Low-temperature Anaerobic Digestion Research Articles

Full-scale study on high-rate low-temperature anaerobic digestion of agro-food wastewater: process performances and microbial community

ABSTRACT The fast-growing global population has led to a substantial increase in food production, which generates large volumes of wastewater during the process. Despite most industrial wastewater being discharged at lower ambient temperatures (<20 °C), majority of the high-rate anaerobic reactors are operated at mesophilic temperatures (>30 °C). High-rate low-temperature anaerobic digestion (LtAD) has proven successful in treating industrial wastewater both at laboratory and pilot scales, boasting efficient organic removal and biogas production. In this study, we demonstrated the feasibility of two full-scale high-rate LtAD bioreactors treating meat processing and dairy wastewater, and the microbial communities in both reactors were examined. Both reactors exhibited rapid start-up, achieving considerable chemical oxygen demand (COD) removal efficiencies (total COD removal >80%) and generating high-quality biogas (CH4% in biogas >75%). Long-term operations (6–12 months) underscored the robustness of LtAD bioreactors even during winter periods (average temperature <12 °C), as evidenced by sustained high COD removal rates (total COD removal >80%). The stable performance was underpinned by a resilient microbial community comprising active acetoclastic methanogens, hydrolytic, and fermentative bacteria. These findings underscore the feasibility of high-rate low-temperature anaerobic wastewater treatment, offering promising solutions to the zero-emission wastewater treatment challenge.

A robust, low-temperature, closed-loop anaerobic system for high-solid mixed farm wastes: advancing agricultural waste management solutions in Canada.

This study investigates the effectiveness of low-temperature (20 ± 1 °C) anaerobic digestion (AD) for two organic multiple farm substrate combinations: Set 1 comprising chicken manure (CM), dairy manure (DM), and waste corn silage (CS) and Set 2 comprising CM, DM, pig manure (PM), and CS. Inoculum adaptation steps were carried out using CM and CM+DM for Set 1 and Set 2, respectively. Over three consecutive operating cycles spanning 245 days with increasing organic loads, 4.3 and 2.8 g VS L-1 d-1 for Sets 1 and 2 during Cycles 1 to 5.1 and 4.6 g VS L-1 d-1for Sets 1 and 2 during Cycle 3, a closed-loop two-stage liquid-solid AD system was employed, with performance assessed via stability ratios of short-chain volatile fatty acids and alkalinity. Results demonstrate that mono-digestion of CM with adapted inoculum yielded the highest biogas production of 424 ± 4 L over 77 days, indicating superior performance by Set 1 during Phase I, whereas a similar performance was observed during Phase 2, where Sets 1 and 2 exhibited highest specific methane yields of 0.233 ± 0.028 and 0.262 ± 0.004 L g-1 VSfed, respectively, over 68 days. Analysis of heavy metal concentrations in digestates revealed a significant decrease compared to initial raw substrate concentrations, highlighting their role as nutrients for microbial growth. This study, the first of its kind, highlights the potential of low-temperature AD systems to manage diverse organic residues/byproducts and offers insights into effective performance monitoring without compromising system integrity.

Effect of bioaugmentation on gas production and microbial community during anaerobic digestion in a low-temperature fixed-bed reactor.

Low temperature is one of the limiting factors for anaerobic digestion in cold regions. To improve the efficiency of anaerobic digestion for methane production in stationary reactors under low-temperature conditions, and to improve the structure of the microbial community for anaerobic digestion at low temperatures. We investigated the effects of different concentrations of exogenous Methanomicrobium (10, 20, 30%) and different volumes of carbon fiber carriers (0, 10, 20%) on gas production and microbial communities to improve the performance of low-temperature anaerobic digestion systems. The results show that the addition of 30% exogenous microorganisms and a 10% volume of carbon fiber carrier led to the highest daily (128.15 mL/g VS) and cumulative (576.62 mL/g VS) methane production. This treatment effectively reduced the concentrations of COD and organic acid, in addition to stabilizing the pH of the system. High-throughput sequencing analysis revealed that the dominant bacteria under these conditions were Acidobacteria and Firmicutes and the dominant archaea were Candidatus_Udaeobacter and Methanobacterium. While the abundance of microorganisms that metabolize organic acids was reduced, the functional abundance of hydrogenophilic methanogenic microorganisms was increased. Therefore, the synergistic effect of Methanomicrobium bioaugmentation with carbon fiber carriers can significantly improve the performance and efficiency of low-temperature anaerobic fermentation systems.

