LAUSR

Abstract Anaerobic digestion is a promising technology for the recovery of energy from waste activated sludge. With the increasing use of ZnO- and TiO2-engineered nanoparticle materials (ENMs) and their accumulation in sludge, their combined effect on anaerobic digestion was investigated. The toxicological mechanism involving Zn2+ bioavailability and reactive oxygen species (ROS: ·OH and H2O2) was initially investigated and confirmed. Results showed that ZnO ENMs inhibited the metabolism of volatile fatty acids (VFAs) and the production of biogas because of Zn2+ and ROS. The metabolic pathway of butyric acid to acetic acid was inhibited. The biogas production in the system 30ZnO and 80ZnO were reduced to 67.4 and 31.5 mL/g VSS, respectively, compared with 86.3 mL/g VSS in the control group. The toxicity of ZnO ENMs on anaerobic digestion was reduced in the presence of TiO2 ENMs because of the reduction of Zn2+ bioavailability. The biogas production 30ZnO+80TiO2 system was 80 mL/g VSS, which approached to the control group. The microbial analysis of a multi-ENM anaerobic digestion system revealed that cell membrane integrity, microbial abundance, and community structure were affected. Zn2+ bioavailability and ROS likely altered microbial communities.