Comparative effects of raw and aged tire microplastics on anaerobic fermentation of excess sludge

Tire microplastics (TMs), as a prominent environmental source of microplastic pollution, are aged and accumulated during their migration through drainage systems and sewage treatment facilities, most of which trapped in excess sludge. Consequently, this study systematically examines the concentration-dependent effects of TMs and aged TMs (ATMs) on anaerobic fermentation of excess sludge. Results showed that TMs and ATMs negatively affected acidogenic fermentation and organic matter hydrolysis. Low TMs concentration (0.002 g/g-VSS) and high ATMs concentration (0.2 g/g-VSS) resulted in concentration reductions of volatile fatty acids, soluble chemical oxygen demand, proteins, and polysaccharides ranged from 16.28 % to 40.40 % relative to the control group. In general, the detrimental effect demonstrated a positive correlation with ATM concentration. Conversely, low TMs concentration exerted a ‌significant inhibitory impact compared with high concentration, which contradicted the hormetic effect. In response to TMs/ATMs stress, the microorganisms employed extracellular polymeric substances (EPS) secretion as a defense mechanism, with pronounced‌ variations examined in protein content within tightly bound EPS and polysaccharide content in loosely bound EPS. Comprehensive mechanistic analysis revealed that although the incorporation of TMs/ATMs into fermentation system improved electron transfer efficiency through elevating electron transport system activity and sludge conductivity, TMs demonstrated superior interspecies electron transfer (IET) enhancement compared to ATMs, coupled with material-dependent suppression of hydrolytic and acidogenic bacterial consortia, which potentially altering the acid generation and microbial dynamics. Overall, our study provides critical insights for optimizing TMs control in sludge treatment by leveraging their differential profiles toward IET and bacterial consortia.