Sulfate-reducing Bioreactor Research Articles

Sequential hydrotalcite precipitation, microbial sulfate reduction and in situ hydrogen sulfide removal for neutral mine drainage treatment

This study proposed and examined a new process flowsheet for treating neutral mine drainage (NMD) from an open-pit gold mine. The process consisted of three sequential stages: (1) in situ hydrotalcite (HT) precipitation; (2) low-cost carbon substrate driven microbial sulfate reduction; and (3) ferrosol reactive barrier for removing biogenic dissolved hydrogen sulfide (H2S). For concept validation, laboratory-scale columns were established and operated for a 140-days period with key process performance parameters regularly measured. At the end, solids recovered from various depths of the ferrosol column were analysed for elemental composition and mineral phases. Prokaryotic microbial communities in various process locations were characterised using 16S rRNA gene sequencing. Results showed that the Stage 1 HT-treatment substantially removed a range of elements (As, B, Ba, Ca, F, Zn, Si, and U) in the NMD, but not nitrate or sulfate. The Stage 2 sulfate reducing bioreactor (SRB) packed with 70 % (v/v) Eucalyptus woodchip, 1 % (w/v) ground (<1 mm) dried Typha biomass, and 10 % (w/v) NMD-pond sediment facilitated complete nitrate removal and stable sulfate removal of ca. 50 % (50 g-SO4 m−3 d−1), with an average H2S generation rate of 10 g-H2S m−3d−1. The H2S-removal performance of the Stage 3 ferrosol column was compared with a synthetic amorphous Fe-oxyhydroxide-amended sand control column. Although both columns facilitated excellent (95–100 %) H2S removal, the control column only enabled a further ca. 10 % sulfate reduction, giving an overall sulfate removal of 56 %. In contrast, the ferrosol enabled an extra 99.9 % sulfate reduction in the SRB effluent, leading to a near complete sulfate removal. Overall, the process successfully eliminated a range of metal/metalloid contaminants, nitrate, sulfate (2500 mg-SO4 L−1 in the NMD to <10 mg-SO4 L−1 in the final effluent) and H2S (>95 % removal). Further optimisation is required to minimise release of ferrous iron from the ferrosol barrier into the final effluent.

Replacing rice bran with low-molecular-weight substrates affected the performance and metabolic feature of sulfate-reducing bioreactors treating acid mine drainage

The sulfate-reducing bioreactor presents an auspicious approach for eradicating toxic metals from acid mine drainage (AMD). In earlier studies, the incorporation of rice bran as a substrate in bioreactors was observed to foster the proliferation of a diverse range of microorganisms, consequently leading to the undesirable development of biofilms atop the bioreactor. To selectively stimulate sulfate-reducing bacteria (SRB) within the bioreactor, we examined the consequences of introducing three low-molecular-weight substrates in lieu of rice bran under 12 varying conditions. The sulfate reduction performance was marginally impacted by the incorporation of lactate with corn steep liquor (CSL) compared to ethanol or glucose with CSL, and the microbial composition in the presence of lactate differed significantly from those containing ethanol or glucose. The integration of ethanol and glucose with/without CSL efficiently lowered oxidation-reduction potential and promoted the proliferation of Desulfosporosinus-related SRB (relative abundance: ∼26.6%). Given that the employment of glucose initially activated fermentative bacteria such as Clostridium spp., ethanol emerged as an efficacious alternative substrate to rice bran, which directly fosters the accumulation of Desulfosporosinus sp. without encouraging the growth of other microorganisms that contribute to the unwanted formation of biofilms within the sulfate-reducing bioreactors employed in AMD treatment.

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.

Treatment of Organic and Sulfate/Sulfide Contaminated Wastewater and Bioelectricity Generation by Sulfate-Reducing Bioreactor Coupling with Sulfide-Oxidizing Fuel Cell.