Molecular insights informing factors affecting low temperature anaerobic applications: Diversity, collated core microbiomes and complexity stability relationships in LCFA-fed systems

Fats, oil and grease, and their hydrolyzed counterparts-long chain fatty acids (LCFA) make up a large fraction of numerous wastewaters and are challenging to degrade anaerobically, more so, in low temperature anaerobic digestion (LtAD) systems. Herein, we perform a comparative analysis of publicly available Illumina 16S rRNA datasets generated from LCFA-degrading anaerobic microbiomes at low temperatures (10 and 20 °C) to comprehend the factors affecting microbial community dynamics. The various factors considered were the inoculum, substrate and operational characteristics, the reactor operation mode and reactor configuration, and the type of nucleic acid sequenced. We found that LCFA-degrading anaerobic microbiomes were differentiated primarily by inoculum characteristics (inoculum source and morphology) in comparison to the other factors tested. Inoculum characteristics prominently shaped the species richness, species evenness and beta-diversity patterns in the microbiomes even after long term operation of continuous reactors up to 150 days, implying the choice of inoculum needs careful consideration. The generalised additive models represented through beta diversity contour plots revealed that psychrophilic bacteria RBG-13-54-9 from family Anaerolineae, and taxa WCHB1–41 and Williamwhitmania were highly abundant in LCFA-fed microbial niches, suggesting their role in anaerobic treatment of LCFAs at low temperatures of 10–20 °C. Overall, we showed that the following bacterial genera: uncultured Propionibacteriaceae, Longilinea, Christensenellaceae R7 group, Lactivibrio, candidatus Caldatribacterium, Aminicenantales, Syntrophus, Syntrophomonas, Smithella, RBG-13-54-9, WCHB1–41, Trichococcus, Proteiniclasticum, SBR1031, Lutibacter and Lentimicrobium have prominent roles in LtAD of LCFA-rich wastewaters at 10–20 °C. This study provides molecular insights of anaerobic LCFA degradation under low temperatures from collated datasets and will aid in improving LtAD systems for treating LCFA-rich wastewaters.

Effects of environmental factors on low temperature anaerobic digestion of pig manure

In this study, we investigated the inhibitory effect of low temperature on biogas fermentation system. Biogas fermentation inoculum was domesticated at 4 °C, and subsequently inoculated into pig manure feedstock in a batch biogas fermentation process. A low-temperature biogas fermentation system was maintained at 9 °C, and its abiotic factors, bacterial community, and archaeal community were determined. The results showed that (1) the biogas fermentation lasted for 160 days, with a total gas production of 19,150 ml, including N2 production of 11,370 ml, CO2 production of 3,534 ml, and CH4 production of 3,031 ml. (2) The average relative abundance of the primary dominant bacterium operational taxonomic unit (OTU) was 30%, with 100% similarity to Pseudomonas caeni , atypical denitrifying bacterium. The average relative abundance of the secondary dominant bacterium OTU was 20.36%, with 99% similarity to Clostridium cellulovorans, a typical cellulose- and hemicellulose-degrading bacterium. The average relative abundance of the primary dominant methanogenic archaeon OTU was 7.22%, with 99% similarity to Methanosphaera cuniculi, a typical methylotrophic methanogen, and the secondary dominant methanogenic archaeon OTU had an average relative abundance of 4.15%, with 98% similarity to Methanocorpusculum sinense, a typical hydrogenotrophic methanogen. (3) The primary dominant bacterial genus Pseudomonas exhibited highly significant negative correlations with the dominant biogas-producing microorganisms such as Clostridium, Terrisporobacter, Turicibacter, Methanosphaera, and Methanocorpusculum. We concluded that (1) high concentrations of abiotic factors such as ammonia nitrogen, acetic acid, and propionic acid were important indicators of relatively poor operation of low-temperature biogas fermentation systems, while (2) high numbers of MCG archaea and Pseudomonas were important biotic factors showing comparatively poor operation of the system.

