Lignocellulosic biowastes as carrier material and slow release electron donor for sulphidogenesis of wastewater in an inverse fluidized bed bioreactor

Sulfate removal rate and metal recovery as settling precipitates in bioreactors: Influence of electron donors

Four down-flow structured bed bioreactors were operated targeting biological sulfate-reduction and metal recovery. Three different electron donors were tested: glycerol (R1), lactate (R2), sucrose (R3), and a blend of the previous three (R4) with an increasing copper influent load (5, 15, and 30 mg Cu²⁺.L^-1). Copper inhibited sulfate-reduction in R1 (15 mg Cu²⁺.L^-1) and R3 (5 mg Cu²⁺.L^-1), but the fermentative activity was not affected. R2 and R4 were not inhibited by the copper influent concentration. R2 provided the highest sulfate reduction rate (1767.3 ± 240.1 mg SO₄^2-.L.day^-1). Nonetheless, the accumulation of settling precipitates was 22 % higher in R4 than in R2, indicating the former yielded the highest metal recovery as settling precipitates (24.8 g FSS.L^-1, 25 % Fe²⁺, 5% Cu²⁺). 16S rRNA sequencing showed highest diversity of sulfate-reducing bacteria in R2. A predominance of sulfate-reducing and fermentative bacteria with more similarity was observed between microbial populations in R1 and R4, despite the difference in toxicity thresholds. Hence, the electron donor influenced not only the biological sulfate reduction, but also metal toxicity thresholds and metal recovery as settling precipitates.

Pre-treatment and temperature effects on the use of slow release electron donor for biological sulfate reduction

Lignocellulosic materials can be used as slow release electron donor (SRED) for biological sulfate reduction, potentially enhancing the subsequent metal sulfide precipitation. Lignocellulosic materials require a pre-treatment step in other biotechnological applications, but pre-treatment strategies for its use as a SRED for biological sulfate reduction have not yet been tested. Three pre-treatments strategies (mechanical, acid, and mechanical followed by acid pre-treatment) were tested to enhance electron donor release from brewery spent grain (BSG), and compared to a non-pre-treated control. Mechanical pre-treatment provided the highest sulfate removal rate (82.8 ± 8.8 mg SO₄^2-.(g TVS.day)^-1), as well as the highest final sulfide concentration (441.0 ± 34.4 mg.L^-1) at mesophilic conditions. BSG submitted to mechanical pre-treatment was also assessed under psychrophilic and thermophilic conditions. Under mesophilic and psychrophilic conditions, both sulfate reduction and methane production occurred. Under psychrophilic conditions, the sulfate reduction rate was lower (25 ± 2.0 mg SO₄^2-.(g TVS.day)^-1), and the sulfide formation depended on lactate addition. A metal precipitation assay was conducted to assess whether the use of SRED enhances metal recovery. Zinc precipitation and recovery with chemical or biogenic sulfide from the BSG batches were tested. Sulfide was provided in a single spike or slowly added, mimicking the effect of SRED. ZnS was formed in all conditions, but better settling particles were obtained when sulfide was slowly added, regardless of the sulfide source.

Role of cost-effective organic carbon substrates in bioremediation of acid mine drainage-impacted soil of Malanjkhand Copper Project, India: a biostimulant for autochthonous microbial populations

Development of an efficient bioremediation strategy for the mitigation of low pH (3.21), high dissolved SO₄^2- (6285 mg/L), and Fe (7292 mg/kg)-rich acid mine drainage-impacted soil (AIS) was investigated through amendment of readily available organic carbon substrates (rice husk, compost, leaf litter, and grass clippings). An organic carbon mixture (OCM) formulated by mixing the test substrates was used to biostimulate microbial processes (SO₄^2-/Fe³⁺reduction) necessary for efficient attenuation of the hazards imposed by AIS. OCM amendment in calcium carbonate-treated AIS enhanced reductive processes and removed dissolved SO₄^2- and Fe³⁺ considerably raising the pH close to neutrality. 16S rRNA gene amplicon sequencing performed with total DNA and RNA elucidated the microbial population dynamics of treated AIS. Metabolically active populations comprised of fermentative (Clostridium sensu stricto 1 and Fonticella), iron-reducing (Acidocella, Anaeromyxobacter, and Clostridium sensu stricto 1), and sulfate-reducing (Desulfovibrio, Desulfotomaculum, Desulfosporosinus, and Desulfobacteraceae) bacteria. Microbial guilds obtained highlighted the synergistic role of cellulolytic, fermentative, and SO₄^2-/Fe³⁺-reducing bacteria in attenuation of hazardous contaminants. Quantitative PCR analysis well supported the role of OCM in stimulating the indigenous bacterial populations, including those harboring the dissimilatory sulfite reductase (dsrB) gene and involved actively in SO₄^2- reduction. The study demonstrated the suitability of locally available organic substrates as a low-cost and efficient biostimulation agent for in situ bioremediation of acid mine drainage (AMD)-impacted soil system.

