Effect of electron donor source on the treatment of Cr(VI)-containing textile wastewater using sulfate-reducing fluidized bed reactors (FBRs)

Impacts of phosphorus and nitrogen absence on microbial diversity and sulfate removal in anaerobic batch reactors

Sulfate-rich effluents have been successfully treated in anaerobic reactors using sulfate-reducing bacteria (SRB). Many authors have demonstrated that these systems require nitrogen and phosphorous supplementation to achieve high sulfate removal rates. However, the resource ratio theory assumes that some species can be dominant according to the nutritional relations used or even without external nutrient supplementation. Thus, this study evaluated the SRB communities in batch reactors without external nitrogen and phosphorus sources based on most probable number (MPN) quantification, denaturing gradient gel electrophoresis (DGGE) analyses and sequencing. The sulfate and chemical oxygen demand (COD) removal and kinetic parameters were also determined. After 100 days of operation, the sulfate and COD removal achieved 71.8 ± 10% and 86.5 ± 10%, respectively. The SRB population increased from 8.10⁶ to 4 × 10¹² MPN 100 mL^-1, and the richness of SRB bands was much higher at the end of the experiment compared to the inoculum. In addition, the sequenced bands from SRB-DGGE showed similarities to Desulfacinum infernum, Desulfobulbus sp, Syntrophobacter and Desulfomicrobium aestuarii-related sequences. Therefore, biological treatment of acid mine drainage wastewater was effective in the absence of nutrients, lowering costs and providing high sulfate removal efficiency.

Extracellular electron transfer-dependent Cr(VI)/sulfate reduction mediated by iron sulfide nanoparticles

The slow electron transfer rate is a bottleneck to the biological wastewater treatment. This study evaluated the concomitant biotransformation and nonenzymatic reduction of Cr(VI) mediated by sulfate reducing bacteria (SRB), especially for the reinforcing Cr(VI) reduction via accelerating the electron transfer by the in-situ biosynthesized iron sulfide nanoparticles (FeS NPs). The kinetic results showed that 10 mg/L Cr(VI) was completely removed by pre-cultured FeS NPs within 7 h with k_Cr(VI) of 2.6 × 10^-4 s^-1, one magnitude higher than that without FeS NPs. Despite its competing electron to postpone sulfate reduction, the reduction of Cr(VI) was markedly improved via nonenzymatic reactions by the sulfide, the product of sulfate reduction. In the reinforcing system (bio-FeS NP@SRB), the bio-FeS NPs served as an electronic bypass conduit for CoQ could significantly amplify the electron flux, and switch the Cr(VI) reduction from intracellular space to extracellular environment, which had a great detoxification effect on the microorganisms, eventually markedly promoted electron transfer extracellularly and the reduction of Cr(VI). After the long-term acclimatization, Desulfovibrio became the dominant bacteria at the genus level and accounted for the relative abundance of 32%. This study provides an alternative to use biogenic FeS NPs for Cr(VI) remediation.

Highly efficient Cr(VI) removal from industrial electroplating wastewater over Bi2S3 nanostructures prepared by dual sulfur-precursors: Insights on the promotion effect of sulfate ions

In this work, different Bi₂S₃ nanostructures were prepared from various single and dual sulfide precursors via a solvothermal method. It was found that Bi₂S₃ nanostructures prepared from dual sulfur precursors of L-cysteine and ammonium sulfide exhibited highest Cr(VI) removal ability with maximum Cr(VI) removal capacity of 148.95 mg/g in Cr(VI) solution (pH = 2). More importantly, the removal capacity strikingly increased to 223.33 and 240.25 mg/g in two kinds of actual industrial electroplating wastewater. By analyzing the components of actual electroplating wastewater and the results of control experiments in the absence and presence of different ions in Cr(VI) solution, it was found that SO₄^2- played a critical role in the Cr(VI) removal over Bi₂S₃. The addition of SO₄^2- could promote the conversion of Cr(VI) to Cr(III) on the surface of Bi₂S₃, thus leading to the enhanced Cr(VI) removal ability in actual electroplating wastewater. The Bi₂S₃ maintained its original Cr(VI) removal ability after four cycles in the electroplating wastewater, indicating the moderate reuse ability of the sample. This work not only demonstrated an highly efficient nanomaterials for the Cr(VI) removal in industrial electroplating wastewater, but also provided an insight on the influence of the components in wastewater on Cr(VI) removal.

