Sulfate reduction relative to methane production in high-rate anaerobic digestion: microbiological aspects

Emerging Strategies for Enhancing Propionate Conversion in Anaerobic Digestion: A Review

Anaerobic digestion (AD) is a triple-benefit biotechnology for organic waste treatment, renewable production, and carbon emission reduction. In the process of anaerobic digestion, pH, temperature, organic load, ammonia nitrogen, VFAs, and other factors affect fermentation efficiency and stability. The balance between the generation and consumption of volatile fatty acids (VFAs) in the anaerobic digestion process is the key to stable AD operation. However, the accumulation of VFAs frequently occurs, especially propionate, because its oxidation has the highest Gibbs free energy when compared to other VFAs. In order to solve this problem, some strategies, including buffering addition, suspension of feeding, decreased organic loading rate, and so on, have been proposed. Emerging methods, such as bioaugmentation, supplementary trace elements, the addition of electronic receptors, conductive materials, and the degasification of dissolved hydrogen, have been recently researched, presenting promising results. But the efficacy of these methods still requires further studies and tests regarding full-scale application. The main objective of this paper is to provide a comprehensive review of the mechanisms of propionate generation, the metabolic pathways and the influencing factors during the AD process, and the recent literature regarding the experimental research related to the efficacy of various strategies for enhancing propionate biodegradation. In addition, the issues that must be addressed in the future and the focus of future research are identified, and the potential directions for future development are predicted.

Effect of polyhydroxyalkanoates on the microbial reductive dechlorination of polychlorinated biphenyls and competing anaerobic respirations in a marine microbial culture

The effect of polyhydroxyalkanoates (PHAs) with different composition on the reductive dechlorination activity of a polychlorinated biphenyls (PCBs) dechlorinating marine microbial community and on the activity of sulfate-reducing (SRB) and methanogenic bacteria (MB), were investigated in marine sediment microcosms and compared with the main monomer, 3-hydroxybutyric acid (3HB). Despite PHAs were fermented more slowly than 3HB, all electron donors stimulated constantly sulfate-reduction, methanogenesis and, only transiently, PCB reductive dechlorination. No relevant differences were observed with different compositions of PHAs. According to electron balances, the majority of the supplied electrons (50 %) were consumed by SRB and to less extent by MB (9-31 %), while a small percentage (0.01 %) was delivered to OHRB. In the studied conditions PHAs were confirmed as potential slow‑hydrogen releasing compounds in marine environment but their fermentation rate was sufficiently high to mainly stimulate the competitors of organohalide respring bacteria for electron donors.

In-Vitro Assessment of the Corrosion Potential of an Oral Strain of Sulfate-Reducing Bacteria on Metallic Orthodontic Materials

AIM

Orthodontic literature is scant when it comes to microbial corrosion. The oral prevalence of many bacteria which are capable of causing microbial corrosion is reported in the dental literature. The aim of this study is to experimentally determine the corrosive potential of an oral strain of Sulfate-reducing bacteria.

MATERIALS AND METHODS

Stainless steel (SS) bracket, stainless steel archwire, NiTi archwire, Titanium molybdenum (TMA) archwire, and titanium miniscrew were immersed in five media which included Artificial saliva (group I), Sulfate rich artificial saliva (group II), API agar medium specific for SRB (group III), AS + API medium+ bacterial strain (group IV), SRAS+ API medium+ bacterial strain (group V). The materials were then subjected to Scanning electron microscopy and energy-dispersive X-ray analysis (EDX).

RESULTS

Materials in groups I, II, and III did not show any surface changes whereas materials in groups IV and V which contained the bacteria showed surface changes which were erosive patches suggestive of corrosion. EDX analyses were in line with similar findings.

CONCLUSION

This in vitro study suggested that the oral strain of Sulfate-reducing bacteria was able to induce corrosive changes in the experimental setup.

Review of Remediation Solutions for Acid Mine Drainage Using the Modified Hill Framework

This paper reviews the Acid Mine Drainage (AMD) remediation potential and operational costs of twelve existing AMD remediation methods against Class 0 and Class I AMD geochemical characteristics as defined in the Modified Hill Framework. Of the twelve remediation options reviewed in this study, eleven required additional process steps either for further treatment to achieve the discharge limits or for the safe management of hazardous waste by-products. Chemical desalination showed the greatest potential with high quality treated water and operational costs between USD 0.25 and USD 0.75 per cubic meter treated. The management of the toxic metal and sulphide by-products remains a key challenge that requires further research for sustainable mine water remediation. Further development of end-to-end methods suitable for Class 0 AMD with economical operational costs is recommended in order to effectively address the ongoing environmental challenges posed by AMD globally.

Optimization of biochemical sulfide potential (BSP) assay for anaerobic biodegradability assessment

The anaerobic biodegradability assessment (biodegradation extent and kinetics) of organic wastes is critical for optimum design and evaluating treatment efficiencies for anaerobic treatment technologies. The biochemical sulfide potential (BSP) assay has previously demonstrated the advantages of its time efficiency and measurement accuracy for biologically assessing substrate degradability, while its application is limited by undefined operational parameters. In this study, the BSP assay was further optimized through a systematic investigation of a critical parameter, inoculum-to-substrate ratio (ISR), and the applicable kinetic model to unravel the potential use of BSP assays for anaerobic waste treatment. Under two series of experimental scenarios, the common ISR ranges of 0.5-4.0 (based on the traditional BMP assay) and extreme ISRs (as low as 0.1) were studied, in which the advantage of a BSP assay on extreme ISRs was highlighted. Meanwhile, the underlying cause and mechanism for biodegradability discrepancies under different ISRs (0.1-6.0) were further investigated. The extracellular polymeric substance (EPS) characterization of residual organics and the two-substrate first-order hydrolysis model analyses revealed that the hydrolysis process of slowly-biodegradable organics fraction was hindered under improper ISR conditions. Furthermore, the Cone model was evaluated as more appropriate for biodegradation kinetics analysis in BSP assays among the five common kinetic models (i.e., Exponential, Fitzhugh, Cone, Transference, and modified Gompertz models). Overall, the results provide fundamental guidance on designing consistent BSP assays and put a step forward in standardizing the BSP assay for anaerobic biodegradability assessments.

