Optimization of sulfate reduction and methanogenesis via phase separation in a two-phase internal circulation reactor for the treatment of high-sulfate organic wastewater

To enhance the performance of the internal circulation (IC) reactor when treating high-sulfate organic wastewater, a laboratory-scale two-phase IC reactor with distinct phase separation capabilities was designed, and the sulfate reduction and methanogenesis processes were optimized by segregating the reactor into two specialized reaction zones. The results demonstrated that the first and second reaction areas of the two-phase IC reactor could be maintained at 4.5-6.0 and 7.5-8.5, respectively, turning them into the specialized phase for sulfate reduction and methanogenesis. Through phase separation, the two-phase IC reactor achieved a COD degradation and sulfate reduction efficiency of more than 80% when the influent sulfate concentration exceeded 5,000 mg/L, which were 32.32% and 16.04% higher than that before phase separation. Functional analyses indicated a greater activity of both the dissimilatory and assimilatory sulfate reduction pathways in the acidogenic phase, largely due to a rise in the relative abundance of the genera Desulfovibrio, Bacteroides, and Lacticaseibacillus, the primary carriers of sulfate reduction functional genes. In contrast, all the acetoclastic, hydrogenotrophic, and methylotrophic methanogenesis pathways were inhibited in the acidogenic phase but thrived in the methanogenic phase, coinciding with shifts in the genus Methanothrix, which harbors the mcrA, mcrB, and mcrG genes essential for the final transformation step of all three methanogenesis pathways.

Insights into the effects of operating parameters on sulfate reduction performance and microbial pathways in the anaerobic sequencing batch reactor

Sulfate-reducing bacteria (SRB)-based anaerobic process has aroused wide concern in the treatment of sulfate-containing wastewater. Chemical oxygen demand-to-sulfate ratio (COD/SO42-) and HRT are two key factors that affect not only the anaerobic treatment performance but also the activity of SRB. In this study, an anaerobic sequencing batch reactor was constructed, and the effects of different operating parameters (COD/SO42-, HRT) on the relationship of sulfate (SO42-) reduction performance, microbial communities, and metabolic pathways were comprehensively investigated. The results indicated that the SO42- removal rates could achieve above 95% under different operating parameters. Bioinformatics analysis revealed that microbial community changed with reactor operation. At the genus level, the enrichment of Propionicclava and Peptoclostridium contributed to the establishment of a homotrophic relationship with Desulfobulbus, the dominant SRB in the reactor, which indicated that they took vital part in maintaining the structural and functional stability of the bacterial community under different operating parameters. In particular, an increasing trend of the relative abundance of functional genes encoding dissimilatory sulfate reduction was detected with the increase of COD/SO42-, which indicated high SO42- reduction potentials. This knowledge will help to reveal the mechanism of the effect of operating parameters on the anaerobic sulfate removal process, thus providing effective guidance for the targeted regulation of anaerobic sequencing batch bioreactors treating SO42--containing wastewater.

Biogas production by anaerobic co-digestion of sugarcane biorefinery byproducts: Comparative analyses of performance and microbial community in novel single-and two-stage systems

Anaerobic co-digestion (AcD) of sugarcane biorefinery byproducts (hemicelluloses hydrolysate (HH), vinasse, yeast extract and sugarcane bagasse fly ashes was evaluated using new anaerobic reactors fed with organic loading rates (OLR) from 0.9 to 10.8 gCODL^-1d^-1. The best results were obtained in a two-stage system when the OLR was 5.65 gCODL^-1d^-1, leading to a total chemical oxygen demand (COD) removal of 87.6 % and methane yield of 243NmLCH₄gCODr^-1. Microbial community analyses of sludge from both systems (one and two-stages) revealed structural changes and relationship among the main genus found (Clostridium (62.8%), Bacteroides(11.3 %), Desulfovibrio (19.1 %), Lactobacillus(67.7 %), Lactococcus (22.5%), Longilinea (78%), Methanosaeta (19.2 %) and Syntrophus (18.9 %)) with processes performance, kinetic and hydrodynamic parameters. Moreover, biomass granulation was observed in the novel structured anaerobic reactor operated at single stage due to sugarcane bagasse fly ash addition.

