Responses of SNEDPR-AGS system under long-term exposure of polyethylene terephthalate microplastics for treating low C/N wastewater: granular effect and microbial structure

Abstract

The removal of nutrients in wastewater treatment plants can be significantly impacted by carbon limitations, especially for treating low carbon to nitrogen ratio (C/N) wastewater, which can markedly increase operational costs. Simultaneous nitrification, endogenous denitrification, and phosphorus removal combined with aerobic granular sludge (SNEDPR-AGS) has emerged as one of the optimal processes for treating low C/N wastewater owing to its high carbon utilization efficiency; however, the long-term effect of microplastics (MPs) on this system remains unclear. This study investigated the granular effect and microbial response of an SNEDPR-AGS system for treating low C/N wastewater under long-term exposure (180 d) to polyethylene terephthalate microplastics (PET-MPs). The results showed that the integrity of the AGS structure was disrupted significantly as the PET-MP concentration increased, with clear AGS cracks appearing on days 180, 124, and 74 after exposure to 1, 10, and 100 mg/L of PET-MPs, respectively. Additionally, the addition of PET-MPs also inhibited denitrification and phosphorus removal due to a decrease in the relative abundance of functional genes (napAB, nirK/nirS, ppk1, ppk2, and ppx). Notably, both chemometric and high-throughput sequencing results indicated that the metabolic form of the system would shift from a polyphosphate-accumulating metabolism to a glycogen-accumulating metabolism. The reason may be that PET-MP stress inhibited the relative abundance of functional genes related to carbon, glycogen, phosphorus, and energy metabolism pathways in Candidatus Accumulibacter and Dechloromonas, but promoted their relative abundance of Candidatus Competibacter. Flow cytometry and molecular docking simulations have also demonstrated the direct toxic effects of PET-MPs on the SNEDPR-AGS system. The biological enhancement and functional recovery of damaged SNEDPR-AGS systems must be further investigated in future studies.

Environmental Implication

STPs are difficult to remove microplastics, and the vast majority of microplastics are trapped in the sludge system. It is necessary to evaluate the long-term effects on typical sludge systems. This study investigated the effects of long-term exposure (180 days) to PET-MPs on the SNEDPR-AGS system. Even in AGS systems with strong tolerance, the sludge structure will be destroyed, and the C, N, P metabolic processes of (D)PAOs will be inhibited. The metabolism of the system shifted from PAM to GAM, thereby impacting the stable operation of the SNEDPR-AGS system. This threat is more severe in traditional activated sludge processes.