Microbial Mechanisms for Remediation of Hexavalent Chromium and their Large-Scale Applications; Current Research and Future Directions

The increase of anthropogenic activities has led to the pollution of the environment by heavy metals, including chromium (Cr). There are two common oxidative states of Cr that can be found in industrial effluents the trivalent chromium Cr(III) and the hexavalent chromium Cr(VI). While the hexavalent chromium Cr(VI) is highly toxic and can trigger serious human health issues, its reduced form, the trivalent chromium Cr(III), is less toxic and insoluble. Leather tanning is an important industry in many developing countries and serves as a major source of Cr(VI) contamination. Globally, tannery factories generate approximately 40 million m3 of Cr-containing wastewater annually. While the physico-chemical treatments of tannery wastewater are not safe, produce toxic chemicals and require large amounts of chemical inputs, bioremediation using chromium-resistant bacteria (CRB) is safer, efficient and does not produce toxic intermediates. Chromium-resistant bacteria (CRB) utilise three mechanisms for Cr(VI) removal: biotransformation, biosorption and bioaccumulation. This review will evaluate the three Cr(VI) detoxification mechanisms used by bacteria, their limitations and assess their applications for large-scale remediation of Cr(VI). This can be helpful for understanding the nature of Cr(VI) remediation mechanisms used by bacteria, therefore, bridging the gap between laboratory findings and industrial application of microorganisms for Cr(VI) removal.

Chemical-Assisted Microbially Mediated Chromium (Cr) (VI) Reduction Under the Influence of Various Electron Donors, Redox Mediators, and Other Additives: An Outlook on Enhanced Cr(VI) Removal

Chromium (Cr) (VI) is a well-known toxin to all types of biological organisms. Over the past few decades, many investigators have employed numerous bioprocesses to neutralize the toxic effects of Cr(VI). One of the main process for its treatment is bioreduction into Cr(III). Key to this process is the ability of microbial enzymes, which facilitate the transfer of electrons into the high valence state of the metal that acts as an electron acceptor. Many underlying previous efforts have stressed on the use of different external organic and inorganic substances as electron donors to promote Cr(VI) reduction process by different microorganisms. The use of various redox mediators enabled electron transport facility for extracellular Cr(VI) reduction and accelerated the reaction. Also, many chemicals have employed diverse roles to improve the Cr(VI) reduction process in different microorganisms. The application of aforementioned materials at the contaminated systems has offered a variety of influence on Cr(VI) bioremediation by altering microbial community structures and functions and redox environment. The collective insights suggest that the knowledge of appropriate implementation of suitable nutrients can strongly inspire the Cr(VI) reduction rate and efficiency. However, a comprehensive information on such substances and their roles and biochemical pathways in different microorganisms remains elusive. In this regard, our review sheds light on the contributions of various chemicals as electron donors, redox mediators, cofactors, etc., on microbial Cr(VI) reduction for enhanced treatment practices.

Optimization of nitrate and selenate reduction in an ethanol-fed fluidized bed reactor via redox potential feedback control

Electron donors are a major cost-factor in biological removal of oxyanions, such as nitrate and selenate from wastewater. In this study, an online ethanol dosing strategy based on feedback control of oxidation-reduction potential (ORP) was designed to optimize the performance of a lab-scale fluidized bed reactor (FBR) in treating selenate and nitrate (5 mM each) containing wastewater. The FBR performance was evaluated at various ORP setpoints ranging between -520 mV and -240 mV (vs. Ag/AgCl). Results suggested that both nitrate and selenate were completely removed at ORPs between -520 mV and -360 mV, with methylseleninic acid, selenocyanate, selenosulfate and ammonia being produced at low ORPs between -520 mV and -480 mV, likely due to overdosing of ethanol. At ORPs between -300 mV and -240 mV, limited ethanol dosing resulted in an apparent decline in selenate removal whereas nitrate removal remained stable. Resuming the ORP to -520 mV successfully restored complete selenate reduction. An optimal ORP of -400 mV was identified for the FBR, whereby selenate and nitrate were nearly completely removed with a minimal ethanol consumption. Overall, controlling ORP via feedback-dosing of the electron donor was an effective strategy to optimize FBR performance for reducing selenate and nitrate in wastewater.