Phase Separation in Anaerobic Digestion: A Potential for Easier Process Combination?

The flexibilization of bioenergy production has the potential to counteract partly other fluctuating renewable energy sources (such as wind and solar power). As a weather-independent energy source, anaerobic digestion (AD) can offer on-demand energy supply through biogas production. Separation of the stages in anaerobic digestion represents a promising strategy for the flexibilization of the fermentative part of biogas production. Segregation in two reactor systems facilitates monitoring and control of the provision of educts to the second methanogenic stage, thus controlling biogas production. Two-stage operation has proven to reach similar or even higher methane yields and biogas purities than single-stage operation in many different fields of application. It furthermore allows methanation of green hydrogen and an easier combination of material and energy use of many biogenic raw and residual biomass sources. A lot of research has been conducted in recent years regarding the process phase separation in multi-stage AD operation, which includes more than two stages. Reliable monitoring tools, coupled with effluent recirculation, bioaugmentation and simulation have the potential to overcome the current drawbacks of a sophisticated and unstable operation. This review aims to summarize recent developments, new perspectives for coupling processes for energy and material use and a system integration of AD for power-to-gas applications. Thereby, cell physiological and engineering aspects as well as the basic economic feasibility are discussed. As conclusion, monitoring and control concepts as well as suitable separation technologies and finally the data basis for techno-economic and ecologic assessments have to be improved.

Stimulation of Biomethane Productivity in Anaerobic Digestion Using Electro-Conductive Carbon-Nanotube Hollow-Fiber Media

The production of biogas was promoted via direct interspecies electron transfer (DIET) by employing electro-conductive carbon-nanotube hollow-fiber media (CHM) in anaerobic digestion. Experimental results showed a positive effect of CHM presence on CH4 productivity with 34% higher CH4 production rate than that of in the presence of non-electroconductive polymeric hollow fiber media. An increased CH4 production rate was due to the shift in the microbiome with more abundant Pelobacter (10.0%), Geobacter (6.9%), and Methanosaeta (15.7%), which play key roles in promoting CH4 production via syntrophic metabolism associated with DIET. Microscopic morphology analysis, using confocal laser scanning microscopy and scanning electron microscopy, exhibited that several living cells were attached with electro-conductive pili on the CHM surface, thereby facilitated electron transport between microbial cells.

Sulfate reducing bacteria-based wastewater treatment system integrated with sulfide fuel cell for simultaneous wastewater treatment and electricity generation

This study aimed to design a sulfate-reducing bacteria (SRB)-based wastewater treatment system (SWTS) integrated with a sulfide fuel cell (SFC) as an alternative to the energy-intensive aerobic wastewater treatment process. The result showed that the COD/sulfate ratio and hydraulic retention time (HRT) were two important parameters in a SWTS. The highest COD and sulfate removal efficiency rates were at a HRT of 4 h at a COD/sulfate ratio of 0.67, reaching 83 ± 0.2% and 84 ± 0.4% with sulfate removal rates of 4.087 ± 32 mg SO₄^2-/d, respectively. A microbial analysis revealed that the dominance of nine OTUs belonging to SRB closely affected the high sulfate removal efficiency in the SWTS. At the HRT of 8 h, voltage of 0.02 V and a power density level of 130 mW/m² were obtained with sulfide removal efficiency of 99 ± 0.5%. These results overall demonstrate that SRB can serve as a green and effective route for wastewater treatment.

