Upgrade the high-load anaerobic digestion and relieve acid stress through the strategy of side-stream micro-aeration: biochemical performances, microbial response and intrinsic mechanisms

Optimizing anaerobic digestion: Benefits of mild temperature transition from thermophilic to mesophilic conditions

Anaerobic digestion (AD) plays a significant role in renewable energy recovery. Upgrading AD from thermophilic (50-57 °C) to mesophilic (30-38 °C) conditions to enhance process stability and reduce energy input remains challenging due to the high sensitivity of thermophilic microbiomes to temperature fluctuations. Here we compare the effects of two decreasing-temperature modes from 55 to 35 °C on cell viability, microbial dynamics, and interspecies interactions. A sharp transition (ST) is a one-step transition by 20 °C d-1, while a mild transition (MT) is a stepwise transition by 1 °C d-1. We find a greater decrease in methane production with ST (88.8%) compared to MT (38.9%) during the transition period. ST mode overproduced reactive oxygen species by 1.6-fold, increased membrane permeability by 2.2-fold, and downregulated microbial energy metabolism by 25.1%, leading to increased apoptosis of anaerobes by 1.9-fold and release of intracellular substances by 2.9-fold, further constraining methanogenesis. The higher (1.6 vs. 1.1 copies per gyrA) metabolic activity of acetate-dependent methanogenesis implied more efficient methane production in a steady mesophilic, MT-mediated system. Metagenomic binning and network analyses indicated that ST induced dysbiosis in keystone species and greatly enhanced microbial functional redundancy, causing loss of microbial syntrophic interactions and redundant metabolic pathways. In contrast, the greater microbial interconnections (average degrees 44.9 vs. 22.1) in MT at a steady mesophilic state suggested that MT could better maintain necessary system functionality and stability through microbial syntrophy or specialized pathways. Adopting MT to transform thermophilic digesters into mesophilic digesters is feasible and could potentially enhance the further optimization and broader application of practical anaerobic engineering.

Influence of oxygen partial pressure on homoacetogenesis and promotion of acetic acid accumulation through low pH regulation under microaerobic conditions

Homoacetogenesis is an important pathway for bio-utilization of CO2; however, oxygen is a key environmental influencing factor. This study explored the impact of different initial oxygen partial pressures (OPPs) on homoacetogenesis, while implementing low pH regulation enhanced acetic acid (HAc) accumulation under microaerobic conditions. Results indicated that cumulative HAc production increased by 18.2% in 5% OPP group, whereas decreases of 31.3% and 56.0% were observed in 10% and 20% OPP groups, respectively, compared to the control group. However, hydrogenotrophic methanogens adapted to microaerobic environment and competed with homoacetogens for CO2, thus limiting homoacetogenesis. Controlling influent pH 5.0 per cycle increased cumulative HAc production by 18.3% and 18.2% in 5% and 10% OPP groups, respectively, compared with the control group. Consequently, regulating low pH effectively inhibited methanogenic activity under microaerobic conditions, thus increasing HAc production. This study was expected to expand the practical application of homoacetogenesis in bio-utilization of CO2.

Micro-aeration and leachate recirculation for the acceleration of landfill stabilization: Enhanced hydrolytic acidification by facultative bacteria

The long duration of landfill stabilization is one of the challenges faced by municipalities. In this paper, a combination of micro-aeration and leachate recirculation is used to achieve rapid degradation of organic matter in landfill waste. The results showed that the content of volatile fatty acids (VFAs) in the hydrolysis phase increased significantly and could enter the methanogenic phase quickly. Until the end of the landfill, the removal rates of chemical oxygen demand (COD), total phosphorus (TP) and ammonia nitrogen (NH4+-N) by micro-aeration and leachate recirculation reached 80.17 %, 48.30 % and 48.56 %, respectively, and the organic matter degradation rate reached 50 %. Micro-aeration and leachate recirculation enhanced the abundance of facultative hydrolytic bacteria such as Rummeliibacillus and Bacillus and the oxygen tolerance of Methanobrevibacter and Methanoculleus. Micro-aeration and leachate recirculation improved the organic matter degradation efficiency of landfill waste by promoting the growth of functional microorganisms.

Impact of sandwich-type composite anodic membrane on membrane fouling and methane recovery from sewage sludge and food waste via electrochemical anaerobic membrane bioreactor

Membrane fouling presents a big challenge for the real-world implementation of anaerobic membrane bioreactors (AnMBRs) in digesting high-solid biowastes. In this study, an electrochemical anaerobic membrane bioreactor (EC-AnMBR) with a novel sandwich-type composite anodic membrane was designed and constructed for controlling membrane fouling whilst improving the energy recovery. The results showed that EC-AnMBR produced a higher methane yield of 358.5 ± 74.8 mL/d, rising by 12.8% compared to the AnMBR without applied voltage. Integration of composite anodic membrane induced a stable membrane flux and low transmembrane pressure through forming an anodic biofilm while total coliforms removal reached 97.9%. The microbial community analysis further provided compelling evidence that EC-AnMBR enriched the relative abundance of hydrolyzing (Chryseobacterium 2.6%) bacteria and methane-producing (Methanobacterium 32.8%) archaea. These findings offered new insights into anti-biofouling performance and provided significant implications for municipal organic waste treatment and energy recovery in the new EC-AnMBR.

