Co-effects of silver nanoparticles and microplastics on nitrifying microorganisms from wastewater treatment plants and their activities

Silver nanoparticles (AgNPs) and microplastics are emerging water contaminants of the decade. They share a similar fate and transport in wastewater treatment plants as they tend to accumulate in sludge of aeration tanks. Since both contaminants have negative effects on microbial growth, the ammonia-oxidizing microorganisms in the aeration tanks are at risk for inhibition and consequently nitrification process fails. This study investigated the effects of AgNPs and microplastics on ammonia-oxidizing activity and community. No inhibition of ammonia oxidation rate was observed at 0.1 mg/L AgNPs. Partial inhibition was found at 0.5 and 1 mg/L AgNPs, while complete inhibition occurred at higher concentrations of 2.5, 5, and 10 mg/L AgNPs. qPCR targeting AOA, AOB, and comammox amoA genes indicated that the numbers of the AOB amoA genes decreased when AgNPs were ≥2.5 mg/L while the comammox amoA genes dropped at ≥0.5 mg/L of AgNPs. Inhibition of AOA was found at AgNP concentrations above 0.5 mg/L but in substantially less compared to AOB and comammox. This study suggests that the three ammonia-oxidizing microorganisms have different responses to AgNP. The co-effect of AgNPs and PVC was studied in microcosms at concentrations of 0.5 mg/L and 500 mg/L, respectively. The results showed that the PVC microplastics had no inhibitory effects on the ammonia oxidation rate. Interestingly, the microcosms, in which the PVC was pre-shaken for 7 days before adding the sludge and AgNPs showed, a faster ammonia oxidation rate than the microcosms containing the sludge and AgNPs with and without fresh (non-shaken) PVC. This suggests that the pre-shaken PVC microplastics may reduce the toxicity of AgNPs. qPCR results indicated that PVC microplastics did not suppress AOA, AOB, and comammox.