Enrichment culture combined with microbial electrochemical enhanced low-temperature anaerobic digestion of cow dung

Enrichment culture combined with the microbial electrochemical system was used to co-enhance the low-temperature (20 °C) anaerobic digestion. The results showed that enrichment culture combined with microbial electrochemical system increased the cumulative methane production in low-temperature anaerobic digestion system by 39.64 % and 133.29 % compared to single and no enrichment culture, respectively. Enrichment culture combined with microbial electrochemical system increased the relative abundance of methanogenic archaea (Methanomassiliicoccus, Methanocorpusculum, unclassified Methanomicrobiaceae, Methanobacterium, Methanoculleus, Methanocalculus) and the relative abundance of cold-tolerant hydrolytic acidifying bacteria (unclassified Bacteroidetes, Treponema). The expressions of specific enzyme genes in the methanogenesis pathway were enhanced, including acetyl-CoA synthetase, formylmethanofuran dehydrogenase, methanol cobalamin methyltransferase, etc. These results indicated that enrichment culture combined with microbial electrochemical system enhanced low-temperature anaerobic digestion methanogenesis by altering microbial communities and stimulating enzyme gene expression to affect volatile fatty acids, pH, redox potential, and reducing sugar parameters.

Bioelectrochemically assisting anaerobic digestion enhanced methane production under low-temperature

Bioelectrochemical systems (BES) with carbon brush electrode were developed to improve methanogenic performance of low-temperature (15 °C) anaerobic digestion (AD) in this study. It turned out that the BES system (1.0 V) could significantly improve the CH4 yield, and during the reaction process, propionic acid (∼618.73 mg/L) had a serious accumulation. Metatranscriptomic analysis revealed that methane was mainly produced by acetoclastic methanogenic pathway, and application of voltage could promote its improvement. On carbon brush electrodes, the enhancement of the acetoclastic pathway mainly depended on acetate kinase (Pta) and phosphate acetyltransferase (Ack) to convert acetate to acetyl-CoA. While it was mainly regulated by the acetate-CoA ligase (Acs) in fluid activated sludge. The activity of methanogenesis-related enzymes on the cathode was generally higher than that on the anode, while the enzyme activity on the fluid sludge anode was mostly inhibited. In addition, BES system significantly increased the abundance of methanogens and electrogens, and enhanced the activity of electron transfer related enzymes, which greatly promoted the direct interspecies electron transfer (DIET) of species. Therefore, the BES system could be an attractive alternative strategy to improve the performance of AD when treating low-temperature wastewater.

Low-temperature anaerobic digestion of chicken manure at high organic and nitrogen loads - strategies for controlling short chain fatty acids

Objective of this work was to investigate the technical feasibility of low-temperature, closed-loop two-stage (liquid–solid) anaerobic digesters to treat chicken-manure (TS:68%; NH3:8 g/L) as a sole-feedstock. Effect of pH, temperature, treatment-duration, organic loading rate (OLR) and inoculum-recirculation ratio on short chain fatty acids (SCFA) production was studied. Digesters were operated at 20 ± 1 °C for 282-d over 4 batch-runs (∼70-d/batch) at an OLR of 8.78–4.3 gVS/L/d. Results showed that specific methane yield above 0.6 LCH4/gVS was feasible with a methane concentration > 60%. SCFA speciation of the entire system was monitored through the liquid-digester. Among SCFA indicators, the ratios of propionic-to-acetic acids, (butyric + valeric)-to-acetic acids, and total SCFA-to-alkalinity were observed within the limit, i.e., below 1.4, 0.3 and 0.8, respectively, indicating high-digester stability. This strategy allowed early detection, diagnosis of process failures in high-solids digester in fed-batch mode, and re-evaluation of operating protocol to enrich performance with economic-benefits.