Hydrogen sulfide affects the performance of a methanogenic bioelectrochemical system used for biogas upgrading

Methanogenic bioelectrochemical systems (BESs) can convert carbon dioxide (CO₂) to methane (CH₄) and may be used for anaerobic digester biogas upgrading. However, the effect of hydrogen sulfide (H₂S), a common biogas component, on BES performance is unknown. Thus, the objective of this study was to assess the effect of H₂S addition to the cathode headspace on BES performance at a range of initial gas-phase H₂S concentrations (0-6% v/v), as well as its effect on the anode and cathode microbial communities. As the initial cathode headspace H₂S increased from 0 to 2% (v/v), biocathodic CH₄ production increased by two-fold to 3.56 ± 0.36 mmol/L-d, due to dissolved H₂S transport from the cathode to the anode where H₂S was oxidized. Elemental sulfur and sulfate were H₂S oxidation products detected in the anode. Above 3% initial cathode headspace H₂S, biocathodic CH₄ production declined due to inhibition. A phylotype most closely related to Methanobrevibacter arboriphilus dominated the cathode archaeal communities. In the sulfide-amended BES, a phylotype similar to the exoelectrogen Ochrobactrum anthropi was enriched in both the anode and cathode, whereas phylotypes related to sulfate-reducing and sulfur oxidizing Bacteria were detected in the bioanode. Thus, sulfide transport and oxidation in the anode play an important role in methanogenic BESs treating sulfide-bearing biogas.

A Fixed Bed Pervious Concrete Anaerobic Bioreactor for Biological Sulphate Remediation of Acid Mine Drainage Using Simple Organic Matter

The development of low-operational-cost and low-operational-complexity active sulphate (SO4) reducing bioremediation for Acid Mine Drainage (AMD) is an ongoing pursuit towards sustainable mining. This study introduces a fixed bed pervious concrete anaerobic bioreactor as a second stage AMD remediation process. The study investigated the pH self-regulation capabilities, SO4 remediation capabilities and the rate limiting parameters of the bioreactor using glucose as an organic matter source. The AMD was pre-treated using a permeable reactive barrier. A 21-day trial comprised of an increase in the SO4 loading rate while reducing the organic loading rate was undertaken to identify performance limiting conditions. A daily average SO4 concentration reduction rate of 55.2% was achieved over the initial 13 days of the experiments. The study found that a COD to SO4 ratio and VFA to alkalinity ratio below 5:1 and 0.5:1 respectively were performance limiting. The bioreactor was capable of self-regulating pH within the neutral range of 6.5 and 7.5. The study findings indicate that the bioreactor design can reduce operational costs and operational complexity of active AMD bioremediation.

Biocorrosive behavior of sulphate-reducing bacteria in kerr endodontic files: Determination of the corrosion

Aims:

This study determined the corrosion rate by mass loss caused by oral strains of sulphate-reducing bacteria (SRB) in Kerr endodontic files (KF), aiming the development of a biopharmaceutical that facilitates the removal of endodontic limb fragments from root canals.

Materials and Methods:

Nine new KF were analyzed after immersion in the modified Postgate E culture medium inoculated with Desulfovibrio desulfuricans oral (84 days), Desulfovibrio fairfieldensis in the consortium (84 days) and environmental D. desulfuricans (119 days).

Results:

Optical microscopy revealed corrosion suggestive areas in all files submitted to immersion in SRB cultures, presenting a statistical difference (P < 0.05) between the samples environmental D. desulfuricans and KF control and between oral D. desulfuricans and KF control. Epifluorescence microscopy revealed an active SRB biofilm over the entire metal surface of the KF, as evidenced by the SYTO^® 9 fluorophore.

Conclusion:

SRB were capable of promoting biocorrosion in Kerr type endodontic files, but with low rate.

Sulfate Alters the Competition Among Microbiome Members of Sediments Chronically Exposed to Asphalt

Sulfate-reducing microorganisms (SRMs) often compete with methanogens for common substrates. Due to thermodynamic reasons, SRMs should outcompete methanogens in the presence of sulfate. However, many studies have documented coexistence of these microbial groups in natural environments, suggesting that thermodynamics alone cannot explain the interactions among them. In this study, we investigated how SRMs compete with the established methanogenic communities in sediment from a long-term, electron acceptor-depleted, asphalt-exposed ecosystem and how they affect the composition of the organic material. We hypothesized that, upon addition of sulfate, SRMs (i) outcompete the methanogenic communities and (ii) markedly contribute to transformations of the organic material. We sampled sediments from the test and proximate control sites under anoxic conditions and incubated them in seawater medium with or without sulfate. Abundance and activity pattern of SRMs and methanogens, as well as the total prokaryotic community, were followed for 6 weeks by using qPCR targeting selected marker genes. Some of these genes were also subjected to amplicon sequencing to assess potential shifts in diversity patterns. Alterations of the organic material in the microcosms were determined by mass spectrometry. Our results indicate that the competition of SRMs with methanogens upon sulfate addition strongly depends on the environment studied and the starting microbiome composition. In the asphalt-free sediments (control), the availability of easily degradable organic material (mainly plant-derived) allows SRMs to use a larger variety of substrates, reducing interspecies competition with methanogens. In contrast, the abundant presence of recalcitrant compounds in the asphalt-exposed sediment was associated with a strong competition between SRMs and methanogens, ultimately detrimental for the latter. Our data underpin the importance of the quality of bioavailable organic materials in anoxic environments as a driver for microbial community structure and function.