A wastewater treatment system has been established based on sulfate-reducing and sulfide-oxidizing processes for treating organic wastewater containing high sulfate/sulfide. The influence of COD/SO42- ratio and hydraulic retention time (HRT) on removal efficiencies of sulfate, COD, sulfide and electricity generation was investigated. The continuous operation of the treatment system was carried out for 63 days with the optimum COD/SO42- ratio and HRT. The result showed that the COD and sulfate removal efficiencies were stable, reaching 94.8 ± 0.6 and 93.0 ± 1.3% during the operation. A power density level of 18.0 ± 1.6 mW/m2 was obtained with a sulfide removal efficiency of 93.0 ± 1.2%. However, the sulfide removal efficiency and power density decreased gradually after 45 days. The results from scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) show that sulfur accumulated on the anode, which could explain the decline in sulfide oxidation and electricity generation. This study provides a promising treatment system to scale up for its actual applications in this type of wastewater.

Arsenic (III) and arsenic (V) removal from acidic mine waters using an acidophilic sulfate-reducing bioreactor

Chile accounts for about one-third of the world's copper production; however, high levels of arsenic (As) in its copper concentrates have generated problems and subsequent challenges for the mining industry. In particular, the requirement of additional processing for arsenic removal from aqueous solutions from mining operations. In this technology area, cost effective biotechnological approaches for remediating this metalloid, using acidophilic sulfate reducing bacteria (SRB) can be an attractive alternative to current systems for As removal from acidic mine water. Here we describe the performance of two acidophilic continuous fed bioreactors for the removal of arsenic (III) and arsenic (V). The biofilm-based continuous bioreactors were inoculated with a mixed culture including an acidophilic sulfate reducer (Desulfosporosinus acididurans). Both bioreactors were fed with an acidic synthetic water (pH 2.1) for up to 420 days, with varying concentrations of As3+ and As5+ (2–150 ppm) and using glycerol as carbon and energy source. The results demonstrated that over 95% of both forms of As were removed as arsenic sulfide. The hydraulic retention times (HRT) increased up to 350 h for higher concentrations of As. Considering that the pH of the bioreactor liquor was maintained at pH 4.5 by the addition of the synthetic mine water, the increased HRT was correlated with a decrease in SRB activity. This study demonstrated the feasibility of removing As (III) and As (V) from an extremely acidic water by using a continuous acidophilic SRB bioreactor, expanding the use of this technology for the remediation of this metalloid.

Bio-precipitation of arsenic and antimony in a sulfate-reducing bioreactor treating real acid mine drainage water.

Arsenic (As) and antimony (Sb) from mining sites can seep into aquatic ecosystems by acid mine drainage (AMD). Here, the possibility of concomitantly removing As and Sb from acidic waters by precipitation of sulfides induced by sulfate-reducing bacteria (SRB) was investigated in a fixed-bed column bioreactor. The real AMD water used to feed the bioreactor contained nearly 1 mM As, while the Sb concentrations were increased (0.008 ± 0.006 to 1.01 ± 0.07 mM) to obtain an Sb/As molar ratio = 1. Results showed that the addition of Sb did not affect the efficiency of As bio-precipitation. Sb was removed efficiently (up to 97.9% removal) between the inlet and outlet of the bioreactor, together with As (up to 99.3% removal) in all conditions. Sb was generally removed as it entered the bioreactor. Appreciable sulfate reduction occurred in the bioreactor, which could have been linked to the stable presence of a major SRB operational taxonomic unit affiliated with the Desulfosporosinus genus. The bacterial community included polymer degraders, fermenters, and acetate degraders. Results suggested that sulfate reduction could be a suitable bioremediation process for the simultaneous removal of Sb and As from AMD.