Self-forming dynamic membrane bioreactor for textile industry wastewater treatment

The robustness of anaerobic dynamic membrane bioreactor (AnDMBR) for synthetic textile wastewater treatment was investigated. Textile wastewater may contain high concentrations of NaCl and sulfate, hence their impact on the AnDMBR performance was investigated in detail. A dynamic membrane was formed on a 20-μm pore sized nylon support layer at a constant flux of around 8 LMH. In the absence of sulfate addition, total and filtered (soluble) COD averaged 96 ± 49 mg/L (91% removal) and 75 ± 35 mg/L (93% removal), respectively. Sulfate addition increased total COD in the permeate to 222 ± 68 mg/L (79% removal). Average SS concentration was lower than 30 mg/L in the permeate although its concentration in the bioreactor reached 10 g/L. Throughout the AnDMBR operation dye removal averaged >97%. Sludge filterability, which was assessed by specific resistance to filtration, supernatant filtration, capillary suction time and viscosity, decreased after sulfate addition. Organic and inorganic matters in the dynamic layer were characterized by SEM-EDS and FTIR analyses.

Sequential hydrotalcite precipitation and biological sulfate reduction for acid mine drainage treatment

Hydrotalcite precipitation is a promising technology for the on-site treatment of acid mine drainage (AMD). This technology is underpinned by the synthesis of hydrotalcite that can effectively remove various contaminants. However, hydrotalcite precipitation has only limited capacity to facilitate sulfate removal from AMD. Therefore, the feasibility of coupling biological sulfate reduction with the hydrotalcite precipitation to maximize sulfate removal was evaluated in this study. AMD emanating from a gold mine (pH 4.3, sulfate 2000 mg L^-1, with various metals including Al, Cd, Co, Cu, Fe, Mn, Ni, Zn) was first treated using the hydrotalcite precipitation. Subsequently, biological treatment of the post-hydrotalcite precipitation effluent was conducted in an ethanol-fed fluidized bed reactor (FBR) at a hydraulic retention time (HRT) of 0.8-1.6 day. The hydrotalcite precipitation readily neutralized the acidity of AMD and removed 10% of sulfate and over 99% of Al, Cd, Co, Cu, Fe, Mn, Ni, Zn. The overall sulfate removal increased to 73% with subsequent FBR treatment. Based on 454 pyrosequencing of 16S rRNA genes, the identified genera of sulfate-reducing bacteria (SRB) included Desulfovibrio, Desulfomicrobium and Desulfococcus. This study showed that sulfate-rich AMD can be effectively treated by integrating hydrotalcite precipitation and a biological sulfate reducing FBR.

Effect of chromium (VI) on the multiple nitrogen removal pathways and microbial community of aerobic granular sludge

The frequent appearance of Cr(VI) significantly impacts the microbial metabolism in wastewater. In this study, long-term effects of Cr(VI) on microbial community, nitrogen removal pathways and mechanism of aerobic granular sludge (AGS) were investigated. AGS had strong resistance ability to 1.0 mg/L Cr(VI). 3.0 mg/L Cr(VI) increased the heterotrophic-specific ammonia uptake rate (HSAUR) and heterotrophic-specific nitrate uptake rate (HSNUR) transiently, whereas 5.0 mg/L Cr(VI) sharply decreased the specific ammonia uptake rate (SAUR), specific nitrate uptake rate (SNUR) and simultaneous nitrification denitrification rate (SNDR). It was found that Cr (VI) has a greater inhibitory effect on autotrophic nitrification (ASAUR), and the maximal inhibition rate (IR) was 139.19%. Besides, the inhibition of Cr (VI) on nitrogen removal process belongs to non-competitive inhibition. Cr(VI) had a weaker negative impact on heterotrophic bacteria compared with that on autotrophic bacteria. Denaturing gradient gel electrophoresis analyses suggest that Acidovorax sp., flavobacterium sp., uncultured soil bacterium, uncultured nitrosospira sp., uncultured prokaryote, uncultured β-proteobacterium and uncultured pseudomonas sp. were the dominant species. The inhibition of Cr(VI) on nitrite-oxidizing bacteria was the strongest, followed by ammonia-oxidizing bacteria and denitrifying bacteria. Linear correlations between bacterial count and biomass-specific uptake rate were observed when the Cr(VI) concentration exceeded 3 mg/L. This study revealed the effect of Cr(VI) on nitrification is more serious than that on denitrification. Autotrophic and heterotrophic nitrification, heterotrophic denitrification and simultaneous nitrification denitrification played a significant role on nitrogen removal under Cr(VI) stress.