Evaluating the Effect of pH, Temperature, and Hydraulic Retention Time on Biological Sulphate Reduction Using Response Surface Methodology

Biological sulphate reduction (BSR) has been identified as a promising alternative for treating acid mine drainage. In this study, the effect of pH, temperature, and hydraulic retention time (HRT) on BSR was investigated. The Box–Behnken design was used to matrix independent variables, namely pH (4–6), temperature (10–30 °C), and HRT (2–7 days) with the sulphate reduction efficiency and sulphate reduction rate as response variables. Experiments were conducted in packed bed reactors operating in a downflow mode. Response surface methodology was used to statistically analyse the data and to develop statistical models that can be used to fully understand the individual effects and the interactions between the independent variables. The analysis of variance results showed that the data fitted the quadratic models well as confirmed by a non-significant lack of fit. The temperature and HRT effect were significant (p < 0.0001), and these two variables had a strong interaction. However, the influence of pH was insignificant (p > 0.05).

Impact of corn stover particle size and C/N ratio on reactor performance in solid-state anaerobic co-digestion with dairy manure

Green energy generation from agricultural waste has the potential to minimize dependency on fossil and reduce the resultant environmental impact of this fuel provided anaerobic reactor performance is optimized. Hence, the interactive impact of carbon to nitrogen (C/N) ratio, particle size, and co-digestion of dairy manure (DM) and corn stover (CS) on solid-state anaerobic digester (SSAD) performance was investigated with four treatments (DMCS24S, DMCS24L, DMCS28L, and DMCS32L) in this solid-state study. Novel scanning electron microscope (SEM) image analysis utilized to describe the corn stover using ImageJ indicated that corn stover of particle size 0.18-0.42 mm had lower rough surface texture relative to the 0.42-0.84 mm size. This observation not only influenced the ingestate degradation, the bioconversion rate was negatively affected by 0.18-0.42 mm particle size of corn stover. Notably, increase in C/N ratio led to decrease in total ammonia nitrogen (TAN) and alkalinity concentration (Alk), hence, treatments with the lowest C/N ratio had better reactor performance in terms of suitable process parameters such as Alk, pH, ORP, and TAN. Furthermore, DMCS24L treatment had the highest methane yield (106 mL/g VS) and net methane energy (2.92 MJ/kg). Interestingly, modified Gompertz model gave the best kinetic description of the methane production. This SSAD mesophilic study suggests that corn stover, with particle size of 0.42-0.84 mm, co-digested with dairy manure under a C/N ratio of 24 has the potential to enhance methane yield and optimize reactor performance.Implications: The utilization of agricultural waste for bioenergy generation through solid-state anaerobic digestion could be enhanced through the interactive impact of substrate particle size, carbon-to-nitrogen (C/N) ratio and co-digestion, which has not been previously studied. These ternary factors significantly improved reactor performance and enhanced methane yield when corn stover of 0.42-0.84 mm particle size was co-digested with dairy manure to achieve a C/N ratio of 24.

Recent advances in dissimilatory sulfate reduction: From metabolic study to application

Sulfate-reducing bacteria (SRB) are a group of diverse anaerobic microorganisms omnipresent in natural habitats and engineered environments that use sulfur compounds as the electron acceptor for energy metabolism. Dissimilatory sulfate reduction (DSR)-based techniques mediated by SRB have been utilized in many sulfate-containing wastewater treatment systems worldwide, particularly for acid mine drainage, groundwater, sewage and industrial wastewater remediation. However, DSR processes are often operated suboptimally and disturbances are common in practical application. To improve the efficiency and robustness of SRB-based processes, it is necessary to study SRB metabolism and operational conditions. In this review, the mechanisms of DSR processes are reviewed and discussed focusing on intracellular and extracellular electron transfer with different electron donors (hydrogen, organics, methane and electrodes). Based on the understanding of the metabolism of SRB, responses of SRB to environmental stress (pH-, temperature-, and salinity-related stress) are summarized at the species and community levels. Application in these stressed conditions is discussed and future research is proposed. The feasibility of recovering energy and resources such as biohydrogen, hydrocarbons, polyhydroxyalkanoates, magnetite and metal sulfides through the use of SRB were investigated but some long-standing questions remain unanswered. Linking the existing scientific understanding and observations to practical application is the challenge as always for promotion of SRB-based techniques.

Sulfate-Reducing Bacteria: Biofilm Formation and Corrosive Activity in Endodontic Files

AIM

This study describes the biofilm formation and the corrosive capacity of sulfate-reducing bacteria (SRB) on the metallic structure of used endodontic files.

METHODS

Sulfate-reducing bacteria (SRB) (Desulfovibrio desulfuricans oral and Desulfovibrio fairfieldensis or D. desulfuricans environmental) were inoculated into the culture media (Postgate C culture medium or modified Postgate E culture medium). The biocorrosive potential of these bacteria will be an important component of a biopharmaceutical under development called BACCOR. Afterwards, four used endodontic files (UEFs) were separately inoculated into a specific culture media for 445 days at 30°C in an incubator. The four UEFs were placed in a scanning electron microscope (SEM) and analyzed by the energy-dispersive X-ray spectrometry (EDS).

RESULTS

The confocal laser scanning microscopic images indicate the presence of biofilm in the four samples. The SEM and SEM-EDS revealed the presence of rough, irregular structures adhering along the metallic surface of the used endodontic files, suggesting a mature calcified biofilm with a high concentration of Ca, P, C, and S.

CONCLUSION

The formation of SRB biofilms on used endodontic files shows characteristics that may contribute to the biocorrosion of these files, and the results may also provide complementary data for a biopharmaceutical, which is still under development to assist in the removal of fractured endodontic files inside root channels.

Speciation evolution of zinc and copper during pyrolysis and hydrothermal carbonization treatments of sewage sludges

Thermal and hydrothermal treatments are promising techniques for sewage sludge management that can potentially facilitate safe waste disposal, energy recovery, and nutrient recovery/recycling. Content and speciation of heavy metals in the treatment products affect the potential environmental risks upon sludge disposal and/or application of the treatment products. Therefore, it is important to study the speciation transformation of heavy metals and the effects of treatment conditions. By combining synchrotron X-ray spectroscopy/microscopy analysis and sequential chemical extraction, this study systematically characterized the speciation of Zn and Cu in municipal sewage sludges and their chars derived from pyrolysis (a representative thermal treatment technique) and hydrothermal carbonization (HTC; a representative hydrothermal treatment technique). Spectroscopy analysis revealed enhanced sulfidation of Zn and Cu by anaerobic digestion and HTC treatments, as compared to desulfidation by pyrolysis. Overall, changes in the chemical speciation and matrix properties led to reduced mobility of Zn and Cu in the treatment products. These results provide insights into the reaction mechanisms during pyrolysis and HTC treatments of sludges and can help evaluate the environmental/health risks associated with the metals in the treatment products.