Treatment of Saline Irrigation Water Using a Sulfate-Reducing Bioreactor Coupled with an Iron-Based Desalination Reactor

Recent advances in alternative water desalination technologies have become increasingly common, due in part to expanding water scarcity and the ability to deliver usable water without the high energy cost and advanced infrastructure required of traditional desalination technologies such as reverse osmosis (RO). One such emerging technology is iron-based desalination or the use of nano- (or micron-sized) metals (principally iron) to desalinate water. In this study, iron-based desalination was coupled with passive sulfate reduction technology (SRB) to partially treat saline irrigation water similar in composition to that encountered in the San Joaquin Valley, California, USA. Water scarcity, especially in the southwestern United States, is driving an effort to identify and use less expensive but effective desalination methods to allow the use of saline groundwater, seawater, and impounded saline agricultural drainage and runoff for crop growth. The system described here used a synthetic water mimicking a typical saline irrigation water; this was then routed through a sulfate-reducing bioreactor to remove the sulfate prior to entry into an N-ZVM reactor for removal of Na and Cl. Sulfate at high concentrations can inhibit Na and Cl removal in N-ZVM reactors. The results showed that the sulfate was reduced from 2500 mg/L to less than 250 mg/L in the bioreactor, which allowed the N-ZVM to reduce the Na and Cl by 50%. This allowed the conductivity to decrease from 9.2 mS to about 5 mS. Synthetic irrigation water without sulfate removal was only desalinated by about 10%. The observed change in conductivity, sodium, and chloride content allowed the discharged water to be used for irrigation of many field crops and some vegetables. This paper provides reactor development, manufacture, and performance information including recommendations for continued performance increases.

A sulfate reducing bioreactor controlled by an electrochemical system enables near-zero chemical treatment of metallurgical wastewater

Metallurgical wastewaters are characterized by a low pH (<4), high concentrations of sulfate (15 gSO42− L−1), and metal(loid)s. Current treatment requires the consumption of chemicals such as alkali and high levels of waste sludge generation. In this study, we have shown that combining water electrolysis and sulfate reducing bioreactors enables the in-situ generation of base and H2, eliminating the need for base and electron donor addition, resulting in the near-zero treatment of metallurgical wastewater. By extracting cations from the effluent of the system to the bioreactor, the bioreactor pH could be maintained by the in-situ production of alkali. The current for pH control varied between 112–753 mol electrons per m³ wastewater or 5–48 A m−2 electrode area. High concentrations of sulfate in the influent and addition of CO2 increased the current required to maintain a steady bioreactor pH. On the other hand, a high sulfate reduction rate and increased influent pH lowered the current required for pH control. Moreover, the current efficiency varied from 14% to 91% and increased with higher pH and cation (Na+, NH4+, K+, Mg2+, Ca2+) concentrations in the middle compartment of the electrochemical cell. The salinity was lowered from 70–120 mS cm−1 in the influent to 5–20 mS cm−1 in the system effluent. The energy consumption of the electrochemical pH control varied between 10 and 100 kWh m−3 and was affected by the conductivity of the wastewater. Industrial wastewater was treated successfully with an average energy consumption of 39 ± 7 kWh m−3, removing sulfate from 15 g SO42− L−1 to 0.5 ± 0.5 g SO42− L−1 at a reduction rate of 20 ± 1 gSO42− L−1 d−1..Metal(loid)s such as As, Cd, Cu, Pb, Te, Tl, Ni and Zn were removed to levels of 1–50 µg L−1.

Use of propionic acid additions to enhance zinc removal from mine drainage in short residence time, flow-through sulfate-reducing bioreactors

The effectiveness of liquid carbon additions to enhance zinc removal in laboratory-scale short hydraulic residence time (19 h) compost bioreactors receiving synthetic mine water with a high influent zinc concentration (45 mg/L) was investigated. Effective removal of such elevated zinc concentrations could not be sustained by sulfate reduction and/or other attenuation processes without carbon supplementation. Propionic acid addition resulted in improved and sustained performance by promoting the activities of sulfate reducing bacteria, leading to efficient zinc removal (mean 99%) via bacterial sulfate reduction. In contrast, cessation of propionic acid addition led to carbon limitation and the growth of sulfur oxidising bacteria, compromising zinc removal by bacterial sulfate reduction. These research findings demonstrate the potential for modest liquid carbon additions to compost-based passive treatment systems to engineer microbial responses which enhance rates of zinc attenuation in a short hydraulic residence time, enabling remediation of highly polluting mine drainage at sites with limited land availability.

