Drinking water distribution systems (DWDSs) may be contaminated to various degrees when different microorganisms attach to the pipe walls. Understanding the characteristics of biofilms on pipe walls can help prevent and control microbial contamination in DWDSs. The biofilm formation, interspecific interactions, and chlorine resistance of 10 dual-species biofilms in polyethylene (PE) and cast iron (CI) pipes were investigated in this paper. The biofilm biomass (heterotrophic bacterial plate count and crystal violet) of dual species in CI pipes is significantly higher than that in PE pipes, but the biofilm activity in CI pipes is significantly lower than that in PE pipes. The interspecific interaction of Sphingomonas-containing group presented synergistic or neutral relationship in PE pipes, whereas the interspecific interaction of the Acidovorax-containing group showed a competitive relationship in CI pipes. Although interspecific relationships may help bacteria resist chlorine, the chlorine resistance was more reliant on dual-species groups and pipe materials. In CI pipes, the Microbacterium containing biofilm groups showed better chlorine resistance, whereas in PE pipes, most biofilm groups with Bacillus exhibited better chlorine resistance. The biofilm groups with more extracellular polymeric substance (EPS) secretion showed stronger chlorine resistance. The biofilm in the PE pipe is mainly protected by EPS, while both EPS and corrosion products shield the biofilms within CI pipe. These results supported that dual-species biofilms are affected by pipe materials and interspecific interactions and provided some ideas for microbial control in two typical pipe materials.

High-solid anaerobic digestion (HS-AD) is one of the most efficient and popular solid-waste-treatment and energy-recovery technologies. However, it is significantly influenced by composition and substrate characteristics, with the carbon-to-nitrogen ratio (C/N ratio) and biodegradability being important. In this study, the substrate was adjusted and compounded to investigate the methanogenesis performance with the substrate C/N ratio (2.82–82.72) and biodegradability (refractory and easily degraded) variation during HS-AD. The results showed that the highest methane yield (MY) was achieved with white meat (273.02 mL/g-VS), which was 2.22 times higher than that of substrates with higher C/N ratios. However, this enhanced methane productivity was accompanied by elevated total ammonia nitrogen (TAN) concentrations, which substantially increased the risk of system instability. For a high C/N ratio (>50:1), the difference between the C/N ratio and biodegradability had little influence on MY, and under a low C/N ratio (<5:1), the methane production rate was higher. For low C/N substrates, acetic (14.53–76.82%) accounted for the highest total volatile fatty acids (VFAs) during HS-AD, and for high C/N ratio substrates, propionic (46.44–82.66%) had a higher proportion (r = 0.94). An increased C/N ratio decreased total ammonia nitrogen (TAN) and alkalinity (p < 0.05). Variations in the C/N ratio and biodegradability led to differences in the microbial composition.