4% to 30.1%. 44.0%, and 80.0%, respectively. Moreover, dye biotransformation was also adversely affected in the presence of ZnO-NPs as revealed by the increase in total aromatic amines formation and the decrease in activities of various oxido-reductive enzymes such as veratryl alcohol oxidase, lignin peroxidase, and azo reductase.

Sulfate reducing bacterial community and in situ activity in mature fine tailings analyzed by real time qPCR and microsensor

Sulfate reducing bacteria (SRB) play significant roles in anaerobic environments in oil sands mature fine tailings (MFTs). Hydrogen sulfide (H2S) is produced during the biological sulfate reduction process. The production of toxic H2S is one of the concerns because it may hinder the landscape remediation efficiency of oil sands tailing ponds. In present study, the in situ activity and the community structure of SRB in MFT and gypsum amended MFT in two settling columns were investigated. Combined techniques of H2S microsensor and dissimilatory sulfite reductase β-subunit (dsrB) genes-based real time quantitative polymerase chain reaction (qPCR) were applied to detect the in situ H2S and the abundance of SRB. A higher diversity of SRB and more H2S were observed in gypsum amended MFT than that in MFT, indicating a higher sulfate reduction activity in gypsum amended MFT; in addition, the activity of SRB varied as depth in both MFT and gypsum amended MFT: the deeper the more H2S produced. Long-term plans for tailings management can be assessed more wisely with the information provided in this study.

Influence of co-substrate on textile wastewater treatment and microbial community changes in the anaerobic biological sulfate reduction process

This study investigated the anaerobic treatment of sulfate-rich synthetic textile wastewater in three sulfidogenic sequential batch reactors (SBRs). The experimental protocol was designed to examine the effect of three different co-substrates (lactate, glucose, and ethanol) and their concentrations on wastewater treatment performance. Sulfate reduction and dye degradation were improved when lactate and ethanol were used as electron donors, as compared with glucose. Moreover, under co-substrate limited concentrations, color, sulfate, and chemical oxygen demand (COD) removal efficiencies were declined. By reducing co-substrate COD gradually from 3000 to 500mg/L, color removal efficiencies were decreased from 98.23% to 78.46%, 63.37%, and 69.10%, whereas, sulfate removal efficiencies were decreased from 98.42%, 82.35%, and 87.0%, to 30.27%, 21.50%, and 10.13%, for lactate, glucose, and ethanol fed reactors, respectively. Fourier transform infrared spectroscopy (FTIR) and total aromatic amine analysis revealed lactate to be a potential co-substrate for further biodegradation of intermediate metabolites formed after dye degradation. Pyrosequencing analysis showed that microbial community structure was significantly affected by the co-substrate. The reactor with lactate as co-substrate showed the highest relative abundance of sulfate reducing bacteria (SRBs), followed by ethanol, whereas the glucose-fed reactor showed the lowest relative abundance of SRB.

Effect of process variables on the sulfate reduction process in bioreactors treating metal-containing wastewaters: factorial design and response surface analyses.

The individual and combined effect of the pH, chemical oxygen demand (COD) and SO4 (2-) concentration, metal to sulfide (M/S(2-)) ratio and hydraulic retention time (HRT) on the biological sulfate reduction (SR) process was evaluated in an inverse fluidized bed reactor by factorial design analysis (FDA) and response surface analysis (RSA). The regression-based model of the FDA described the experimental results well and revealed that the most significant variable affecting the process was the pH. The combined effect of the pH and HRT was barely observable, while the pH and COD concentration positive effect (up to 7 and 3gCOD/L, respectively) enhanced the SR process. Contrary, the individual COD concentration effect only enhanced the COD removal efficiency, suggesting changes in the microbial pathway. The RSA showed that the M/S(2-) ratio determined whether the inhibition mechanism to the SR process was due to the presence of free metals or precipitated metal sulfides.

Spontaneous electrochemical treatment for sulfur recovery by a sulfide oxidation/vanadium(V) reduction galvanic cell

Sulfide is the product of the biological sulfate reduction process which gives toxicity and odor problems. Wastewaters or bioreactor effluents containing sulfide can cause severe environmental impacts. Electrochemical treatment can be an alternative approach for sulfide removal and sulfur recovery from such sulfide rich solutions. This study aims to develop a spontaneous electrochemical sulfide oxidation/vanadium(V) reduction cell with a graphite electrode system to recover sulfide as elemental sulfur. The effects of the internal and external resistance on the sulfide removal efficiency and electrical current produced were investigated at different pH. A high surface area of the graphite electrode is required in order to have as less internal resistance as possible. In this study, graphite powder was added (contact area >633 cm2) in order to reduce the internal resistance. A sulfide removal efficiency up to 91% and electrical charge of more than 400 C were achieved when using five graphite rods supplemented with graphite powder as the electrode at an external resistance of 30 Ω and a sulfide concentration of 250 mg L−1.