Comparison of two acidophilic sulfidogenic consortia for the treatment of acidic mine water.

Sulfate-reducing bioreactors are a biotechnological alternative for the treatment of acid mine drainage (AMD). In this study, two separate bioreactors with pH and temperature-controlled (Bio I and II) were operated with two different acidophilic microbial consortia to determine their efficiencies in sulfate removal from a synthetic acidic mine water. The bioreactors were operated for 302days in continuous flow mode under the same parameters: fed with a sulfate solution of ∼30mM with a pH of 2.5, the temperature at 30°C, stirred gently at 40rpm and using a continuous stream of nitrogen to help remove the H2S produced in the bioreactor. The glycerol consumption, acetate production, and sulfate removal were monitored throughout the course of the experiment. The community composition and potential metabolic functional groups were analyzed via 16S rRNA partial gene sequencing. Bio I consortium reduced the sulfate, achieving a range of sulfate concentration from 4.7 to 19mM in the effluent liquor. The removal of sulfate in Bio II was between 5.6 and 18mM. Both bioreactors' communities showed the presence of the genus De sulfosporosinus as the main sulfate-reducing bacteria (SRB). Despite differences in microbial composition, both bioreactors have similar potential metabolism, with a higher percentage of microorganisms that can use sulfate in respiration. Overall, both bioreactors showed similar performance in treating acidic mine water containing mostly sulfate using two different acidophilic sulfidogenic consortia obtained from different global locations.

Remediation technologies for acid mine drainage: Recent trends and future perspectives

Acid mine drainage (AMD) is a highly acidic solution rich in heavy metals and produced by mining activities. It can severely inhibit the growth of plants, and microbial communities and disturb the surrounding ecosystem. In recent years, the use of different bioremediation technologies to treat AMD pollution has received widespread attention due to its environment-friendly and low-cost nature. Various active and passive remediation technologies have been developed for the treatment of AMD. The active treatment involves the use of different chemical compounds while passive treatments utilize natural and biological processes like constructed wetlands, anaerobic sulfate-reducing bioreactors, anoxic limestone drains, vertical flow wetlands, limestone leach beds, open limestone channels, and various organic materials. Moreover, different nanomaterials have also been successfully employed in AMD treatment. There are also reports on certain plant growth-promoting rhizobacteria (PGPR) which have the potential to enhance the growth and productivity of plants under AMD-contaminated soil conditions. PGPR applied to plants with phytoremediation potential called PGPR-assisted phytoremediation has emerged as an economical and environment-friendly approach. Nevertheless, various approaches have been tested and employed, all the approaches have certain limitations in terms of efficiency, secondary pollution of chemicals used for the remediation of AMD, and disposal of materials used as sorbents or as phytoextractants as in the case of PGPR-assisted phytoremediation. In the future, more research work is needed to enhance the efficiency of various approaches employed with special attention to alleviating secondary pollutants production and safe disposal of materials used or biomass produced during PGPR-assisted phytoremediation.

Insights into the microbial response mechanisms to ciprofloxacin during sulfur-mediated biological wastewater treatment using a metagenomics approach