Coexistence and competition of sulfate-reducing and methanogenic populations in an anaerobic hexadecane-degrading culture

BACKGROUND

Over three-fifths of the world's known crude oil cannot be recovered using state-of-the-art techniques, but microbial conversion of petroleum hydrocarbons trapped in oil reservoirs to methane is one promising path to increase the recovery of fossil fuels. The process requires cooperation between syntrophic bacteria and methanogenic archaea, which can be affected by sulfate-reducing prokaryotes (SRPs). However, the effects of sulfate on hydrocarbon degradation and methane production remain elusive, and the microbial communities involved are not well understood.

RESULTS

In this study, a methanogenic hexadecane-degrading enrichment culture was treated with six different concentrations of sulfate ranging from 0.5 to 25 mM. Methane production and maximum specific methane production rate gradually decreased to 44 and 56% with sulfate concentrations up to 25 mM, respectively. There was a significant positive linear correlation between the sulfate reduction/methane production ratio and initial sulfate concentration, which remained constant during the methane production phase. The apparent methanogenesis fractionation factor (αapp) gradually increased during the methane production phase in each treatment, the αapp for the treatments with lower sulfate (0.5-4 mM) eventually plateaued at ~1.047, but that for the treatment with 10-25 mM sulfate only reached ~1.029. The relative abundance levels of Smithella and Methanoculleus increased almost in parallel with the increasing sulfate concentrations. Furthermore, the predominant sulfate reducer communities shifted from Desulfobacteraceae in the low-sulfate cultures to Desulfomonile in the high-sulfate cultures.

CONCLUSION

The distribution of hexadecane carbon between methane-producing and sulfate-reducing populations is dependent on the initial sulfate added, and not affected during the methane production period. There was a relative increase in hydrogenotrophic methanogenesis activity over time for all sulfate treatments, whereas the total activity was inhibited by sulfate addition. Both Smithella and Methanoculleus, the key alkane degraders and methane producers, can adapt to sulfate stress. Specifically, different SRP populations were stimulated at various sulfate concentrations. These results could help to evaluate interactions between sulfate-reducing and methanogenic populations during anaerobic hydrocarbon degradation in oil reservoirs.

Analysing the mechanisms of sludge digestion enhanced by iron

Carbon-neutral operation of wastewater treatment plants (WWTPs) requires enhancing anaerobic digestion (AD) of excess sludge for a higher energy conversion efficiency. Among others, iron has been identified to function on enhancing methane production in AD. As an industrial residual, waste iron scraps (WISs) have been reported as potentially enhancing CH₄ production in AD. With this study, the mechanisms of AD enhanced by WISs are analysed in a two-phase process: acidogenic phase (AP) and methanogenic phase (MP). Semi-continuous tests substantially excluded ORP reduction and hydrogen-evolution corrosion induced by WISs in enhancing CH₄ production, although WISs (10 g Fe/L) could indeed increase CH₄ production by 10.1% and 21.4% when added in AP and MP respectively. Detection on both FISH and enzymatic activities of involved microorganisms revealed that the stimulating effects of WISs on anaerobes (both catabolism and anabolism) could play an important (96.3%) role in enhancing CH₄ production, which would facilitate hydrolysis of refractory organics and improvement of electron transport rate (ETR).

Performance of CSTR-EGSB-SBR system for treating sulfate-rich cellulosic ethanol wastewater and microbial community analysis

Performance and microbial community composition were evaluated in a two-phase anaerobic and aerobic system treating sulfate-rich cellulosic ethanol wastewater (CEW). The system was operated at five different chemical oxygen demand (COD)/SO₄^2- ratios (63.8, 26.3, 17.8, 13.7, and 10.7). Stable performance was obtained for total COD removal efficiency (94.5%), sulfate removal (89.3%), and methane production rate (11.5 L/day) at an organic loading rate of 32.4 kg COD/(m³·day). The acidogenic reactor made a positive contribution to net VFAs production (2318.1 mg/L) and sulfate removal (60.9%). Acidogenic bacteria (Megasphaera, Parabacteroides, unclassified Ruminococcaceae spp., and Prevotella) and sulfate-reducing bacteria (Butyrivibrio, Megasphaera) were rich in the acidogenic reactor. In the methanogenic reactor, high diversity of microorganisms corresponded with a COD removal contribution of 83.2%. Moreover, methanogens (Methanosaeta) were predominant, suggesting that these organisms played an important role in the acetotrophic methanogenesis pathway. The dominant aerobic bacteria (Truepera) appeared to have been responsible for the COD removal of the SBR. These results indicate that dividing the sulfate reduction process could effectively minimize sulfide toxicity, which is important for the successful operation of system treating sulfate-rich CEW.

Wastes valorization from Rhodosporidium toruloides NCYC 921 production and biorefinery by anaerobic digestion

Yeast production and biomass biorefinery processes for lipid and carotenoid extraction generate residues that can be used as substrates for anaerobic digestion. Glucose and carob pulp syrups were used as carbon sources to produce the yeast biomass. The yeast cultivation broth, yeast biomass residues (after carotenoid and lipid extraction) and the carob pulp solid residues obtained from the extraction of sugars were used to produce biogas by applying different Substrate/Inoculum ratios (S/I of 0.5 and 0.75). For all the residues studied, the digestions at the S/I ratio of 0.75 provided higher biogas yields than those carried out at the S/I ratio of 0.5. The best results in terms of biogas production and methane yield were observed for the yeast residue digestion at S/I of 0.75 (65.9mL, 333.7mLg^-1VS^-1 substrate). As monitored through flow cytometry, its bacterial consortium showed the lowest proportion of injured cells.

