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Anaerobic digestion (AD) with high-solid content of agricultural waste often leads to the accumulation of volatile fatty acids (VFAs) due to the imbalance of the functions of hydrolytic acidification bacteria and methanogenic archaea. This study developed a novel dry–wet AD of high-solid content was developed to make acidogenic bacteria and methanogenic archaea having higher collaboration ability. The results showed that the dry–wet AD system achieved a dynamic balance and synergistic effect between hydrolytic acidification and the methane production process, which shortened the lag phase to a minimum of 1.21 days. Additionally, the system exhibited a highest volumetric biogas production rate of 1.76 L L−1 d−1 when CM:CS and the initial pH were 6:4, 8.0, respectively. The total VFA concentration reached a maximum of 4.69 g L−1 d−1 and acetic acid (HAc) and butyric acid (HBu) were dominant VFAs (88.75%–93.27%) in the leachate in hydrolytic acidification phase. Metagenomic analysis indicated Pseudomonadales, Clostridiales, Bacteroidales , Spirochaetales, and Methanosaeta were dominant microorganism in hydrolytic acidification phase and methanogenic phase, respectively. The gene abundance of key enzymes in the acetoclastic methanogenic pathway significantly exceeded that of the hydrotrophic methanogenic pathway. This study provides a novel AD technology and insights into the energy conversion of agricultural waste. [Display omitted] • A novel dry–wet AD for agricultural waste treatment was investigated. • A highest methane production was achieved at a SR of 6:4 and an initial pH of 8.0. • HAc and HBu maintained an average of 88.75%–93.27% in the intermediates. • Synergistic relationship between Clostridiales and Methanosaeta was evaluated. • Acetoclastic metabolic pathway was dominant in the dry–wet AD system. [ABSTRACT FROM AUTHOR]

Acetic acid (HAc) is an essential intermediate for improving the efficiency of biogas production in anaerobic digestion (AD) or for the production of liquid chemicals. The relationships between the mechanisms and microbial contributions of targeted regulation of HAc production in high-solids AD remains unclear. This study represents a breakthrough in revealing the mechanism of pH directed regulation of HAc production and microbial interactions. The regulation of the initial spray pH effectively improved the substrate hydrolysis rate and increased the concentration of HAc, with an average HAc percentage of total organic acids was 66.20 %. Microecological analysis revealed that rare species played a vital role in acidification, with all key acetogenic microbes identified as rare species. Additionally, the construction of a bacterial–environmental factor co-occurrence network revealed that lowering the initial spray pH enhanced the synergistic interactions among bacterial communities, and proportion of positive interactions among bacteria was 82.23 %. The gene abundance of key enzymes in the acetate-type metabolic pathway dominated in the acidogenesis phase. Furthermore, a regression model was established between the HAc indicator microbe (Acinetobacter) and HAc yields. Overall, the results of this study indicate that pH regulation plays an important role in strengthening the production of HAc and rare species significantly contributed to HAc production. [Display omitted] • The concentration of acetic acid increased significantly at a pH of 5.5. • Rare species dominate the acidogenic key microbial species; • Changes in initial pH facilitated synergistic cooperation of acetogenic microbes. • Contributions of key rare species to VFA yield were consistent with RF predictions. • Relationships between the response of key rare species and pH were established. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effects of activated pyrochar on AD performance were investigated. • A methane production rate of 1.6 L/L/d was obtained with CO 2 -activated pyrochar. • Pyrochar mainly played a strengthening role in the early and middle stages of AD. • Cornstalk pyrochar-N 2 had a positive correlation with isoleucyl-alanine. In this study, the effects of CO 2 -activated/non-activated pyrochars (PCs) from cornstalk, cotton straw, and rice straw on anaerobic digestion (AD) performances and microbial characteristics were investigated. The maximum biogas production rate (2.2 L/L/d) with a methane content of 73% was obtained from the AD with CO 2 -activated cotton straw PC. The activated PC mainly played a strengthening role in the early and middle stages of AD. Specifically, the cornstalk PC could greatly relieve acid inhibition, and cotton straw PC had a significantly positive effect on the regulation of ammonia nitrogen concentration. The rare genera like Verrucomicrobia had obvious differences among groups of AD with PCs. Regarding differential metabolites, cornstalk PC-N 2 displayed a positive correlation with isoleucyl-alanine, while exhibiting a negative correlation with deoxyinosine, and the corresponding relative expression levels were + 3.0 and −2.4, respectively. Overall, gas-activated PCs could promote methane production and affect the composition of microbial community. [ABSTRACT FROM AUTHOR]