The fate and removal of ciprofloxacin, a class of fluoroquinolone antibiotic, during sulfur-mediated biological wastewater treatment has been recently well documented. However, little is known regarding the genetic response of microorganisms to ciprofloxacin. Here, a lab-scale anaerobic sulfate-reducing bioreactor was continuously operated over a long term for ciprofloxacin-contaminated wastewater treatment to investigate the response of the microorganisms to ciprofloxacin by adopting a metagenomics approach. It was found that total organic carbon (TOC) removal and sulfate reduction were promoted by approximately 10% under ciprofloxacin stress, along with the enrichment of functional genera (e.g., Desulfobacter, Geobacter) involved in carbon and sulfur metabolism. The metagenomic analytical results demonstrated that ciprofloxacin triggered the microbial SOS response, as demonstrated by the up-regulation of the multidrug efflux pump genes (8–125-fold higher than that of the control) and ciprofloxacin-degrading genes (4–33-fold higher than that of the control). Moreover, the contents of ATP, NADH, and cytochrome C, as well as related functional genes (including genes involved in energy generation, electron transport, carbon metabolism, and sulfur metabolism) were markedly increased under ciprofloxacin stress. This demonstrated that the carbon and sulfur metabolisms were enhanced for energy (ATP) generation and electron transport in response to ciprofloxacin-induced stress. Interestingly, the microbes tended to cooperate while being subjected exposure to exogenous ciprofloxacin according to the reconstructed metabolic network using the NetSeed model. Particularly, the species with higher complementarity indices played more pivotal roles in strengthening microbial metabolism and the SOS response under long-term ciprofloxacin stress. This study characterized the response mechanisms of microorganisms to ciprofloxacin at the genetic level in sulfur-mediated biological wastewater treatment. These new understandings will contribute the scientific basis for improving and optimizing the sulfur-mediated bioprocess for antibiotics-laden wastewater treatment.

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.

Mechanistic insights into bio-stabilization of lead (II) in flue gas by a sulfate-reducing bioreactor

This study investigated bio-stabilization of divalent lead (Pb2+) in the simulated flue gas by a sulfate-reducing bioreactor (SRBR). The SRBR achieved Pb2+ bio-removal effectively (95.7–98.7%). Metagenomic and metaproteomic analyses revealed that bacterial heavy metal resistance, sulfate bio-reduction/dephosphorylation-driven Pb2+ bioprecipitation, Pb2+ bio-complexation by extracellular polymeric substances (EPS) and thiols, Pb2+ biosorption by cell wall and cell membrane, as well as Pb2+ bio-expulsion by bacterial efflux and secretion system synergistically actualized flue gas Pb2+ biodetoxification. Lead speciation analysis by sequential extraction processes followed by inductively coupled mass spectrometry and further characterization by a series of spectroscopic and spectrometric techniques comprehensively verified that well-crystalized Galena (PbS) and Pyromorphite (Pb5(PO4)3Cl) cell surface-deposited microparticles in the form of residual lead (Res-Pb, 53.4–73.7%) were the major products in Pb2+ bio-stabilization. Besides, organic matter-bound lead (OM-Pb, 4.1–12.6%) and humic acids-bound lead (HA-Pb, 9.0–23.1%) were the second major products. Flue gas Pb2+ mostly bioprecipitated with biogenic sulfides and phosphates to form PbS and Pb5(PO4)3Cl, partly complexed by humic acids, EPS and thiols to form HA-Pb and OM-Pb, and was partly immobilized by cell wall and cell membrane. The weakly stable OM-Pb and HA-Pb may thermodynamically convert into the less mobile, highly chemically stable and less bioavailable products: PbS and Pb5(PO4)3Cl via long-term equilibration. Therefore, SRBR effectively achieved bio-removal, bio-stabilization and bio-recovery of Pb2+ in flue gas, which may provide a feasible and environment-friendly biotechnique for flue gas Pb2+ decontamination.