Anaerobic digestion of sulfate-acidified cattle slurry: One-stage vs. two-stage

Two strategies to include acidified cattle manure (AcCM) in co-digestion with normal cattle manure (CM) are presented in this work. The strategies are a single thermophilic (50 °C) continuous stirred tank reactor (CSTR) anaerobic digestion and a two-step (65 °C + 50 °C) CSTR process. In both strategies, two different inclusion levels of H2SO4-acidified CM (10% and 20%) in co-digestion with normal CM were tested and compared with a control CSTR fed only CM. Important enhancement of methane (CH4) yield and solid reductions were observed in the thermophilic one-step CSTR working with 10% AcCM. However, a higher inclusion level of AcCM (20%) caused volatile fatty acid accumulation in the reactor and a more than 30% reduction in CH4 production. In terms of CH4 production, when 10% of AcCM was co-digested with 90% of CM, the two-step anaerobic co-digestion yielded less than the single step. During the first step of the two-step CSTR process, acidogenesis and a partial sulfate reduction were achieved. However, sulfide stripping between the first and the second step must be promoted in order to advance this technology.

Improvement of the degradation of sulfate rich wastewater using sweetmeat waste (SMW) as nutrient supplement

External dosing of sweetmeat waste (SMW) dosing into exhausted upflow packed bed bioreactor (PBR) resulted in prompt reactivation of SO4(2-) removal. Different SMW concentrations in terms of chemical oxygen demand (COD)/SO4(2-) ratios (1, 2, 4 and 8) were introduced into four identical PBR where process stability was found within 3 weeks of operation. SO4(2-) removal was proportional to COD/SO4(2-) ratios up to 4 at which maximum sulfate removal (99%) was achieved at a rate of 607 mg/d. The value of COD consumption:SO4(2-)removal was much higher at ratio 4 than 8 whereas, ratio 2 was preferred over all. Net effluent acetate concentration profile and total microbial population attached to the reactor matrices were corresponding to COD/SO4(2-) ratio as 4>8>2>>1. Sulfate reducing bacteria (SRB) population was found to be inversely proportional to COD/SO4(2-) ratio in which acetate oxidizing SRB and fermentative bacteria were the dominant.

Effect of influent COD/SO4(2-) ratios on UASB treatment of a synthetic sulfate-containing wastewater

The effect of the chemical oxygen demand/sulfate (COD/SO4(2-)) ratio on the anaerobic treatment of synthetic chemical wastewater containing acetate, ethanol, and sulfate, was investigated using a UASB reactor. The experimental results show that at a COD/SO4(2-) ratio of 20 and a COD loading rate of 25.2gCODL(-1)d(-1), a COD removal of as high as 87.8% was maintained. At a COD/SO4(2-) ratio of 0.5 (sulfate concentration 6000mgL(-1)), however, the COD removal was 79.2% and the methane yield was 0.20LCH4gCOD(-1). The conversion of influent COD to methane dropped from 80.5% to 54.4% as the COD/SO4(2-) ratio decreased from 20 to 0.5. At all the COD/SO4(2-) ratios applied, over 79.4% of the total electron flow was utilized by methane-producing archaea (MPA), indicating that methane fermentation was the predominant reaction. The majority of the methane was produced by acetoclastic MPA at high COD/SO4(2-) ratios and both acetoclastic and hydrogenthrophic MPA at low COD/SO4(2-) ratios. Only at low COD/SO4(2-) ratios were SRB species such as Desulfovibrio found to play a key role in ethanol degradation, whereas all the SRB species were found to be incomplete oxidizers at both high and low COD/SO4(2-) ratios.

Methane production from acid hydrolysates of Agave tequilana bagasse: evaluation of hydrolysis conditions and methane yield

Evaluation of diluted acid hydrolysis for sugar extraction from cooked and uncooked Agave tequilana bagasse and feasibility of using the hydrolysates as substrate for methane production, with and without nutrient addition, in anaerobic sequencing batch reactors (AnSBR) were studied. Results showed that the hydrolysis over the cooked bagasse was more effective for sugar extraction at the studied conditions. Total sugars concentration in the cooked and uncooked bagasse hydrolysates were 27.9 g/L and 18.7 g/L, respectively. However, 5-hydroxymethylfurfural was detected in the cooked bagasse hydrolysate, and therefore, the uncooked bagasse hydrolysate was selected as substrate for methane production. Interestingly, results showed that the AnSBR operated without nutrient addition obtained a constant methane production (0.26 L CH4/g COD), whereas the AnSBR operated with nutrient addition presented a gradual methane suppression. Molecular analyses suggested that methane suppression in the experiment with nutrient addition was due to a negative effect over the archaeal/bacterial ratio.

Genetic resources for methane production from biomass described with the Gene Ontology

Methane (CH₄) is a valuable fuel, constituting 70–95% of natural gas, and a potent greenhouse gas. Release of CH₄ into the atmosphere contributes to climate change. Biological CH₄ production or methanogenesis is mostly performed by methanogens, a group of strictly anaerobic archaea. The direct substrates for methanogenesis are H₂ plus CO₂, acetate, formate, methylamines, methanol, methyl sulfides, and ethanol or a secondary alcohol plus CO₂. In numerous anaerobic niches in nature, methanogenesis facilitates mineralization of complex biopolymers such as carbohydrates, lipids and proteins generated by primary producers. Thus, methanogens are critical players in the global carbon cycle. The same process is used in anaerobic treatment of municipal, industrial and agricultural wastes, reducing the biological pollutants in the wastes and generating methane. It also holds potential for commercial production of natural gas from renewable resources. This process operates in digestive systems of many animals, including cattle, and humans. In contrast, in deep-sea hydrothermal vents methanogenesis is a primary production process, allowing chemosynthesis of biomaterials from H₂ plus CO₂. In this report we present Gene Ontology (GO) terms that can be used to describe processes, functions and cellular components involved in methanogenic biodegradation and biosynthesis of specialized coenzymes that methanogens use. Some of these GO terms were previously available and the rest were generated in our Microbial Energy Gene Ontology (MENGO) project. A recently discovered non-canonical CH₄ production process is also described. We have performed manual GO annotation of selected methanogenesis genes, based on experimental evidence, providing “gold standards” for machine annotation and automated discovery of methanogenesis genes or systems in diverse genomes. Most of the GO-related information presented in this report is available at the MENGO website (http://www.mengo.biochem.vt.edu/).

