Do Microplastics Affect Biological Wastewater Treatment Performance? Implications from Bacterial Activity Experiments

Microplastics have been ubiquitously detected in the wastewater treatment plants, while their effects on the activities of wastewater treatment bacteria have never been evaluated. This study investigated the effects of polyester (PES), polyethylene (PE), and polyvinyl chloride (PVC) microplastics (100–1200 μm; 50–10 000 particles/L) on the activities of ammonium-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), denitrifiers, and polyphosphate-accumulating organisms (PAOs). The activities of AOB and NOB without microplastics addition are 6.3 ± 0.3 and 4.0 ± 0.4 mg N/g MLVSS/h (MLVSS: mixed liquor volatile suspended solids), which are similar (p > 0.05) to their activities (5.2 ± 0.7 to 6.8 ± 0.8 and 3.7 ± 0.9 to 5.1 ± 0.8 mg N/g MLVSS/h) with microplastics addition. Similarly, the activities of denitrifiers and PAOs without microplastics addition (14.1 ± 1.1 mg N/g MLVSS/h and 29.2 ± 0.9 mg P/g MLVSS/h) are comparable (p > 0.05) to those with microplastics addition (12.8 ± 1.2 to 15.1 ± 0.5 mg N/g MLVSS/h and 28.0 ± 1.1 to 29.7 ± 2.4 mg P/g MLVSS/h). The results demonstrated that microplastics do not significantly affect the activities of AOB, NOB, denitrifiers, and PAOs, and therefore the effect of microplastics on the wastewater treatment performance should not be overemphasized.