Impact of microplastics on the treatment performance of constructed wetlands: Based on substrate characteristics and microbial activities

Presence of microplastics (MPs) in wastewater has posed a huge ecosystem risk. Constructed wetlands (CWs) can effectively intercept MPs, while with MPs accumulation the response of CWs' performance is still unclear. In order to evaluate those effects, we conducted a 370-day experiment using CW microcosms fed with different levels (0, 10, 100, and 1000 μg/L) of polystyrene (PS) MPs (diameter: 50-100 μm). Results showed that nitrogen removal efficiency was increased (by 3.9%-24.7%) during the first 60 days and then decreased (by 7.1%-41.3%) with MPs accumulating, but no obvious change in COD and TP removal was observed. From further analysis, MPs accumulation changed the biofilm composition (TOC content increased from 41.4% to 52.7%), substrate porosity (electrical resistivity increased by 1.2-2.4 folds), and oxygen mass transfer (|KLa,O2| increased from 3.5% to 18.6%). Moreover, the microbial dynamics presented a higher abundance of nitrifiers (Nitrospira and Nitrosomonas) during the 60-day experiment and a lower abundance in the last days, while denitrifiers (Thauera, Thiobacillus, and Anaerolinea) had a high relative abundance throughout the experiment, being consistent with the variation of nitrification and denitrification rates. Finally, structural equation model analysis proved that due to the changes of substrate characteristics and microbial compositions and activities, the obvious decrease in nitrification efficiency was a direct reason for the decline of nitrogen removal during 370-day MPs accumulation. Overall, our study first prove that MPs accumulation can cause a series of changes in physicochemical and microbial characteristics of substrate, and ultimately affect the nitrogen-transforming process in CWs. Although our conclusions were based on the lab-scale CWs being different from the real wetlands, we hope that the conclusions can provide the effective regulatory strategies to guide the control of MPs in the actual wetlands.