Polylactic acid microplastics increase N2O emissions from mainstream biological nitrogen removal systems: overlooked concern.

Despite being marketed as environmentally benign alternatives for conventional synthetic polymers, biodegradable plastics inevitably generate large amounts of microplastics, posing currently overlooked yet potentially significant environmental risks. This study selected polylactic acid microplastics (PLA-MPs) as model biodegradable microplastics to investigate their effects on nitrous oxide (N2O) emissions from a mainstream biological nitrogen removal system. Long-term exposure to PLA-MPs at environmentally relevant level (0.5 mg/L) increased the N2O emission factor by 28.9%, and inhibited maximum nitrification and denitrification activities by 13.4%-20.9% and 4.9%-16.7%, respectively, while the nitrate reduction rate increased by 9.9%. Under elevated stress (5 mg/L), the N2O emissions increased by 32.4%, but the extent of maximum activity inhibition was comparable to that at 0.5 mg/L. Isotopic analysis revealed that PLA-MPs promoted the NH2OH oxidation pathway and consequently enhanced the N2O-producing capacity of nitrifiers, while nitrite accumulation during nitrification remained largely unaffected. During denitrification, however, PLA-MPs disrupted the balance of nitrogen oxide reduction steps, leading to elevated accumulation of both nitrite and N2O. PLA‑MPs altered the composition and structure of extracellular polymeric substances, which was accompanied by increased sludge particle size and hydrophobicity. Concurrently, shifts in the relative abundances of key microorganisms and enzyme-encoding genes involved in electron transfer, electron consumption, and metal transmembrane transport were observed. Together, these changes in sludge properties and microbial community likely explained the elevated N2O emissions under PLA-MP exposure, providing new insights into the long-term impacts of biodegradable microplastics on engineered wastewater systems.