Impact of the Hydraulic Retention Time on the Performance of a Sulfidogenic Bioreactor

Treatment of acidic Fe (II)- and sulfate-rich mine waters represents a major problem in many areas of the world. Therefore, a process was developed which utilises naturally occurring sulfate-reducing microorganisms for the elimination of sulfate and of part of the acidity from the acidic mine water. In order to improve the performance of this biological sulfate reduction process an in-depth analysis of the microbial diversity and activity in dependence of the hydraulic retention time (HRT) and other process parameters used to run the bioreactors was undertaken. This comparison demonstrated a positive correlation between shorter HRT and increasing sulfate reduction rates. The improvement in performance with decreasing HRT was paralleled by an increase of the total enzymatic activity (measured as hydrolase activity) of the microbial community and of the biomass (measured as protein concentration) in the bioreactors. A partial taxonomic identification of the microbial community in the bioreactors was achieved via nucleotide sequence analysis of a clone library of PCR-amplified 16S rRNA gene fragments prepared from a sample of the microbial biofilm in the bioreactor. Additionally, the genetic fingerprint technique T-RFLP was used to assess temporal changes of the microbial community in the biofilm within the reactor.

Precipitation of heavy metals from coal ash leachate using biogenic hydrogen sulfide generated from FGD gypsum

Investigations were undertaken to utilize flue gas desulfurization (FGD) gypsum for the treatment of leachate from the coal ash (CA) dump sites. Bench-scale investigations consisted of three main steps namely hydrogen sulfide (H(2)S) production by sulfate reducing bacteria (SRB) using sulfate from solubilized FGD gypsum as the electron acceptor, followed by leaching of heavy metals (HMs) from coal bottom ash (CBA) and subsequent precipitation of HMs using biologically produced sulfide. Leaching tests of CBA carried out at acidic pH revealed the existence of several HMs such as Cd, Cr, Hg, Pb, Mn, Cu, Ni and Zn. Molasses was used as the electron donor for the biological sulfate reduction (BSR) process which produced sulfide rich effluent with concentration up to 150 mg/L. Sulfide rich effluent from the sulfate reduction process was used to precipitate HMs as metal sulfides from CBA leachate. HM removal in the range from 40 to 100% was obtained through sulfide precipitation.

Influence of electron donors on the growth and activity of sulfate-reducing bacteria

Because of the difficulty in disposing bioleaching solutions of nickel ore, an alternative remediation treatment, the selective precipitation of metals using H2S produced biologically by sulfate-reducing bacteria (SRB), has attracted extensive attention. SRB oxidize some carbon compounds anaerobically by reducing sulfate, in which electron donors are essential for the growth of SRB and the efficiency of biological sulfate reduction process. In this paper, influence of some electron donors on the growth and activity of SRB was studied. The carbon compounds tested included organic acids, alcohols, volatile fatty acid salts and carbohydrates. Results showed that sulfate-reducing activity decreases in the order of organic acids, volatile fatty acid salts, alcohols and carbohydrates. Considering the rate of sulfate removal and operating cost, the use of formic acid as an energy source in our experiments was found to be the most effective, resulting in 97% sulfate removal.

Inhibition of hydrogen sulfide generation from disposed gypsum drywall using chemical inhibitors

Disposal of gypsum drywall in landfills has been demonstrated to elevate hydrogen sulfide (H 2S) concentrations in landfill gas, a problem with respect to odor, worker safety, and deleterious effect on gas-to-energy systems. Since H 2S production in landfills results from biological activity, the concept of inhibiting H 2S production through the application of chemical agents to drywall during disposal was studied. Three possible inhibition agents – sodium molybdate (Na 2MoO 4), ferric chloride (FeCl 3), and hydrated lime (Ca(OH) 2) – were evaluated using flask and column experiments. All three agents inhibited H 2S generation, with Na 2MoO 4 reducing H 2S generation by interrupting the biological sulfate reduction process and Ca(OH) 2 providing an unfavorable pH for biological growth. Although FeCl 3 was intended to provide an electron acceptor for a competing group of bacteria, the mechanism found responsible for inhibiting H 2S production in the column experiment was a reduction in pH. Application of both Na 2MoO 4 and FeCl 3 inhibited H 2S generation over a long period (over 180 days), but the impact of Ca(OH) 2 decreased with time as the alkalinity it contributed was neutralized by the generated H 2S. Practical application and potential environmental implications need additional exploration.

Study of anaerobic lactate metabolism under biosulfidogenic conditions

Biological sulfate reduction (BSR) has been reported to have potential for the treatment of acid mine drainage (AMD). The provision of a suitable carbon source and electron donor for this process remains a challenge. Lactate offers potential advantages as carbon source and electron donor in the biological sulfate reduction process. As this substrate is utilized by both fermentative bacteria and oxidative sulfate-reducing bacteria (SRB), the effect of feed sulfate concentration on the lactate pathways utilized under biosulfidogenic conditions was investigated. Studies were carried out in chemostat bioreactors across a range of residence times, using an enriched culture of SRB. The stoichiometry of biological sulfate reduction was affected by feed sulfate concentration and dilution rate. Incomplete oxidation of lactate was dominant at low feed sulfate concentration (1.0 g/L), while the yield of propionate from lactate metabolism increased at feed sulfate concentrations of 2.5–10.0 g/L, indicating the occurrence of lactate fermentation. Furthermore, at each sulfate feed concentration, in the range 2.5–10.0 g/L, the ratio in which lactate was metabolized by the oxidative and fermentative pathways varied with varying dilution rates. Lactate oxidation was higher at a feed sulfate concentration of 10.0 g/L relative to 2.5 and 5.0 g/L. The volumetric lactate utilization rate was enhanced by increasing the feed sulfate concentration. However, the proportion of total lactate consumed that was channelled into providing electrons for other activities apart from sulfate reduction also increased over the range of increasing sulfate concentrations studied and appeared to be a function of residual lactate and sulfide concentrations.