Hydrogen production from biomass through integration of anaerobic digestion and biogas dry reforming

Hydrogen is a clean fuel for heat and power generation and can serve as a feedstock for chemical synthesis. This study assesses the hydrogen production from biomass (e.g., cow manure) through integrating psychrophilic anaerobic digestion and dry methane reforming. For the first time, a rigorous model is developed for the low-temperature anaerobic digestion process by implementing the complex kinetics of the fermentation bioreactions. The produced biogas from the anaerobic digestion process is fed to the reforming process for hydrogen production. The kinetics of the dry methane reforming and water gas shift reactions over Co-Ni-Al2O3 catalyst are incorporated into the model. Validating the results of the proposed process using experimental data shows <5% relative deviation. The effects of total solids content, organic loading rate, hydraulic retention time, and digestate recirculation fraction on biogas and CH4 yield are investigated. The optimum values of operating parameters in the anaerobic digestion process as well as the dry methane reforming process are obtained. The process is designed to achieve the highest CH4-to-H2 conversion and the lowest energy consumption. A 48.07 kg/h biogas could produce 8.11 kg/h H2. The biomass-to-H2 process offers an energetic efficiency of 72.85%, revealing its superiority to similar processes, such as steam and auto-thermal reforming. The proposed process highlights a high potential for CO2 emission reduction (e.g., 398,736 t/y), compared to the direct biogas combustion for electricity production. Economic analysis shows that the cost of biogas-to-H2 production is 1.39 USD/kg H2 for a plant capacity of 45.5 kg/h H2.

Determining the Microbial Source of Methane Production in Anaerobic Digestion Systems Through High-Throughput Sequencing Technology

Anaerobic digestion is widely used to simultaneously generate biogas while treating different organic wastes. It is difficult to determine the source of CH4 from the complex microbial community structure using traditional microbiological pure culture techniques. Therefore, this study aimed to elucidate the microbial source of CH4 in low-temperature anaerobic digestion systems using the recently developed high-throughput sequencing technology. Herein, anaerobic digestion microbes were domesticated at 15°C and then inoculated into pig manure-containing raw materials in a batch anaerobic digester to form a low-temperature anaerobic digestion system with fermentation controlled at 15°C. Several analytical approaches including abiotic factor analysis and biotic factor analysis (high-throughput sequencing) were applied to investigate the abiotic factors, bacterial communities, and archaeal communities in the low-temperature anaerobic digestion system. The results showed that: 1) The anaerobic digestion lasted for 120 days, with 68.65 L total gas production and 31.19 L CH4 production. 2) The relative abundances of the primary and secondary dominant bacterial operational taxonomic units ranged from 8.02 to 22.84% and 5.62–17.09%, respectively, with 99% similarities to Clostridium cellulovorans (a typical cellulose- and hemicellulose-degrading bacterium) and Terrisporobacter petrolearius (a representative fermentation bacterium), respectively. Moreover, the relative abundance of the primary dominant methanogenic archaeal operational taxonomic unit ranged from 1.03 to 16.85%, with 98% similarity to Methanobacterium beijingense, which is a typical hydrogenotropic methanogen. Based on the low-temperature CH4-producing metabolic pathways of bacterial and methanogenic operational taxonomic units, Methanobacterium beijingense was found to be the primary functional microbe for CH4 production in the 15°C anaerobic digestion system.

The changes of microbial abundance and functional genes in bioelectrochemistry at 15 ℃

Bioelectrochemistry system (BES) provides a new strategy for the improvement of low temperature anaerobic digestion (AD) for methane production. In this study, the BES with carbon brushes as electrodes was operated at 15 °C to analyze the changes of microbial abundance and functional genes. The microbial community structure showed that the application of voltage had no effect to type and relative abundance of bacteria based on 16S-rRNA sequencing. Metatranscriptomic analyses revealed that significantly enhanced the abundance of fermentation bacteria in the fluid activated sludge, and the relative abundance of biosynthesis of other secondary metabolites (61.56%) was also highly expressed. More than that, the abundance of electricigens (Pseudomonas, Syntrophomonas) was also enriched in the BES system. These electricigens could interact with methanogens to promote electron transfer by direct interspecies electron transfer (DIET). The expression level of functional enzyme related to electron transport was increased at the anode (30.60%), but decreased at the cathode (−33.84%). Analysis of methanogens found that the application of voltage decreased the relative abundance of Methanosarcina and promoted the enrichment of Methanothrix. At 15 ℃, acetoclastic was the mainly methanogenic pathway. What’s more, the application of voltage could enhance the acetoclastic and inhibited the hydrogenotrophic and the methylotrophic pathway. Further, the acetoclastic pathway was changed from acetate kinase (EC: 2.7.2.1) and phosphate acetyltransferase (EC: 2.3.1.8) to acetate-CoA ligase (EC: 6.2.1.1) by applied voltage. These results are conducive to the further development of low-temperature AD treatment technology.