Exploring the potential of anaerobic sulfate reduction process in treating sulfonated diazo dye: Microbial community analysis using bar-coded pyrosequencing

Anaerobic decolorization and biotransformation of azo dye was investigated in a sulfate-reducing environment. Batch reactor studies were performed with mixed cultures of anaerobic sulfate-reducing bacteria (SRBs) enriched from anaerobic digester sludge. Complete sulfate and color removal were achieved in batch experiments with different initial dye concentrations (50-2500mg/L) and 1000mg/L of sulfate. Induction of various oxidoreductive enzyme activities such as phenol oxidase, veratryl alcohol oxidase, lignin peroxidase, and azo reductase was studied to understand their involvement in dye metabolism under anoxic environment. The degradation of Cotton Red B was confirmed using high-performance liquid chromatography and gas chromatography-mass spectroscopy. Sulfidogenic sludge demonstrated excellent dye degradation and mineralization ability, producing aniline and 1,4-diamino benzene as metabolites. A barcoded 16S rRNA gene-pyrosequencing approach was used to assess the bacterial diversity in the sludge culture and a phylogenetic tree was constructed for sulfate-reducing bacteria.

Cr(VI) Adsorption on Red Mud Modified by Lanthanum: Performance, Kinetics and Mechanisms

Water pollution caused by the highly toxic metal hexavalent chromium (Cr(VI)) creates significant human health and ecological risks. In this study, a novel adsorbent was used to treat Cr(VI)-containing wastewater; the adsorbent was prepared using red mud (RM) generated from the alumina production industry and the rare earth element lanthanum. This study explored adsorption performance, kinetics, and mechanisms. Results showed that the adsorption kinetics of the RM modified by lanthanum (La-RM), followed the pseudo-second-order model, with a rapid adsorption rate. Cr(VI) adsorption was positively associated with the absorbent dose, pH, temperature, and initial Cr(VI) concentration; coexisting anions had little impact. The maximum Cr(VI) adsorption capacity was 17.35 mg/g. Cr(VI) adsorption on La-RM was a mono-layer adsorption pattern, following the Langmuir isotherm model. Thermodynamic parameters showed the adsorption was spontaneous and endothermic. The adsorption of Cr(VI) on La-RM occurred as a result of LaOCl formation on the RM surface, which in turn further reacted with Cr(VI) in the wastewater. This study highlighted a method for converting industrial waste into a valuable material for wastewater treatment. The novel absorbent could be used as a potential adsorbent for treating Cr(VI)-contaminating wastewater, due to its cost-effectiveness and high adsorption capability.

The effect of acidic pH and presence of metals as parameters in establishing a sulfidogenic process in anaerobic reactor

The successful use of anaerobic reactors for bioremediation of acid mine drainage has been shown in systems with neutral pH. However, the choice of an efficient and suitable process for such wastewater must consider the capability of operating at acidic pH and in the presence of metals. This work studies the performance of an anaerobic batch reactor, under conditions of varying initial pH for its efficiencies in sulfate removal and metal precipitation from synthetic acid mine drainage. The chemical oxygen demand/sulfate (COD/SO4(2-)) ratio used was 1.00, with ethanol chosen as the only energy and carbon source. The initial pH of the synthetic drainage was progressively set from 7.0 to 4.0 to make it as close as possible to that of real acid mine drainage. Metals were also added starting with iron, zinc, and finally copper. The effectiveness of sulfate and COD removal from the synthetic acid mine drainage increased as the initial pH was reduced. The sulfate removal increased from 38.5 ± 3.7% to 52.2 ± 3%, while the removal of organic matter started at 91.7 ± 2.4% and ended at 99 ± 1%. These results indicate that the sulfate reducing bacteria (SRB) community adapted to lower pH values. The metal removal observed was 88 ± 7% for iron, 98.0 ± 0.5% for zinc and 99 ± 1% for copper. At this stage, an increase in the sulfate removal was observed, which reaches up to 82.2 ± 5.8%. The kinetic parameters for sulfate removal were 0.22 ± 0.04 h(-1) with Fe, 0.26 ± 0.04 h(-1) with Fe and Zn and 0.44 ± 0.04 h(-1) with Fe, Zn, and Cu.

