Comparison of nanosilver removal by flocculent and granular sludge and short- and long-term inhibition impacts

Abstract

The growing production and application of silver nanoparticles (Ag NPs) increases the chance of these particles entering wastewater treatment plants and interacting with activated sludge. In this paper, the removal of Ag NPs by flocculent and granular sludge was studied, and the short- and long-term inhibitory impacts of Ag NPs on the sludge were compared. Results showed that both forms of sludge contributed to removal of Ag NPs with 30–58% and 2.5–9.4% removal by the flocculent sludge and the granular sludge, respectively, at Ag NP dosage of 1–8 mg/L. Exposure to Ag NPs had greater inhibitive effects on the flocculent sludge than the granular sludge. Short-term (12 h) exposure to Ag NPs at 1, 10, 50 and 100 mg/L reduced the ammonia oxidizing rate of the flocculent sludge by 21.0–24.9%, while no inhibition was found for the granular sludge; the oxygen uptake rate of the flocculent sludge was inhibited at Ag NP concentrations as low as 1 mg/L, while that of the granular sludge was only affected at much higher Ag NP concentrations (50 and 100 mg/L). The denitrification rate, however, was not inhibited for either sludge. After long-term (22 day) exposure to 5 and 50 mg/L of Ag NPs, flocculent sludge was significantly inhibited on ammonia oxidizing rate, denitrification rate and oxygen uptake rate, but the microbial activity of granular sludge was not inhibited. Exposure to Ag NPs resulted in oxidative stress and damage of bacterial cell integrity for both flocculent and granular sludge as was determined by generation of reactive oxygen species and release of lactate dehydrogenase (LDH). The toxic effect of Ag NPs on sludge was mediated via both ROS-dependent and ROS-independent pathways, and both small (<10 nm) and large (>10 nm) Ag NPs contributed to it. Overall, granular sludge demonstrated stronger resistance to the toxicity of Ag NPs than flocculent sludge, while flocculent sludge was more efficient in removing Ag NPs.