Bioremediation of Aquaculture Wastewater with Algal-Bacterial Biofilm Combined with the Production of Selenium Rich Biofertilizer

The discharge of aquaculture wastewater and the excessive selenium in aquaculture effluent caused by selenium addition to aquatic feed are posing a serious risk for the marine environment. In this study, batch tests were carried out to investigate the feasibility of utilizing algal–bacterial biofilm for the treatment of selenium-rich aquaculture wastewater. The effects of four different types of commercial biofilm carriers on the attached growth of biofilms and the contaminant removal capacity were examined. The braided cotton biofilm carrier had the best performance on biofilm growth, while in an exponential growth period the dry weight density of the biofilm was above 2.0 g L−1. By utilizing the braided cotton carrier with a hydraulic retention time (HRT) of 6 days, the removal rate of N and P from the raw aquaculture wastewater was 88.5 ± 6.2% and 99.8 ± 0.2%, respectively. After that, the effects of different initial wastewater load ratios (IWLR) and HRT on the effluent quality of the treatment process were studied. The decrease in IWLR and the extension of HRT could improve the treatment performance. The effluent N, P and Se concentrations in the group with 50% IWLR and 6-day HRT were 0.75 ± 0.10 mg L−1, 0.015 ± 0.02 mg L−1, 35.2 ± 3.2 μg L−1, respectively, indicating an effective removal of the main contaminants. The algal–bacterial biofilm harvested from the batch test was rich in N, P and Se, where the Se content was 21.8 ± 3.4 mg kg−1, which has the potential to be used as an Se-rich biofertilizer.

Mine drainage: Treatment technologies and rare earth elements

The recent research and development on mine drainage published in 2018 was summarized in this annual review. In particular, this review was focused on two main aspects of mine drainage: (a) advances in treatment technologies and (b) rare earth elements in mine drainage and its recovery. The first section covers passive treatment technologies and active treatment options, including physiochemical treatment and biological treatment. The second section includes the characterization of rare earth elements in mine drainage and recovery technologies. Due to the importance of rare earth elements and the growing interest in their recovery from mine drainage, rare earth elements are reported as a separate section for the first time in this review. PRACTITIONER POINTS: Advances in treatment technologies for mine drainage are reviewed. Rare earth elements in mine drainage and its recovery are summarized. Reviewed technologies include passive, active, advanced and novel processes.

Insights into biofilm carriers for biological wastewater treatment processes: Current state-of-the-art, challenges, and opportunities

Biofilm carriers play an important role in attached growth systems for wastewater treatment processes. This study systematically summarizes the traditional and novel biofilm carriers utilized in biofilm-based wastewater treatment technology. The advantages and disadvantages of traditional biofilm carriers are evaluated and discussed in light of basic property, biocompatibility and applicability. The characteristics, applications performance, and mechanism of novel carriers (including slow-release carriers, hydrophilic/electrophilic modified carriers, magnetic carriers and redox mediator carriers) in wastewater biological treatment were deeply analyzed. Slow release biofilm carriers are used to provide a solid substrate and electron donor for the growth of microorganisms and denitrification for anoxic and/or anaerobic bioreactors. Carriers with hydrophilic/electrophilic modified surface are applied for promoting biofilm formation. Magnetic materials-based carriers are employed to shorten the start-up time of bioreactor. Biofilm carriers acting as redox mediators are used to accelerate biotransformation of recalcitrant pollutants in industrial wastewater.

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.

Effect of ammonium, electron donor and sulphate transient feeding conditions on sulphidogenesis in sequencing batch bioreactors

This work aimed to study the effect of transient feeding conditions on sulphidogenesis in 8 sequencing batch bioreactors (SBR). SBR L1 and H1, operated under steady-state conditions were used as the control reactors, while four SBR were tested under transient feeding conditions using moderate (L2 and L3, feast and famine: 2.5 and 0 g SO₄^2-·L^-1) and high (H2 and H3, feast and famine: 15 and 0 g SO₄^2-·L^-1) loads. The sulphate removal efficiency (RE) was ≥90% in SBR L2, L3 and H1. The NH₄⁺ famine conditions resulted in a higher sulphate RE (≥40% H3) compared to feast conditions (≤20% H2). Besides, the sulphidogenic first-order kinetic constant was 4% larger and the use of electron donor was 16.6% more efficient under NH₄⁺ famine conditions. Sulphidogenesis is robust to transient feeding conditions, but not when applying high loading rates (SBR H2 and H3).