Sulfate/sulfide removal from wastewater by lab-scale microbial fuel cell

Sulfate/sulfide-containing wastewater is a widespread environmental contaminant resulting from human activities. These pollutants have negative impact on natural ecosystems and human beings. Biological sulfate/sulfide removal can be achieved by reducing sulfate to sulfide with sulfate-reducing bacteria and then oxidizing sulfide to elemental sulfur (So) with sulfide oxidizing bacteria. In sulfate/sulfide contaminant wastewater lacking electron acceptor for sulfide oxidization, excess sulfide will be produced and accumulated in the treatment system. Therefore, microbial fuel cells (MFCs) have been shown to be a promising technique for the removal of sulfate/sulfide pollutants in wastewater. In this study, a lab-scale MFCs has been developed based on the activity of sulfate-reducing bacterium Desulfovibrio. sp. The results showed that sulfate and sulfide removal efficiencies of 74 - 82% (78 ± 2,5%) and 80.8 - 89.1% (~85,6 ± 3,1%) were achieved, respectively, from the 5th day to the 14th day of operation. The voltage of 0.02V and power density of 7.2 to 7.8 mW/m2 was obtained. In this study, sulfide oxidizing-based MFC integrated with sulfate-reducing bioreactor, representing the feasibility of simultaneous sulfate/sulfide pollutants removal and electricity generation in MFCs. This provides a promising treatment system to scale up for its actual applications in sulfate/sulfide removal.

Passive Treatment of Circumneutral Mine Drainage from the St. Louis Mine Tunnel, Rico CO: Part 2—Vertical Biotreatment Train Pilot Study

AbstractThis is the second of three papers dealing with metal-bearing circumneutral mine drainage from the inactive Rico-Argentine mine site located at an elevation of ≈ 2740 m (9000 feet) in the San Juan mountain range in southwestern Colorado. This paper evaluates two years of mine drainage treatment using a passive system that included a vertical-flow engineered biotreatment cell. The collapsed St. Louis Tunnel (SLT) discharges circumneutral mine water from several sources that contains elevated concentrations of Cd, Cu, Fe, Mn, Zn. A demonstration-scale 114 L/min (30 gpm) gravity-flow passive treatment system was installed, consisting of a settling basin (utilizing coagulant addition to improve suspended solids settling efficiency), an anaerobic sulfate-reducing bioreactor, and an aeration cascade for effluent polishing. The treatment system generally met target treatment goals for Cd, Cu, Fe, and Pb. Nanophase ZnS in system effluent decreased the frequency of meeting total Zn project treatment goals. Unexpectedly high levels of Mn removal were observed in both the anaerobic bioreactor and the aeration cascade. Large seasonal variations in influent metals concentrations and pH present the greatest challenge in managing system performance.

High rate production of concentrated sulfides from metal bearing wastewater in an expanded bed hydrogenotrophic sulfate reducing bioreactor

Metallurgical wastewaters contain high concentrations of sulfate, up to 15 g L−1. Sulfate-reducing bioreactors are employed to treat these wastewaters, reducing sulfates to sulfides which subsequently co-precipitate metals. Sulfate loading and reduction rates are typically restricted by the total H2S concentration. Sulfide stripping, sulfide precipitation and dilution are the main strategies employed to minimize inhibition by H2S, but can be adversely compromised by suboptimal sulfate reduction, clogging and additional energy costs. Here, metallurgical wastewater was treated for over 250 days using two hydrogenotrophic granular activated carbon expanded bed bioreactors without additional removal of sulfides. H2S toxicity was minimized by operating at pH 8 ± 0.15, resulting in an average sulfate removal of 7.08 ± 0.08 g L−1, sulfide concentrations of 2.1 ± 0.2 g L−1 and peaks up to 2.3 ± 0.2 g L−1. A sulfate reduction rate of 20.6 ± 0.9 g L−1 d−1 was achieved, with maxima up to 27.2 g L−1 d−1, which is among the highest reported considering a literature review of 39 studies. The rates reported here are 6–8 times higher than those reported for other reactors without active sulfide removal and the only reported for expanded bed sulfate-reducing bioreactors using H2. By increasing the influent sulfate concentration and maintaining high sulfide concentrations, sulfate reducers were promoted while fermenters and methanogens were suppressed. Industrial wastewater containing 4.4 g L−1 sulfate, 0.036 g L−1 nitrate and various metals (As, Fe, Tl, Zn, Ni, Sb, Co and Cd) was successfully treated with all metal(loid)s, nitrates and sulfates removed below discharge limits.