Improving mesophilic anaerobic digestion of food waste by side-stream thermophilic reactor: Activation of methanogenic, key enzymes and metabolism

Anaerobic digestion (AD) is a favorable way to convert organic pollutants, such as food waste (FW), into clean energy through microbial action. This work adopted a side-stream thermophilic anaerobic digestion (STA) strategy to improve a digestive system's efficiency and stability. Results showed that the STA strategy brought higher methane production as well as higher system stability. It quickly adapted to thermal stimulation and increased the specific methane production from 359 mL CH₄/g·VS to 439 mL CH₄/g·VS, which was also higher than 317 mL CH₄/g·VS from single-stage thermophilic anaerobic digestion. Further exploration of the mechanism of STA using metagenomic and metaproteomic analysis revealed enhanced activity of key enzymes. The main metabolic pathway was up-regulated, while the dominant bacteria were concentrated, and the multifunctional Methanosarcina was enriched. These results indicate that STA optimized organic metabolism patterns, comprehensively promoted methane production pathways, and formed various energy conservation mechanisms. Further, the system's limited heating avoided adverse effects from thermal stimulation, and activated enzyme activity and heat shock proteins through circulating slurries, which improved the metabolic process, showing great application potential.

Microaerobic condition as pretreatment for improving anaerobic digestion: A review

Pretreatment of waste before anaerobic digestion (AD) has been extensively studied during the last decades. One of the biological pretreatments studied is the microaeration. This review examines this process, including parameters and applications to different substrates at the lab, pilot and industrial scales, to guide further improvement in large-scale applications. The underlying mechanisms of accelerating hydrolysis and its effects on microbial diversity and enzymatic production were reviewed. In addition, modelling of the process and energetic and financial analysis is presented, showing that microaerobic pretreatment is commercially attractive under certain conditions. Finally, challenges and future perspectives were also highlighted to promote the development of microaeration as a pretreatment before AD.

The greenhouse effect of antibiotics: The influence pathways of antibiotics on methane release from freshwater sediment

The impact of antibiotics on methane (CH₄) release from sediment involves both CH₄ production and consumption processes. However, most relevant studies lack a discussion of the pathways by which antibiotics affect CH₄ release and do not highlight the role played by the sediment chemical environment in this influence mechanism. Here, we collected field surface sediments and grouped them with various antibiotic combination concentration gradients (50, 100, 500, 1000 ng g^-1) under a 35-day indoor anaerobic constant temperature incubation. We found that the positive effect of antibiotics on sediment CH₄ release potential appeared later than the positive effect on sediment CH₄ release flux. Still, the positive effect of high-concentration antibiotics (500, 1000 ng g^-1) occurred with a lag in both processes. Also, the positive effect of high-concentration antibiotics was significantly higher than low-concentration antibiotics (50, 100 ng g^-1) in the later incubation period (p < 0.05). We performed a multi-collinearity assessment of sediment biochemical indicators, followed by a generalized linear model with negative binomial regression (GLM-NB) to obtain essential variables. In particular, we conducted the interaction analysis on CH₄ release potential and flux regression for the influence pathways construction. The partial least-squares path modeling (PLS-PM) demonstrated that the positive effect of antibiotics on CH₄ release (Total effect = 0.2579) was primarily attributed to their effect on the sediment chemical environment (Direct effect = 0.5107). These findings greatly expand our understanding of the antibiotic greenhouse effect in freshwater sediment. Further studies should more carefully consider the effects of antibiotics on the sediment chemical environment, and continuously improve the mechanistic studies of antibiotics on sediment CH₄ release.

Effect of Fe0 particle size on buffering characteristics and biohydrogen production in high-load photo fermentation system of corn stover

The aim of this work was to study the effects of Fe0 particle sizes (700 nm, 100 nm and 50 nm) addition on biohydrogen production, liquid culture characteristics and photosynthetic bacterial respond in the high-load photo fermentation system of corn stover within the concentration range of 200-1500 mg/L. Results showed that Fe0 with particle size of 700 nm had a better promotion effect on hydrogen production than 100 nm and 50 nm. The highest hydrogen yield of 74.32 ± 3.48 mL/g TS and hydrogen production rate of 3.31 ± 0.11 mL/g·h TS corn stover were obtained at 1000 and 1500 mg/L Fe0-700 nm, which were significantly increased by 92.88 % and 133.88 % compared with the control group. Further analysis indicated that Fe0 addition effectively alleviated pH drop, enhanced nitrogenase activity, promoted cell growth, and accelerated the consumption of acetic acid and butyric acid.