Processing High-Solid and High-Ammonia Rich Manures in a Two-Stage (Liquid-Solid) Low-Temperature Anaerobic Digestion Process: Start-Up and Operating Strategies.

Globally, livestock and poultry production leads to total emissions of 7.1 Gigatonnes of CO2-equiv per year, representing 14.5% of all anthropogenic greenhouse gas emissions. Anaerobic digestion (AD) is one of the sustainable approaches to generate methane (CH4) from manure, but the risk of ammonia inhibition in high-solids AD can limit the process. Our objective was to develop a two-stage (liquid–solid) AD biotechnology, treating chicken (CM) + dairy cow (DM) manure mixtures at 20 °C using adapted liquid inoculum that could make livestock farming more sustainable. The effect of organic loading rates (OLR), cycle length, and the mode of operation (particularly liquid inoculum recirculation-percolation mode) was evaluated in a two-stage closed-loop system. After the inoculum adaptation phase, aforementioned two-stage batch-mode AD operation was conducted for the co-digestion of CM + DM (Total Solids (TS): 48–51% and Total Kjeldahl Nitrogen (TKN): 13.5 g/L) at an OLR of 3.7–4.7 gVS/L.d. Two cycles of different cycle lengths (112-d and 78-d for cycles 1 and 2, respectively) were operated with a CM:DM mix ratio of 1:1 (w/w) based on a fresh weight basis. Specific methane yield (SMY) of 0.35 ± 0.11 L CH4g/VSfed was obtained with a CH4 concentration of above 60% for both the cycles and Soluble Chemical Oxygen Demand (CODs) and volatile solid (VS) reductions up to 85% and 60%, respectively. For a comparison purpose, a similar batch-mode operation was conducted for mono-digestion of CM (TS: 65–73% and TKN: 21–23 g/L), which resulted in a SMY of 0.52 ± 0.13 L CH4g/VSfed. In terms of efficiency towards methane-rich biogas production and ammonia inhibitions, CM + DM co-digestion showed comparatively better quality methane and generated lower free ammonia than CM mono-digestion. Further study is underway to optimize the operating parameters for the co-digestion process and to overcome inhibitions and high energy demand, especially for cold countries.

Enhancing anaerobic digestion for rural wastewater treatment with granular activated carbon (GAC) supplementation

Notwithstanding many efforts to increase the efficiency of anaerobic digestion at low-temperature (winter) conditions, a cost-effective and efficient method is lacking. This study proposes a low-cost method of low-temperature (<35 °C) anaerobic digestion of wastewater, involving supplementation with granular activated carbon (GAC). Supplementation with GAC was found to reduce the lag time by 29.8% (from 15.1 to 10.6 days) and increase the maximum methane production rate by 23.4% (from 6.4 to 7.9 mL/day) at 25 °C. Network analysis demonstrated a strong co-occurrence of Syntrophobacteriales and hydrogenotrophic methanogens (Methanobacteriaceae; WSA2; Methanoregulaceae). GAC supplementation can drastically reduce the time required for organic matter decomposition and methane production, thereby increase the efficiency of wastewater treatment.

Reactor configuration influences microbial community structure during high-rate, low-temperature anaerobic treatment of dairy wastewater

Low temperature anaerobic digestion remains in its infancy, despite increasing interest for the treatment of complex wastewaters. In this study, the feasibility of low-temperature anaerobic treatment of dairy wastewater was assessed during a 443-day laboratory-scale bioreactor trial. The bioreactors were operated in triplicate at organic loading rates of 7.5–9 kgCODm−3d−1 throughout five operational phases. The structure of the microbial community was analysed using quantitative real-time PCR and amplicon sequencing of 16S rRNA genes from DNA and rRNA. The results indicated that low-temperature treatment of dairy wastewater is feasible at 15 °C, but that reactor configuration remains extremely important. The upflow anaerobic sludge bed (UASB) configuration out-performed the expanded granular sludge bed (EGSB)-based configurations. Decreased temperatures resulted in significant reductions in microbiome diversity. Methanosaeta was identified as a dominant genus throughout the trial, while Lactococcus was identified as an important bacterial genus at low-temperatures. However, the relative abundance of Lactococcus was significantly influenced by reactor configuration.

