Is aceticlastic methanogen composition in full-scale anaerobic processes related to acetate utilization capacity?

Carbon-based conductive materials enhance biomethane recovery from organic wastes: A review of the impacts on anaerobic treatment

Amongst the most important sustainable waste management strategies, anaerobic biotechnology has had a central role over the past century in the management of high-pollution load sources, such as food, agricultural and municipal wastes. During anaerobic digestion (AD), valuable by-products such as digestate and biogas are produced. Biogas (mainly composed of methane) is generated through a series of reactions between bacteria and archaea. Enhancement of AD process with higher methane yield, accelerated methane production rate, and shorter start-up time is possible via tapping into a novel methanogenic pathway discovered a decade ago. This fundamentally new concept that is a substitute to interspecies hydrogen transfer is called direct interspecies electron transfer (DIET). DIET, a thermodynamically more feasible way of electron transfer, has been proven to occur between bacteria and methanogens. It is well-documented that amendment of carbon-based conductive materials (CCMs) can stimulate DIET via serving as an electrical conduit between microorganisms. Therefore, different types of CCMs such as biochar and activated carbon have been amended to a variety of AD reactors and enhancement of process performance was reported. In this review, a comparative analysis is presented for enhancement of AD performance in relation to major CCM related factors; electrical conductivity, redox properties, particle size and dosage. Additionally, the impacts of AD operational conditions such as organic loading rate and temperature on CCM amended reactors were discussed. Further, the changes in microbial communities of CCM amended reactors were reviewed and future perspectives along with challenges for CCM application in AD have been provided.

PCR-DGGE Analysis on Microbial Community Structure of Rural Household Biogas Digesters in Qinghai Plateau

To investigate contribution of environmental factor(s) to microbial community structure(s) involved in rural household biogas fermentation at Qinghai Plateau, we collected slurry samples from 15 digesters, with low-temperature working conditions (11.1-15.7 °C) and evenly distributed at three counties (Datong, Huangyuan, and Ledu) with cold plateau climate, to perform polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and further sequencing. The bacterial communities in the total 15 digesters were classified into 38 genera with Mangroviflexus (12.1%) as the first dominant, and the archaeal communities into ten genera with Methanogenium (38.5%) as the most dominant. For each county, the digesters with higher biogas production, designated as HP digesters, exclusively had 1.6-3.1 °C higher fermentation temperature and the unique bacterial structure composition related, i.e., unclassified Clostridiales for all the HP digesters and unclassified Marinilabiliaceae and Proteiniclasticum for Ledu HP digesters. Regarding archaeal structure composition, Methanogenium exhibited significantly higher abundances at all the HP digesters and Thermogymnomonas was the unique species only identified at Ledu HP digesters with higher-temperature conditions. Redundancy analysis also confirmed the most important contribution of temperature to the microbial community structures investigated. This report emphasized the correlation between temperature and specific microbial community structure(s) that would benefit biogas production of rural household digesters at Qinghai Plateau.

Detection of acidification limit in anaerobic membrane bioreactors at ambient temperature

High-volume, low-strength industrial wastewaters constitute a large potential for biogas production, which could be realized by membrane bioreactors operating at the ambient temperature of the wastewater. However, the start-up of low-temperature anaerobic processes using unadapted inoculum can be sensitive to overloading, which results in acidification. This study assessed if a novel acidification limit test can be used to identify stable organic loading rates as well as process over-loading. The test is based on easy-to-apply batch experiments for determination of the hydrolysis rate constant and the specific methanogenic activity of the acetotrophic and hydrogenotrophic pathways. For evaluation, two anaerobic membrane bioreactors, treating synthetic dairy wastewater at an ambient temperature of 24 °C, were used with a slow or a rapid start-up regime, respectively. Tests for hydrolysis rate and methanogenic activity were performed throughout the experiment and were used to calculate acidification limits for each system throughout the start-up. The acidification limit test was able to successfully identify both stable operation of one reactor and process failure of the other reactor as the organic loading rate increased. The reactor failure was caused by over-loading the acetotrophic pathway and coincided with microbial changes observed in real-time PCR and moving window analysis. Overall, the acidification limit tests seem promising as an easy applicable method for estimating what organic loading rate can be utilized, without risking acidification of anaerobic systems.