Effective co-treatment of synthetic acid mine drainage and domestic sewage using multi-unit passive treatment system supplemented with silage fermentation broth as carbon source

A multi-unit passive treatment system was constructed for co-treatment of synthetic acid mine drainage (AMD) and domestic sewage supplemented with silage fermentation broth as carbon source. AMD and domestic sewage mixing pretreatment (unit 1) improved influent quality with pH increase, metals removal and nutrients supplement. The generated metal-rich sludge in unit 1 retained the metals (69.95% of Fe, 97.36% of Cu, 96.53% of Cd, 72.52% of Zn, and 8.59% of Mn) of influent prior to entering subsequent bioreactors. Silage fermentation broth performed well to promote bacterial sulfate reduction in sulfate reducing bioreactor system (unit 2). Residual metals (Mn) and organic/nutrient pollutants were further polished in surface-flow aerobic wetland (unit 3), where relatively high pH (7.4–8.6), aerobic condition, potential Mn-oxidizing bacteria, limestone layer and low concentrations of Fe(II) (0.04–3.5 mg/L) favored the efficient removal of Mn. After 210-day continuous flow-through experiment, this passive treatment system demonstrated the efficient performance, increasing pH from 2.5 to 8.0 with removal of metals (99%), sulfate and organic/nutrient pollutants. Diverse sulfate reducing bacteria including complete organic oxidizers (e.g. Desulfobacter) and incomplete organic oxidizers (e.g. Desulfovibrio) promoted sulfate reduction and organic/nutrient pollutants removal. Ammonia oxidizing bacteria (e.g. Nitrosomonas) and nitrite oxidizing bacteria (e.g. unidentified_Nitrospiraceae) were the potential nitrifiers for ammonia removal. Collaboration of anaerobic denitrifiers (e.g. Denitratisoma) and potential heterotrophic nitrifying and aerobic denitrifiers (HN-AD) achieved effective nitrate removal. This multi-unit treatment system with domestic sewage and silage fermentation broth as stimulation substrates provided an attractive option for AMD treatment.

Dynamics of Microbial Communities during the Removal of Copper and Zinc in a Sulfate-Reducing Bioreactor with a Limestone Pre-Column System.

Biological treatment using sulfate-reducing bacteria (SRB) is a promising approach to remediate acid rock drainage (ARD). Our purpose was to assess the performance of a sequential system consisting of a limestone bed filter followed by a sulfate-reducing bioreactor treating synthetic ARD for 375 days and to evaluate changes in microbial composition. The treatment system was effective in increasing the pH of the ARD from 2.7 to 7.5 and removed total Cu(II) and Zn(II) concentrations by up to 99.8% and 99.9%, respectively. The presence of sulfate in ARD promoted sulfidogenesis and changed the diversity and structure of the microbial communities. Methansarcina spp. was the most abundant amplicon sequence variant (ASV); however, methane production was not detected. Biodiversity indexes decreased over time with the bioreactor operation, whereas SRB abundance remained stable. Desulfobacteraceae, Desulfocurvus, Desulfobulbaceae and Desulfovibrio became more abundant, while Desulfuromonadales, Desulfotomaculum and Desulfobacca decreased. Geobacter and Syntrophobacter were enriched with bioreactor operation time. At the beginning, ASVs with relative abundance <2% represented 65% of the microbial community and 21% at the end of the study period. Thus, the results show that the microbial community gradually lost diversity while the treatment system was highly efficient in remediating ARD.

Dynamics of Microbial Communities during the Removal of Copper and Zinc in a Sulfate-Reducing Bioreactor with a Limestone Pre-Column System

Dynamics of Microbial Communities during the Removal of Copper and Zinc in a Sulfate-Reducing Bioreactor with a Limestone Pre-Column System