Biodegradable Microplastics Increase N₂O Emission from Denitrifying Sludge More Than Conventional Microplastics

Despite the increasing concern about the impacts of microplastics on wastewater treatment, the underlying mechanism by which microplastics affect nitrous oxide (N2O) accumulation during denitrification is still underexplored. In particular, effects of biodegradable microplastics (BMPs) on sewage sludge systems are largely overlooked. Previous studies often used one type of polymer as model microplastics, far from a real-world scenario of various microplastics occurring simultaneously. This work assesses the toxic influences of microplastics by chronically adding four typical BMPs versus four conventional nonbiodegradable microplastics (NBMPs) to denitrifying sludge. Our results showed that both BMPs and NBMPs suppressed denitrification performance, intensified electron competition, regulated electron distribution, and consequently promoted N2O accumulation at a chemical oxygen demand:nitrate ratio of <4:1. Importantly, more severe impacts were observed in the reactor with BMPs. A subsequent mechanistic study revealed that BMPs significantly decreased the relative abundances of denitrifiers and key genes involved in the electron transport and consumption system (ETCS), which might be related to the significantly varied extracellular polymeric substance components. In contrast, NBMPs damaged bacterial membranes and directly caused more dead cells by overproducing reactive oxygen species, hence disrupting the ETCS. Overall, this work suggested that microplastics in wastewater, especially BMPs, could disrupt denitrification and potentially increase greenhouse gas emission.