Investigation of pre-treatments improving low-temperature anaerobic digestion of waste activated sludge

This work analyzed the feasibility of pre-treatments to improve the anaerobic digestion (AD) of waste activated sludge (WAS) at 20°C. We investigated different physicochemical pre-treatments (thermal at 115°C, thermo-alkaline at pH 10 and 70°C and ozonation at 190mg-O3L−1) by comparing their performances about COD solubilization and sludge disintegration rate. Best performances were obtained by thermo-alkaline pre-treatment, followed by thermal and ozonation; results were consistent with literature. Pre-treated WAS was fed to 12 1-L anaerobic digesters operated in semi-continuous mode. Thermal and thermo-alkaline reactors produced biogas yields (0.30–0.36m3kg−1 VS in standard conditions, 65–70 % methane) analogous to mesophilic conditions. The economic assessment of the scale-up of the whole process demonstrated that thermo-alkaline pre-treatment made AD at 20°C economically profitable for WAS generated by a 20,000 PE WWTP.

Low-temperature anaerobic digestion enhanced by bioelectrochemical systems equipped with graphene/PPy- and MnO2 nanoparticles/PPy-modified electrodes

Bioelectrochemical systems (BESs) with graphene (Gr)/polypyrrole (PPy)- and MnO2 nanoparticles (NPs)/PPy-modified electrodes were developed to enhance low-temperature anaerobic digestion (LTAD) of low-strength wastewater. At 20 °C, the chemical oxygen demand removal efficiencies and CH4 yield of the BESs with Gr/PPy (R2) and MnO2 NPs/PPy (R3)-modified electrodes were 12.7% and 25.6%, and 43.9% and 66.3%, respectively, higher than those of the control (R1, without modification). Although the performance of all reactors decreased as temperature dropping to 12 °C, the CH4 yield rates of R2 and R3 were still 22.8% and 39.0% higher than that of R1. Further analysis indicated that the modified electrodes might stimulate the metabolic activity of the anaerobic digester sludge. Scanning electron microscopy observation showed that the modified electrodes had higher specific surface area, favoring the attachment and formation of dense biofilms on the surface of electrodes. 16S rRNA gene-sequencing results demonstrated that H2-consuming methanogens dominated in the BESs and the influence of Gr/PPy and MnO2 NPs/PPy differed on the microbial community structure of biofilms. These findings justify the wider use of Gr and MnO2 NPs in electrode modification to assist LTAD using BESs.

Cold adaptation and replicable microbial community development during long-term low-temperature anaerobic digestion treatment of synthetic sewage.

ABSTRACTThe development and activity of a cold-adapting microbial community was monitored during low-temperature anaerobic digestion (LtAD) treatment of wastewater. Two replicate hybrid anaerobic sludge bed-fixed-film reactors treated a synthetic sewage wastewater at 12°C, at organic loading rates of 0.25–1.0 kg chemical oxygen demand (COD) m−3 d−1, over 889 days. The inoculum was obtained from a full-scale anaerobic digestion reactor, which was operated at 37°C. Both LtAD reactors readily degraded the influent with COD removal efficiencies regularly exceeding 78% for both the total and soluble COD fractions. The biomass from both reactors was sampled temporally and tested for activity against hydrolytic and methanogenic substrates at 12°C and 37°C. Data indicated that significantly enhanced low-temperature hydrolytic and methanogenic activity developed in both systems. For example, the hydrolysis rate constant (k) at 12°C had increased 20–30-fold by comparison to the inoculum by day 500. Substrate affinity also increased for hydrolytic substrates at low temperature. Next generation sequencing demonstrated that a shift in a community structure occurred over the trial, involving a 1-log-fold change in 25 SEQS (OTU-free approach) from the inoculum. Microbial community structure changes and process performance were replicable in the LtAD reactors.

