Influence of silver nanoparticles on nitrification kinetics and ammonla oxidation in activated sludge

Nitrification is widely applied process for biological removal of nitrogen from wastewaters. The process comprises of two-steps: ammonia oxidation and nitrite oxidation. Ammonia (NH₃) is oxidized to nitrite (NO₂⁻) by ammonia-oxidizing microorganisms (AOM) and then nitrite is subsequently oxidized to nitrate (NO₃⁻) by nitrite-oxidizing microorganisms (NOM). Of which, ammonia oxidation is known as a rate-limiting and more sensitive step because of the microorganism behaviors thus plays an essential important role in any wastewater treatment facilities. Ammonia oxidizing microorganisms are reported to be very sensitive to non-growth substrates including toxic substances. Silver nanoparticles (AgNPs) are one of the toxic substances that have significantly increased in production and use, recently. In this study, the inhibitory effect of AgNPs on nitrification and ammonia oxidation in enriched nitrifying activated sludge was investigated by using respirometric method. The initial concentrations of AgNPs and ammonia ranged from 0.25 to 10 mg/L and 14 to 280 mg/L, respectively, for both processes. Under the presence of AgNPs, the maximum oxygen uptake rate (OUR[subscript max]) and half saturation constant (Ks) of both nitrification and ammonia oxidation were declined. The effect behavior of AgNPs on the both processes was proved to follow uncompetitive inhibition model. The kinetic parameters were estimated for half saturation constants (K[subscript s]) without AgNPs for nitrification and ammonia oxidation and were 5.42 and 15.88 mgNH₄⁺- N/L while the inhibition coefficients (K[subscript i]) were 8.3 and 21.7mg/L, respectively. Increasing AgNPs from 0.25-10 mg/L inhibited 8 ± 4.8% - 38 ± 14.2 and 16 ± 8.2% - 38 ± 11.3% respiration rate of nitrifying and ammonia oxidizing activity, respectively within 0.5h. Silver nanoparticles only partially influenced on nitrification and ammonium oxidation in the manner that higher AgNPs resulted in higher inhibition of respiration rate. Silver nanoparticle and ammonia concentration could synergize inhibitory effect on nitrifying and ammonia oxidizing activity. At high ammonium concentration (280mg/L), nitrifying and ammonia oxidizing activity were inhibited by AgNPs up to 48 and 41% within 0.5h, respectively. The findings indicated that AgNPs could harm nitrogen removal in wastewater treatment plants. Thus, AgNPs should be considered as hazardous substance and special attention should be made in the management of discharge of AgNPs into the environments through enhancing implementation of regulations of production and use.