Performances of anaerobic and aerobic membrane bioreactors for the treatment of synthetic textile wastewater

This study aims at comparatively evaluating anaerobic and aerobic MBRs for the treatment of azo-dye containing synthetic wastewater. Also, the filtration performances of AnMBR and AeMBR were compared under similar operating conditions. In both MBRs, high COD removal efficiencies were observed. Although almost complete color removal was observed in AnMBR, only partial (30-50%) color removal was achieved in AeMBR. AnMBR was successfully operated up to 9 L/(m(2)h) (LMH) and no chemical cleaning was required at 4.5 LMH for around 50 days. AeMBR was operated successfully up to 20 LMH. The filtration resistance of AnMBR was generally higher compared to AeMBR although reversible fouling rates were comparable. In both MBRs, offline chemical cleaning with NaOCl and sulfuric acid almost completely removed irreversible fouling and the resistances of chemically cleaned membranes were close to those of new membranes.

Detoxifying of high strength textile effluent through chemical and bio-oxidation processes

Small-scale textile industries (SSTIs) in India struggled for the economic and environmental race. A full-scale common treatment plant (CETP) working on the principle of destabilising negative charge colloidal particles and bio-oxidation of dissolved organic failed to comply with Inland Surface Waters (ISW) standards. Thus, presence of intense colour and organics with elevated temperature inhibited the process stability. Bench scale treatability studies were conducted on chemical and biological processes for its full-scale apps to detoxify a high strength textile process effluent. Colour, SS and COD removals from the optimised chemical process were 88%, 70% and 40%, respectively. Heterotrophic bacteria oxidised COD and BOD more than 84% and 90% at a loading rate 0.0108kgm(-3)d(-1) at 3h HRT. The combined chemical and bio-oxidation processes showed a great promise for detoxifying the toxic process effluent, and implemented in full-scale CETP. The post-assessment of the CETP resulted in detoxify the toxic effluent.

Treatment of azo dye-containing synthetic textile dye effluent using sulfidogenic anaerobic baffled reactor

This study aims at investigating azo dye reduction performance of a sulfidogenic anaerobic baffled reactor (ABR) for around 400 days. ABR was operated at 30 °C in a temperature-controlled room and hydraulic retention time (HRT) was kept constant at 2 days. The robustness of ABR was assessed under varying azo dye loadings and COD/sulfate ratios. Additionally, oxygen was supplied (1-2 L air/m(3)reactor min) to the last compartment to investigate the removal of azo dye breakdown products. ABR performed well in terms of COD, sulfate and azo dye removals throughout the reactor operation. Maximum azo dye, COD and sulfate removals were 98%, 98% and 93%, respectively, at COD/sulfate ratio of 0.8. Aeration created different redox conditions in last compartment, which enhanced the removal of COD and breakdown products. The adverse effects of aeration on azo dye reduction were eliminated thanks to the compartmentalized structure of the ABR.

Removal of chromium and lead by a sulfate-reducing consortium using peat moss as carbon source

The effect of pre-treated peat moss on the ability of a sulfate-reducing microbial consortium to remove chromium and lead in solution was evaluated. The most active bacterial community (235.7 mmol H2S/g VSS) was selected from among eight consortia. The peat moss was pre-treated with different HCl concentrations and contact times. The best combination of treatments was 20% HCl for 10 min. The constant substrate affinity Ks was 740 mg COD/L and the ratio COD/SO4(2-) was 0.71. At pH 5, higher production of biogenic sulfide was observed. The up-flowpacked bed bioreactor operated at a flow of 8.3 mL/min for 180 h to obtain removal efficiency (by sulfate-reducing activity) of 90% lead and 65% chromium. It is important to consider that peat moss is a natural adsorbent that further influences the removal efficiency of metal ions.

Microbial monitoring by molecular tools of an upflow anaerobic filter treating abattoir wastewaters

The performance of anaerobic digestion of abattoir wastewaters (AW) in an upflow anaerobic filter (UAF) was investigated under mesophilic (37°C) and thermophilic (55°C) conditions. The effects of increasing temperature on the performance of the UAF and on the dynamics of the microbial community of the anaerobic sludge were studied. The results showed that chemical oxygen demand (COD) removal efficiency of 90% was achieved for organic loading rates (OLRs) up to 4.5g CODL(-1)d(-1) in mesophilic conditions, while in thermophilic conditions, the highest OLRs of 9 g CODL(-1)d(-1) led to the efficiency of 72%. The use of molecular and microbiological methods to recover microbial populations involved in this process showed that fermentative bacteria were the prominent members of the sludge microbial community. Three novel strains were identified as Macellibacteroides fermentans, Desulfotomaculum peckii and Defluviitalea saccharophila.