In the process of anaerobic digestion (AD), the poor buffer capacity of the system could lead to the imbalance of the supply and demand of methanogens for organic matters, which may subsequently result in the decrease of biogas production, and even the instability of the whole AD system. In this study, key factors of sudden change were simulated based on the frequent instability problems in biogas projects, and subsequent instability properties, early-warning indicators, and microbial community characteristics were investigated. On the whole, the stable early-warning indicators were biogas production rate and content, pH, total alkalinity, bicarbonate alkalinity, and the sum of volatile fatty acids (VFAs). The threshold value of VFAs for AD instability was 10,000 mg/L, under which the biogas production rate reduced from 0.377 to 0.038 L/L/d, CH 4 content decreased from 62.8% to 33.6%, and the proportion of Methanosaeta dropped from 74% to 51.6%. Furthermore, Methanosaeta abundance was greatly sensitive to the sudden variation of VFAs, therefore this kind of archaea could be considered as a warning microbial indicator for acid inhibition. There has been an obvious difference in the functional abundances between bacteria and archaea. This study could provide theoretical guidance for stable operation of biogas projects. [Display omitted] • Crucial sudden changes were simulated to intervene stable anaerobic digestion. • Biogas production rate and content was confirmed as reliable early-warning indicator. • Methanosaeta could be used as a warning microbial indicator for acid inhibition. • The functional abundances of bacteria and archaea had an obvious difference. [ABSTRACT FROM AUTHOR]

Ensilage is a commonly used method of preserving energy crops for biogas production. However, aerobic deterioration of silage is an inevitable problem. This study investigated the effect of aerobic deterioration on methane production and microbial community dynamics through anaerobic digestion (AD) of maize stalk silage, following 9 days air exposure of silage. After air exposure, hydrolytic activity and methanogenic archaea amount in AD were reduced, decreasing the specific methane yield (SMY); whereas lignocellulose decomposition during exposure improved the degradability of silage in AD and enhanced SMY, partially compensating the dry matter (DM) loss. 29.3% of the DM and 40.7% of methane yield were lost following 0–9 days exposure. Metagenomic analysis showed a shift from Clostridia to Bacteroidia and Anaerolineae in AD after silage deterioration; Methanosaetaceae was the dominant methanogenic archaea. [ABSTRACT FROM AUTHOR]

[Display omitted] • Established a high lignocellulosic dry-wet combined anaerobic digestion system. • The kinetic parameters of the whole anaerobic digestion process were revealed. • Synergism between Clostridiales and methanogens was evaluated via network analysis. • Syntrophy between SFOB and methylotrophic Methanospirillum was observed. • The number of key acidogenic Clostridium was increased by 80.77–107.11%. Dry-wet combined anaerobic digestion is a novel approach for treating lignocellulosic waste by increasing the organic load of reactor while accelerating the conversion of organic acids. Here, we investigated the effect of regulated substrate ratios and initial pH in the dry acidogenesis stage on the bioconversion efficiency of dry-wet combined anaerobic digestion. Our data revealed microbial interactions and further identified key microbes based on microbial co-occurrence network analysis. On day three of acidification, the kinetic hydrolysis rate and acidification yield reached 1.66 and 60.07%, respectively; this was attributed to enhancement of the synergistic effect between Clostridiales and Methanosaeta , which increased the proportion of corn straw in the substrate or lowered the initial spray slurry pH to 5.5–6.5. With increased acidification capacity, acetoclastic methanogens were enriched in the wet methanogenesis stage; the syntrophic effect of Syntrophomonadales , Syntrophobacterales and Methanospirillum , meanwhile, was enhanced, leading to an overall improvement in biogas production. [ABSTRACT FROM AUTHOR]

