Effect of Polypropylene Microplastics Concentration on Wastewater Denitrification

Microplastics have been ubiquitously in the environment, and a large number of microplastics have entered wastewater treatment plants, and their effects on activated sludge denitrification are rarely reported. Polypropylene (PP) microplastics were selected as the research object for denitrification research. Herein, we set different concentrations of PP microplastics to study their effect on sewage denitrification. Several indicators including NO3--N, NO2--N, Total-N (TN), extracellular polymer substances, Zeta potential, and N2O production, etc., were employed to evaluate the denitrification effect. Results demonstrated that the denitrification process was affected by the concentration of PP microplastics. Its concentration may damage the denitrification of sludge, especially in low concentrations, due to its slowing down the NO2--N reduction process. Meanwhile, it will release an amount of greenhouse gas N2O as the concentration is positively correlated. However, the removal of NO3--N is almost unaffected in different concentrations of PP microplastics during the denitrification process. When the concentration of PP microplastics reached 60 mg L-1, the accumulation of NO2--N in the effluent increased significantly. Accordingly, the extracellular polymer and potential indicators also showed that PP microplastics will reduce the flocculation capacity of the sludge, resulting in a decrease in the denitrification efficiency of the sludge overall. When the concentration of PP microplastics exceeded 100 mg L-1, the impact of microplastics on denitrification water quality indicators was relatively small, but it will increase the N2O emission with a negative impact. In addition, the activated sludge denitrification pathway can be affected by the addition of different microplastics and requires further investigations in the future.