Nitrification Inhibition On Nitrifiers And Activate Sequencing Batch Reactor By Silver Nanoparticles

With the increasing utility of nanotechnology and nano-materials, large amount of particles with nanometer sizes have been released to environment. Although some nanoparticles have shown to be toxic and their environmental effect has been drawn attention recently, little is known of the mechanism and level of toxicity. This research thus focused on the nitrification inhibition on nitrifiers and activated sludge treatment process by silver nanoparticles (AgNPs) which is one of the most commonly used nanomaterials in consumer products.Silver nanoparticles (AgNPs) with the diameters of 9, 13, 15 and 23 nm and concentrations range from 0.12 to 4.82 mg/L were used to test the inhibition effect of Nitrosomonas (Ammonia Oxidation Bacteria, short for AOB) and Nitrobacter (Nitrite Oxidation Bacteria, short for NOB). In this research, it was found that Nitrosomonas was much more sensitive than Nitrobacter since the NH4+-N removal efficiency was decreased by nearly 70% by the addition of nanoparticles with size of 23 nm when the silver concentration reached 4.82 mg/L, while NO2--N removal efficiency was merely affected by silver nanoparticles. Nitrification inhibition by silver nanoparticles (AgNPs) on activated sludge process was studied with bench scale sequencing bath reactor (SBR) with four different sizes and four different concentrations of AgNPs. The inhibition effects were not as severe as that of the study with nitrifier but only 30 ~ 40% of reduction of nitrogen removal was monitored throughout the study. This result indicated that the risk of nitrification inhibition due to silver nanoparticles did exist in municipal wastewater treatment process.Further study for the alleviation inhibition was carried out to find the effective alleviating reagent for silver nanoparticles. Inhibition effect by silver nanoparticles could be relieved at a certain extent by inorganic ions rather than by organic ligands, which indicate that, Cl- and S2- could recover about 30% NH4+-N removal capability of Nitrosomonas while organic reagents didn't have evident or even negative effect for relieving the toxicity. With the scanning electron microscope (SEM) imaging, it was observed that the nanoparticles were attached to microbial extracellular polymeric substances and bacteria. It was assumed that the reaction would first happen in the cell membrane between silver nanoparticles and the substance composing membrane. After the reaction, cell membrane would be damaged and then large amounts of particles would then enter the cell, causing the death of cell.