The effect of reducing particle size on physical properties, the methane yield and energy flow were investigated through the biochemical methane potential (BMP) experiment of aerobic-anaerobic digestion (AAD) of rice straw (RS). The whole straw was crushed through four sieves of different aperture sizes (1, 3, 5, and 7 mm) to obtain the actual and non-uniform particle size distribution (PSD). The results indicated that the actual particle sizes were normally or logarithmic normally distributed. Reducing particle size could significantly promote the aerobic hydrolysis and acidification process, increase the content of volatile fatty acids (VFAs) from 4408.78 to 6225.15 mg/L and the degradation of volatile solids (VS) from 40.56% to 50.49%. The results of path analysis suggested that particle size reduction played an important role in improving lignocellulosic degradability, which was the main factor affecting methane production with the comprehensive decision of 0.4616. The maximum methane production obtained at 1 mm sieve size was 176.47 mLCH4g−1 VS. The phyla of Firmicutes (61.5%), Proteobacteria (9.3%), Chloroflexi (8.3%), Bacteroidetes (4.1%), Cyanobacteria/Chloroplast (4.6%) were mainly responsible for VFAs production and lignocellulose degradation. However, the net negative energy balance was observed at the 1 mm sieve size due to the increased energy input. Therefore, the optimum sieve size for AAD was 3 mm.

Lead ion (Pb2+) in wastewater cannot be biodegraded and destroyed. It can easily be enriched in living organisms, which causes serious harm to the environment and human health. Among the existing treatment technologies, adsorption is a green and efficient way to treat heavy metal contamination. Novel KMnO4-treated magnetic biochar (KFBC) was successfully synthesized by the addition of Fe(NO3)3 and KMnO4 treatment during carbonization following Pb2+ adsorption. SEM-EDS, XPS, and ICP-OES were used to evaluate the KFBC and magnetic biochar (FBC) on the surface morphology, surface chemistry characteristics, surface functional groups, and Pb2+ adsorption behavior. The effects of pH on the Pb2+ solution, initial concentration of Pb2+, adsorption time, and influencing ions on the adsorption amount of Pb2+ were examined, and the adsorption mechanisms of FBC and KFBC on Pb2+ were investigated. The results showed that pH had a strong influence on the adsorption of KFBC and the optimum adsorption pH was 5. The saturation adsorption capacity fitted by the model was 170.668 mg/g. The successful loading of manganese oxides and the enhanced oxygen functional groups, as evidenced by XPS and FTIR data, improved KFBC for heavy metal adsorption. Mineral precipitation, functional group complexation, and π-electron interactions were the primary adsorption processes.

With the rapid development of large and medium-sized biogas projects, the high-value utilization of anaerobic fermentation residues has become a hot spot in recent years. In this study, biogas residue from biogas engineering was used as composting raw material, and 0 (CK), 2.5% (T1), 5.0% (T2), 7.5% (T3), and 10.0% (T4) biochar was added to investigate its effects on physico-chemical properties, microbial populations, and maturity degree during the aerobic composting process. Results show that the addition of biochar shortens the time (3 days) to reach the high-temperature period, increases the composting temperature (63.8 °C) and germination index (GI), decreases the electrical conductivity (EC), reduces the loss of C and N elements, and increases the microbial population during composting. These results suggest that biochar can improve the maturity and fertility of compost products, and significantly regulate the structure and function of microbial communities during the composting process.

In this study, biogas residue fermented by biogas engineering was used as compost raw material, and different quality biochar was added in the composting process to explore the effect of biochar on the transformation of heavy metals in the composting process. The composting process was comprehensively analyzed with the potential ecological risk assessment of heavy metals and redundancy analysis. The addition of 10.0% biochar during composting had a strong passivation effect on exchangeable Cu and Cd, with passivation rates of 11.75 and 63.89%, respectively; the addition of 2.5 and 7.5% biochar had strong passivation ability for exchangeable Zn and Pb, and the passivation rates were 15.26 and 45.02%, respectively. At the end of composting, the potential ecological risk indexes of each treatment were T4 (10.0% biochar) > T3 (7.5% biochar) > T2 (5.0% biochar) > T1 (2.5% biochar) > CK (no biochar added). The risk of heavy metal pollution during the aerobic composting of biogas residue was low, which significantly reduced secondary pollution during the composting process.