UASB performance and electron competition between methane-producing archaea and sulfate-reducing bacteria in treating sulfate-rich wastewater containing ethanol and acetate

To find an appropriate method for sulfate-rich wastewater containing ethanol and acetate with COD/sulfate ratio of 1, a UASB reactor was operated for more than 180 days. The influences of HRT (hydraulic retention time) and OLR (organic loading rate) on organics and sulfate removal, gas production, and electrons utilization were investigated. The sludge activity and microorganism composition were also determined. The results indicated that this system removed more than 80% of COD and 30% of sulfate with HRT above 6h and OLR below 12.3 gCOD/L d. Further HRT decrease caused volatile fatty acids accumulation and performance deterioration. Except at HRT of 2h, COD and electron flow were mostly utilized by methane-producing archaea (MPA), and methane yield remained in the range of 0.18-0.24 LCH4/gCOD. Methane was mainly generated by Methanosaeta concilii GP6 with acetate as substrate, whereas sulfate was mainly reduced by incomplete-oxidizing Desulfovibrio species with ethanol as substrate.

Modeling sulfate removal by inhibited mesophilic mixed anaerobic communities using a statistical approach

Optimizing sulfate removal by a mixed anaerobic mesophilic culture fed glucose, linoleic acid (LA) and sulfate under several pH conditions was performed using a three factor three level Box-Behnken design (BBD). Based on the BBD approach, a statistical model was developed to predict the residual sulfate concentration. The LA concentration, initial pH and the COD/SO4(2-) ratio were the three experimental factors under consideration. Increasing the COD/SO4(2-) ratio increased the quantity of sulfate removed. The COD/SO4(2-) ratio showed the largest effect on reducing the sulfate level. Significant interactions between the three experimental factors were confirmed by the surface plots, interaction plot and ANOVA. An analysis of residuals verified accuracy of the model. Acetate and H2 production was dominant in cultures with the pH set at 6.0 and 6.75 and fed LA. After 168 h, butyrate and H2S were associated with the largest quantity of sulfate removed. At a D-optimality value of 1.0, a minimum response (residual sulfate concentration) of 36.2 mg L(-1) was recorded at 1500 mg L(-1) LA with a COD/SO4(2-) ratio of 2.18 and a pH set at 6.0. Based on the conditions under consideration, the model provided a useful approach for predicting the residual sulfate concentration in inhibited mixed anaerobic cultures.

Biological sulfate reduction in the acidogenic phase of anaerobic digestion under dissimilatory Fe (III)--reducing conditions

In this study, a novel approach was developed for sulfate - containing wastewater treatment via dosing Fe₂O₃ in a two - stage anaerobic reactor (A1, S1). The addition of Fe₂O₃ in its second stage i.e. acidogenic sulfate-reducing reactor (S1) resulted in microbial reduction of Fe (III), which significantly enhanced the biological sulfate reduction. In reactor S1, increasing influent sulfate concentration to 1400 mg/L resulted in a higher COD removal (27.3%) and sulfate reduction (57.9%). In the reference reactor without using Fe₂O₃ (S2), the COD and sulfate removal were 15.6% and 29%, respectively. The combined performance of the two-stage anaerobic reactor (A1, S1) also showed a higher COD removal of 74.2%. Denaturing gradient gel electrophoresis (DGGE) and phylogenetic analysis showed that the dominant bacteria with high similarity to IRB species as well as sulfate reducer Desulfovibrio and acidogenic bacteria (AB) were enriched in S1. Quantitative Polymerase Chain Reaction (qPCR) analysis presented a higher proportion of sulfate reducer Desulfovibrio marrakechensis and Fe (III) reducer Iron-reducing bacteria HN54 in S1.

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.

Process performance of anaerobic co-digestion of raw and acidified pig slurry

The effect of incorporating different ratios of acidified pig slurry on methane yield was evaluated in two scales of anaerobic digesters: Thermophilic (50 °C) pilot scale digester (120 l), operating with an average hydraulic retention time of 20 days and thermophilic (52 °C) full-scale digesters (10 and 30 m(3)), operating with an average hydraulic retention time of 30 days. In the lab-scale digester, different inclusion levels of acidified slurry (0-60%) were tested each 15 days, to determine the maximum ratio of acidified to non-acidified slurry causing inhibition and to find process state indicators helping to prevent process failure. In the full-scale digesters, the level of inclusion of the acidified slurry was chosen from the ratio causing methane inhibition in the pilot scale experiment and was carried on in a long-term process of 100 days. The optimal inclusion level of acidified pig slurry in anaerobic co-digestion with conventional slurry was 10%, which promoted anaerobic methane yield by nearly 20%. Higher inclusion levels caused methane inhibition and volatile fatty acids accumulations in both experiments. In order to prevent process failure, the most important traits to monitor in the anaerobic digestion of acidified pig slurry were found to be: sulfate content of the slurry, alkalinity parameters (especially partial alkalinity and the ratio of alkalinity) and total volatile fatty acids (especially acetic and butyric acids).

