Environmental behaviors of exogenous emerging contaminants on the anaerobic digestion of waste activated sludge

Due to the exponential increase and consumption of diverse synthetic chemicals, the massive release of emerging contaminants (ECs) (e.g., nanoparticles, microplastics, etc.) has posed a serious threat to human health and the ecological system. Because of the limited removal efficiency of wastewater treatment plants, these exogenous ECs would ultimately accumulate in the waste activated sludge (WAS), resulting in unexpected impacts on WAS anaerobic digestion, which is a green and promising method for WAS disposal. This work mainly discussed the impacts of exogenous ECs on WAS digestion efficiency, disclosed the underlying mechanisms, and also provided the corresponding promising mitigation approach. Generally, these exogenous ECs exhibited dose-dependent impacts on WAS digestion, and the majority of ECs with low levels could promote WAS digestion performance to some extent through partially or totally accelerating the solubilization, hydrolysis, acidification, and methanogenesis stages. However, they would significantly inhibit WAS digestion efficiency at high levels due to their toxicity. Moreover, methanogens are more vulnerable than hydrolytic and acidogenic bacteria. Besides, the co-occurrence of ECs might aggravate their biotoxicity to WAS digestion, but the related research was limited. Hence, the combined effects of different mixed ECs and the mitigating strategies for typical pollutant inhibition deserve more attention in light of WAS anaerobic digestion.