[Display omitted] • Continuous lab and pilot dry co-digestion system was investigated systematically. • Factor effects were ranked as follows: TS > CD:CS > digestate recirculation ratio. • The biogas production rate reached 3.0 L·L-1·d-1 with optimal parameters. • The increase of CD:CS and TS content inhibited the biogas production. • Proteiniphilum and acetoclastic methanogens could facilitate biogas production. Dry anaerobic digestion (dry-AD) allows high-solid digestion; however, dry-AD application is limited because it is prone to blockage and intermediate inhibition. Here, we reported innovative continuous dry co-digestion systems at both lab and pilot scales. The effects of digestate recirculation ratio, dry mass ratio of cow dung to corn straw (CD:CS), and TS content on the digestion performance were investigated. The effects of the three factors were ranked as follows: TS content > CD:CS > digestate recirculation ratio. The daily biogas production rate reached 0.386 NL/d/g VS with the optimal parameter combination, which was determined to be TS content of 30%, a substrate ratio of 1:3, and a digestate recirculation ratio of 40%. In addition, increasing the CD:CS and TS content increased digestate viscosity, which inhibited biogas production; however, increased abundance of Proteiniphilum and acetoclastic methanogens facilitated biogas production. This study provides empirical support for further application of dry-AD. [ABSTRACT FROM AUTHOR]

• Cottonwood biochar was used as an enhanced mediator for the CSTR-AD of cornstalk. • The highest volumetric biogas production rate with biochar was 1.40 L/L/d. • The CO 2 content decreased by 5.90% while the CH 4 content increased by 7.40%. • Microbial abundance had positive correlations with the environmental parameters. The composition of biogas produced by anaerobic digestion (AD) is typically not ideal due to high CO 2 content. In the study, cottonwood biochar was used as an enhanced mediator for the continuously stirred tank reactor AD of cornstalk. The effects of substrate loading and biochar dosage on biogas composition, volatile fatty acids (VFAs), NH 3 -N, and microbial community characteristics were systematically explored. The results showed that the highest volumetric biogas production rate with biochar was 1.40 L/L/d, at the same time, the CO 2 content in the biogas decreased by 5.90%, while the CH 4 content increased by 7.40%, compared with the values in AD without biochar. Moreover, VFAs were degraded effectively, in particular, the propionic acid concentration decreased by 55.7%. Besides, microbial abundance had positive correlations with environmental parameters. This study could provide valuable information for both the elucidation of strengthening mechanisms of biochar and further large-scale engineering application. [ABSTRACT FROM AUTHOR]

• A horizontal plug flow continuous dry digestion reactor was developed in this study. • More methane and biogas produced at 60% than at 50% digestate recirculation ratios. • Highest biogas production (1.6L/L·d) reached at 60% digestate recirculation ratio. • VadinBC27_wastewater-sludge_group , Methanobacterium proliferated as VFAs decreased. This study conducted a small scale, push flow, continuous, dry anaerobic digestion experiment using 50% and 60% digestate recirculation ratios to investigate the effects of digestate recirculation ratio on continuous dry anaerobic digestion and to analyze microbial community succession. The results showed that the volumetric biogas production rate could reach 1.6 L/L·d during the 60% digestate recirculation ratio. A slight, initial accumulation of volatile fatty acids (VFAs) (maximum concentration 1.6 g/L) was subsequently consumed by the Christensenellaceae R-7 group, and VFA concentrations stabilized at around 20 mg/L. The increased digestate recirculation ratio (60%) promoted both VadinBC27 wastewater-sludge group and Methanobacterium proliferation, and the predominance of those microbial strains may be why VFAs decreased and gas production efficiency improved. Those microbial community changes, fostered by the higher digestate recirculation ratio, are important in continuous dry anaerobic digestion. These results aid continued work aimed at improving continuous, dry anaerobic digestion. [ABSTRACT FROM AUTHOR]