Nitrate and sulfate: Effective alternative hydrogen sinks for mitigation of ruminal methane production in sheep

Twenty male crossbred Texel lambs were used in a 2 × 2 factorial design experiment to assess the effect of dietary addition of nitrate (2.6% of dry matter) and sulfate (2.6% of dry matter) on enteric methane emissions, rumen volatile fatty acid concentrations, rumen microbial composition, and the occurrence of methemoglobinemia. Lambs were gradually introduced to nitrate and sulfate in a corn silage-based diet over a period of 4 wk, and methane production was subsequently determined in respiration chambers. Diets were given at 95% of the lowest ad libitum intake observed within one block in the week before methane yield was measured to ensure equal feed intake of animals between treatments. All diets were formulated to be isonitrogenous. Methane production decreased with both supplements (nitrate: -32%, sulfate: -16%, and nitrate+sulfate: -47% relative to control). The decrease in methane production due to nitrate feeding was most pronounced in the period immediately after feeding, whereas the decrease in methane yield due to sulfate feeding was observed during the entire day. Methane-suppressing effects of nitrate and sulfate were independent and additive. The highest methemoglobin value observed in the blood of the nitrate-fed animals was 7% of hemoglobin. When nitrate was fed in combination with sulfate, methemoglobin remained below the detection limit of 2% of hemoglobin. Dietary nitrate decreased heat production (-7%), whereas supplementation with sulfate increased heat production (+3%). Feeding nitrate or sulfate had no effects on volatile fatty acid concentrations in rumen fluid samples taken 24h after feeding, except for the molar proportion of branched-chain volatile fatty acids, which was higher when sulfate was fed and lower when nitrate was fed, but not different when both products were included in the diet. The total number of rumen bacteria increased as a result of sulfate inclusion in the diet. The number of methanogens was reduced when nitrate was fed. Enhanced levels of sulfate in the diet increased the number of sulfate-reducing bacteria. The number of protozoa was not affected by nitrate or sulfate addition. Supplementation of a diet with nitrate and sulfate is an effective means for mitigating enteric methane emissions from sheep.

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.

A comprehensive microbial insight into two-stage anaerobic digestion of food waste-recycling wastewater

Microbial community structures were assessed in a two-stage anaerobic digestion system treating food waste-recycling wastewater. The reactors were operated for 390 d at 10 different hydraulic retention times (HRTs) ranging from 25 to 4 d. Stable operation was achieved with the overall chemical oxygen demand (COD) removal efficiency of 73.0-85.9% at organic loading rate of up to 35.6 g COD/L·d. Performance of the acidogenic reactors, however, changed significantly during operation. This change coincided with transition of the bacterial community from one dominated by Aeriscardovia- and Lactobacillus amylovorus-related species to one dominated by Lactobacillus acetotolerans- and Lactobacillus kefiri-like organisms. In methanogenic reactors, the microbial community structures also changed at this stage along with the shift from Methanoculleus- to Methanosarcina-like organisms. This trend was confirmed by the non-metric multidimensional scaling joint plot of microbial shifts along with performance parameters. These results indicated that the overall process performance was relatively stable compared to the dynamic changes in the microbial structures and the acidogenic performance.

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.

Biofilm Dynamics and Kinetics during High-Rate Sulfate Reduction under Anaerobic Conditions

The sulfate kinetics in an anaerobic, sulfate-reducing biofilm were investigated with an annular biofilm reactor. Biofilm growth, sulfide production, and kinetic constants (K(m) and V(max)) for the bacterial sulfate uptake within the biofilm were determined. These parameters were used to model the biofilm kinetics, and the experimental results were in good agreement with the model predictions. Typical zero-order volume rate constants for sulfate reduction in a biofilm without substrate limitation ranged from 56 to 93 mumol of SO(4)-cm h at 20 degrees C. The temperature dependence (Q(10)) of sulfate reduction was equivalent to 3.4 at between 9 and 20 degrees C. The measured rates of sulfate reduction could explain the relatively high sulfide levels found in sewers and wastewater treatment systems. Furthermore, it has been shown that sulfate reduction in biofilms just a few hundred micrometers thick is limited by sulfate diffusion into biofilm at concentrations below 0.5 mM. This observation might, in some cases, be an explanation for the relatively poor capacity of the sulfate-reducing bacteria to compete with the methanogenic bacteria in anaerobic wastewater treatment in submerged filters.

Competition and coexistence of sulfate-reducing and methanogenic populations in anaerobic biofilms

The microbial population structure and function of natural anaerobic communities maintained in laboratory fixed-bed biofilm reactors were tracked before and after a major perturbation, which involved the addition of sulfate to the influent of a reactor that had previously been fed only glucose (methanogenic), while sulfate was withheld from a reactor that had been fed both glucose and sulfate (sulfidogenic). The population structure, determined by using phylogenetically based oligonucleotide probes for methanogens and sulfate-reducing bacteria, was linked to the functional performance of the biofilm reactors. Before the perturbation, the methanogenic reactor contained up to 25% methanogens as well as 15% sulfate-reducing bacteria, even though sulfate was not present in the influent of this reactor. Methanobacteriales and Desulfovibrio spp. were the most abundant methanogens and sulfate-reducing bacteria, respectively. The presence of sulfate-reducing bacteria (primarily Desulfovibrio spp. and Desulfobacterium spp.) in the absence of sulfate may be explained by their ability to function as proton-reducing acetogens and/or fermenters. Sulfate reduction began immediately following the addition of sulfate consistent with the presence of significant levels of sulfate-reducing bacteria in the methanogenic reactor, and levels of sulfate-reducing bacteria increased to a new steady-state level of 30 to 40%; coincidentally, effluent acetate concentrations decreased. Notably, some sulfate-reducing bacteria (Desulfococcus/Desulfosarcina/Desulfobotulus group) were more competitive without sulfate. Methane production decreased immediately following the addition of sulfate; this was later followed by a decrease in the relative concentration of methanogens, which reached a new steady-state level of approximately 8%. The changeover to sulfate-free medium in the sulfidogenic reactor did not cause a rapid shift to methanogenesis. Methane production and a substantial increase in the levels of methanogens were observed only after approximately 50 days following the perturbation.