Bio-electrochemically assisting low-temperature anaerobic digestion of low-organic strength wastewater

This study provides a new approach by using bio-electrochemical systems (BES) to improve the performance of anaerobic digestion (AD) for treating low-organic strength wastewater and recovering energy in the form of CH4. The BES had a relatively higher capability for temperature shock. At an applied voltage of 0.4 V, the BES significantly enhanced both the chemical oxygen demand (COD) removal and CH4 yield efficiency compared with those of the control at operational temperatures of 20, 12, and 8 °C. Energy balance evaluation indicated that the net energy obtained from the BES was higher than the energy recovered from the control, suggesting that operation of BES was economically available. Analysis of microbial physiological characteristics elucidated that electrochemical effects stimulated the production of extracellular polymeric substances, suggesting a faster metabolic activity of biomass in the BES. 16S rRNA sequencing showed that genera of H2-utilizing methanogens, Methanobacterium, Methanoregula, and Methanospirillum, dominated the consortia at low temperatures. On the surfaces of cathodes, considerable enrichments of H2-producing bacteria, Acetobacteroides and Anaerolinea were observed, implying the formation of H2-mediated syntrophic communities in the BES. The combined results suggest that operation of BES for low-temperature AD of low-organic strength wastewater will be an attractively potential technology.

Bioelectrochemical enhancement of methane production in low temperature anaerobic digestion at 10 °C

Anaerobic digestion at low temperature is an attractive technology especially in moderate climates, however, low temperature results in low microbial activity and low rates of methane formation. This study investigated if bioelectrochemical systems (BESs) can enhance methane production from organic matter in low-temperature anaerobic digestion (AD). A bioelectrochemical reactor was operated with granular activated carbon as electrodes at 10 °C. Our results showed that bioelectrochemical systems can enhance CH4 yield, accelerate CH4 production rate and increase acetate removal efficiency at 10 °C. The highest CH4 yield of 31 mg CH4-COD/g VSS was achieved in the combined BES-AD system at a cathode potential of −0.90 V (Ag/AgCl), which was 5.3–6.6 times higher than that in the AD reactor at 10 °C. CH4 production rate achieved in the combined BES-AD system at 10 °C was only slightly lower than that in the AD reactor at 30 °C. The presence of an external circuit between the acetate-oxidizing bioanode and methane-producing cathode provided an alternative pathway from acetate via electrons to methane, potentially via hydrogen. This alternative pathway seems to result in higher CH4 production rates at low temperature compared with traditional methanogenesis from acetate. Integration of BES with AD could therefore be an attractive alternative strategy to enhance the performance of anaerobic digestion in cold areas.

Low-temperature anaerobic digestion is associated with differential methanogenic protein expression.

Anaerobic digestion (AD) is an attractive wastewater treatment technology, leading to the generation of recoverable biofuel (methane). Most industrial AD applications, carry excessive heating costs, however, as AD reactors are commonly operated at mesophilic temperatures while handling waste streams discharged at ambient or cold temperatures. Consequently, low-temperature AD represents a cost-effective strategy for wastewater treatment. The comparative investigation of key microbial groups underpinning laboratory-scale AD bioreactors operated at 37, 15 and 7°C was carried out. Community structure was monitored using 16S rRNA clone libraries, while abundance of the most prominent methanogens was investigated using qPCR. In addition, metaproteomics was employed to access the microbial functions carried out in situ. While δ-Proteobacteria were prevalent at 37°C, their abundance decreased dramatically at lower temperatures with inverse trends observed for Bacteroidetes and Firmicutes. Methanobacteriales and Methanosaeta were predominant at all temperatures investigated while Methanomicrobiales abundance increased at 15°C compared to 37 and 7°C. Changes in operating temperature resulted in the differential expression of proteins involved in methanogenesis, which was found to occur in all bioreactors, as corroborated by bioreactors' performance. This study demonstrated the value of employing a polyphasic approach to address microbial community dynamics and highlighted the functional redundancy of AD microbiomes.