Optimizing operation of municipal wastewater treatment plants in China: The remaining barriers and future implications

China's national development strategy now prioritizes environmental protection over economic growth, which has driven a rapid development of China's wastewater sector. In particular, the treatment capacity of municipal wastewater treatment plants (WWTPs) has been substantially strengthened and stricter effluent quality control enforced. However, the operating performance of most WWTPs is still poor and does not meet the sustainable development demands. In this study, the current status of WWTPs operation in China was comprehensively analyzed, the key barriers to improving the plants operating efficiency were identified by taking into account the different plant scales, geographic distribution, discrepancy between cities and counties, and the influence of environmental policies and supplementary facilities. The underdeveloped sewer network was mainly responsible for the low operating ratios (i.e., utilization degree of the designed treatment capacity) of the plants (76% in counties and 85% in cities) especially for those in north China, although the situation is plant specific because a considerable fraction of plants (19%) are still running under overload condition. Other challenges include the high energy consumption of the plants (0.313 kWh/m³), and severely lagged implementation of sludge disposal (up to 40% sludge was still improperly disposed), arising mainly from the poor management on the sewer and sludge. Lastly, several possible directions of improvement to overcome these barriers were discussed. This work may provide valuable implications for optimizing municipal wastewater management in China towards higher efficiency and sustainability.

Increased biodegradability of low-grade coal wastewater in anaerobic membrane bioreactor by adding yeast wastes

Demineralization is required in upgrading low-grade coal to serve as an alternative energy resource for the production of fuel and valuable chemicals but generates a large amount of low-grade coal wastewater (LCWW). The objective of this study was to investigate the effects of a co-substrate on an anaerobic membrane bioreactor (AnMBR) treating LCWW. CH₄ was not produced during the operation fed by LCWW alone. When yeast wastes (YW) were supplemented, there was a gradual increase in the biodegradability of LCWW, achieving 182 CH₄ mL/g COD with 58% COD removal efficiency. The analysis of physicochemical characteristics in the effluent of AnMBR, done by excitation-emission matrix (EEM) and size exclusion chromatography (SEC), showed that the proportion of soluble microbial products (SMPs) and aromatic group with high-molecular weight (>1 kDa) increased. Microbial analysis revealed that the increased dominance of bacteria Comamonas, Methanococcus, and Methanosarcina facilitated biodegradation of LCWW in the presence of YW.

Control of microbial sulfide production by limiting sulfate dispersal in a water-injected oil field

Oil production by water injection often involves the use of makeup water to replace produced oil. Sulfate in makeup water is reduced by sulfate-reducing bacteria to sulfide, a process referred to as souring. In the MHGC field souring was caused by using makeup water with 4mM (384ppm) sulfate. Mixing with sulfate-free produced water gave injection water with 0.8mM sulfate. This was amended with nitrate to limit souring and was then distributed fieldwide. The start-up of an enhanced-oil-recovery pilot caused all sulfate-containing makeup water to be used for dissolution of polymer, which was then injected into a limited region of the field. Produced water from this pilot contained 10% of the injected sulfate concentration as sulfide, but was free of sulfate. Its use as makeup water in the main water plant of the field caused injection water sulfate to drop to zero. This in turn strongly decreased produced sulfide concentrations throughout the field and allowed a decreased injection of nitrate. The decreased injection of sulfate and nitrate caused major changes in the microbial community of produced waters. Limiting sulfate dispersal into a reservoir, which acts as a sulfate-removing biofilter, is thus a powerful method to decrease souring.

Anaerobic digestion of food waste - Challenges and opportunities

The disposal of large amounts of food waste has caused significant environmental pollution and financial costs globally. Compared with traditional disposal methods (i.e., landfilling, incineration, and composting), anaerobic digestion (AD) is a promising technology for food waste management, but has not yet been fully applied due to a few technical and social challenges. This paper summarizes the quantity, composition, and methane potential of various types of food waste. Recent research on different strategies to enhance AD of food waste, including co-digestion, addition of micronutrients, control of foaming, and process design, is discussed. It is envisaged that AD of food waste could be combined with an existing AD facility or be integrated with the production of value-added products to reduce costs and increase revenue. Further understanding of the fundamental biological and physicochemical processes in AD is required to improve the technology.

Monitoring of gas composition in a laboratory biogas plant using cavity enhanced Raman spectroscopy

Cavity-enhanced Raman spectroscopy is a powerful tool for online detection of multiple gases during the process of biogas production.