Long-term effect of microplastics on anammox systems: From macro efficiency to micro metabolic mechanisms and antibiotic resistance genes proliferation

Wastewater treatment plants (WWTPs) are sinks for various types of microplastics (MPs), which have an undeniable impact on anaerobic ammonia oxidation (anammox) systems, yet their differential effects remain elusive. In this study, the long-term effects of non-biodegradable polyvinyl chloride (PVC) MPs and biodegradable polylactic acid (PLA) MPs in anammox systems were investigated from macro efficiency, micro metabolic mechanism, and antibiotic resistance gene proliferation. Results showed that the introduced 0.5 mg L−1 PVC MPs caused a 20.64 % decrease in nitrogen removal efficiency (NRE) compared to the control one, while PLA MPs resulted in a 10.73 % increase in NRE. Accordingly, the relative abundance of Ca. Kuenenia with the introduced PLA MPs was 16.68 % higher than the control group, while it was 10.89 % lower with the introduced PVC MPs. Furthermore, MPs inhibit the reactor performance by inducing intracellular reactive oxygen species (ROS) production in anammox cells. However, PLA MPs, as a slow biodegradable organic, have a reversible negative effect on ROS induction, which further improves nitrogen removal performance and sludge proliferation through dissimilatory nitrate reduction to ammonium (DNRA) and microbial interactions. Nevertheless, this also poses an increased risk of antibiotic resistance genes (ARGs) spread and proliferation in anammox sludge. This study clarified the comprehensive effects of different MPs types in anammox processes, highlighting the feasibility and potential risks in the engineering applications of anammox processes for MPs-contained wastewater treatment.