Sulfate reduction relative to methane production in high-rate anaerobic digestion: technical aspects

The effect of different substrates and different levels of sulfate and sulfide on methane production relative to sulfate reduction in high-rate anaerobic digestion was evaluated. Reactors could be acclimated so that sulfate up to a concentration of 5 g of sulfate S per liter did not significantly affect methanogenesis. Higher levels gave inhibition because of salt toxicity. Sulfate reduction was optimal at a relatively low level of sulfate, i.e., 0.5 g of sulfate S per liter, but was also not significantly affected by higher levels. Both acetoclastic and hydrogenotrophic methane-producing bacteria adapted to much higher levels of free H(2)S than the values reported in the literature (50% inhibition occurred only at free H(2)S levels of more than 1,000 mg/liter). High levels of free H(2)S affected the sulfate-reducing bacteria only slightly. Formate and acetate supported the sulfate-reducing bacteria very poorly. In the high-rate reactors studied, intensive H(2)S formation occurred only when H(2) gas or an H(2) precursor such as ethanol was supplied.

Characterization of microbial trophic structures of two anaerobic bioreactors processing sulfate-rich waste streams

A multi-compartment anaerobic bioreactor, designated the anaerobic migrating blanket reactor (AMBR), did not perform well in terms of chemical oxygen demand (COD) removal after an increase in sulfate load, compared to a conventional upflow anaerobic sludge blanket (UASB) reactor. The trophic structures of the bioreactors were characterized by analyzing the electron flows, formation and consumption of fermentation intermediates and terminal product (methane and hydrogen sulfide) formation. Critical performance parameters were linked to operational perturbations such as increase in sulfate load and changes in flow reversal schemes in the AMBR. Both of these manipulations affected the microbial communities, which were monitored by terminal restriction fragment length polymorphism (T-RFLP) analysis targeting the bacterial and archaeal domains. The less stable AMBR did not produce granular biomass, and in response to increased sulfate concentrations, experienced a reversal in the distribution of hydrogenotrophic methanogens that correlated with a shift in electron flow from butyrate to propionate. As this shift occurred, bacterial populations such as butyrate-producing clostridia, became predominant, thus leading to reactor imbalance. The stable UASB reactor developed and retained granules and maintained a relatively stable archaeal community. Sulfate perturbation led to the selection of a novel bacterial group (Thermotogaceae), which was most likely well adapted to the increasingly sulfidogenic conditions in the bioreactor.

Population dynamics of a single-stage sulfidogenic bioreactor treating synthetic zinc-containing waste streams

Waste streams from industrial processes such as metal smelting or mining contain high concentrations of sulfate and metals with low pH. Dissimilatory sulfate reduction carried out by sulfate-reducing bacteria (SRB) at low pH can combine sulfate reduction with metal-sulfide precipitation and thus open possibilities for selective metal recovery. This study investigates the microbial diversity and population changes of a single-stage sulfidogenic gas-lift bioreactor treating synthetic zinc-rich waste water at pH 5.5 by denaturing gradient gel electrophoresis of 16S rRNA gene fragments and quantitative polymerase chain reaction. The results indicate the presence of a diverse range of phylogenetic groups with the predominant microbial populations belonging to the Desulfovibrionaceae from delta-Proteobacteria. Desulfovibrio desulfuricans-like populations were the most abundant among the SRB during the three stable phases of varying sulfide and zinc concentrations and increased from 13% to 54% of the total bacterial populations over time. The second largest group was Desulfovibrio marrakechensis-like SRB that increased from 1% to about 10% with decreasing sulfide concentrations. Desulfovibrio aminophilus-like populations were the only SRB to decrease in numbers with decreasing sulfide concentrations. However, their population was <1% of the total bacterial population in the reactor at all analyzed time points. The number of dissimilatory sulfate reductase (DsrA) gene copies per number of SRB cells decreased from 3.5 to 2 DsrA copies when the sulfide concentration was reduced, suggesting that the cells' sulfate-reducing capacity was also lowered. This study has identified the species present in a single-stage sulfidogenic bioreactor treating zinc-rich wastewater at low pH and provides insights into the microbial ecology of this biotechnological process.

Effect of sulfide inhibition and organic shock loading on anaerobic biofilm reactors treating a low-temperature, high-sulfate wastewater

To assess the long-term treatment of sulfate- and carbon-rich wastewater at low temperatures, anaerobic biofilm reactors were operated for over 900 days at 20 degrees C and fed wastewater containing lactate and sulfate. Results showed the reactors could be operated at 20 degrees C with a load rate of 1.3 g-chemical oxygen demand (COD)/L x d or less and a sulfur loading rate (SLR) of 0.2 g-S/L x d, with no significant deterioration in performance. With acclimation periods, load rates of 3.4 g-COD/L x d and SLR of 0.3 g/L x d could be tolerated. Effluent dissolved sulfide and hydrogen sulfide levels were approximately 600 and 150 mg-S/L, respectively, during this period. The effect of organic shock loading was also assessed. Reactors appeared to recover from one, but not two, lactate spikes of approximately 5000 mg-COD/L. Long-term stability was achieved in reactors containing large, stable populations of lactate- and propionate-degrading sulfate-reducing bacteria and aceticlastic methanogens.

Application of bacteria involved in the biological sulfur cycle for paper mill effluent purification

In anaerobic wastewater treatment, the occurrence of biological sulfate reduction results in the formation of unwanted hydrogen sulfide, which is odorous, corrosive and toxic. In this paper, the role and application of bacteria in anaerobic and aerobic sulfur transformations are described and exemplified for the treatment of a paper mill wastewater. The sulfate containing wastewater first passes an anaerobic UASB reactor for bulk COD removal which is accompanied by the formation of biogas and hydrogen sulfide. In an aeration pond, the residual CODorganic and the formed dissolved hydrogen sulfide are removed. The biogas, consisting of CH4 (80-90 vol.%), CO2 (10-20 vol.%) and H2S (0.8-1.2 vol.%), is desulfurised prior to its combustion in a power generator thereby using a new biological process for H2S removal. This process will be described in more detail in this paper. Biomass from the anaerobic bioreactor has a compact granular structure and contains a diverse microbial community. Therefore, other anaerobic bioreactors throughout the world are inoculated with biomass from this UASB reactor. The sludge was also successfully used in investigation on sulfate reduction with carbon monoxide as the electron donor and the conversion of methanethiol. This shows the biotechnological potential of this complex reactor biomass.