Unexpected competitiveness of Methanosaeta populations at elevated acetate concentrations in methanogenic treatment of animal wastewater

Acetoclastic methanogenesis is a key metabolic process in anaerobic digestion, a technology with broad applications in biogas production and waste treatment. Acetoclastic methanogenesis is known to be performed by two archaeal genera, Methanosaeta and Methanosarcina. The conventional model posits that Methanosaeta populations are more competitive at low acetate levels (<1 mM) than Methanosarcina and vice versa at higher acetate concentrations. While this model is supported by an extensive body of studies, reports of inconsistency have grown that Methanosaeta were observed to outnumber Methanosarcina at elevated acetate levels. In this study, monitoring of anaerobic digesters treating animal wastewater unexpectedly identified Methanosaeta as the dominant acetoclastic methanogen population at both low and high acetate levels during organic overloading. The surprising competitiveness of Methanosaeta at elevated acetate was further supported by the enrichment of Methanosaeta with high concentrations of acetate (20 mM). The dominance of Methanosaeta in the methanogen community could be reproduced in anaerobic digesters with the direct addition of acetate to above 20 mM, again supporting the competitiveness of Methanosaeta over Methanosarcina at elevated acetate levels. This study for the first time systematically demonstrated that the dominance of Methanosaeta populations in anaerobic digestion could be linked to the competitiveness of Methanosaeta at elevated acetate concentrations. Given the importance of acetoclastic methanogenesis in biological methane production, findings from this study could have major implications for developing strategies for more effective control of methanogenic treatment processes.

Integrated expanded granular sludge bed and sequential batch reactor treating beet sugar industrial wastewater and recovering bioenergy

The exponential rise in energy demand vis-à-vis depletion of mineral oil resources has accelerated recovery of bioenergy from organic waste. In this study, a laboratory-scale anaerobic (An)/aerobic (Ar) system comprising of expanded granular sludge bed (EGSB) reactor coupled to an aerobic sequential batch reactor (SBR) was constructed to treat beet sugar industrial wastewater (BSIW) of chemical oxygen demand (COD) 1665 mg L^-1 while harnessing methane gas. The EGSB reactor generated methane at the rate of 235 mL/g COD added, with considerably higher than previously reported methane content of 86 %. Meanwhile, contaminants were successfully reduced in the combined An/Ar system, realizing a removal rate of more than 71.4, 97.3, 97.7, and 99.3 % of organic matter as total phosphorus, total nitrogen, biological oxygen demand (BOD), and soluble COD, respectively. Microbial community analysis showed that the bacterial genus Clostridium sp. and archaeal genus Methanosaeta sp. dominated the EGSB reactor, while Rhodobacter sp. dominance was observed in the SBR. The obtained experimental results indicate that the integration of expanded granular sludge bed and sequential batch reactor in treating BSIW obtained competitively outstanding performance.

Metagenomic Reconstruction of Key Anaerobic Digestion Pathways in Municipal Sludge and Industrial Wastewater Biogas-Producing Systems

Anaerobic digestion (AD) is a microbial process widely used to treat organic wastes. While the microbes involved in digestion of municipal sludge are increasingly well characterized, the taxonomic and functional compositions of AD digesters treating industrial wastewater have been understudied. This study examined metagenomes from a biogas-producing digester treating municipal sludge in Shek Wu Hui (SWH), Hong Kong and an industrial wastewater digester in Guangzhou (GZ), China, and compared their taxonomic composition and reconstructed biochemical pathways. Genes encoding carbohydrate metabolism and protein metabolism functions were overrepresented in GZ, while genes encoding functions related to fatty acids, lipids and isoprenoids were overrepresented in SWH, reflecting the plants' feedstocks. Mapping of genera to functions in each community indicated that both digesters had a high level of functional redundancy, and a more even distribution of genera in GZ suggested that it was more functionally stable. While fermentation in both samples was dominated by Clostridia, SWH had an overrepresentation of Proteobacteria, including syntrophic acetogens, reflecting its more complex substrate. Considering the growing importance of biogas as an alternative fuel source, a detailed mechanistic understanding of AD is important and this report will be a basis for further study of industrial wastewater AD.