3.2. Observations with activated sludge: The effect of ENP injections
In activated sludge bioreactors, nitrifying microbial communities co-exist with large populations of the heterotrophic bacteria that are responsible for the removal of soluble organic compounds. Numerous studies using different types of ENPs have examined the short-term impact of ENP injection on nitrification rates in such bioreactors (Giao et al., 2012; Gu et al., 2014; Zheng et al., 2011). Injection of AgNPs may cause intracellular damage and ROS production (Giao et al., 2012; Gu et al., 2014). Dong et al. (2017) reported that a shock load of graphite ENPs can damage the AOB cell wall for Nitrosomonas andNitrosospira . Hou et al. (2013) reported that ZnO NP injections (at 5 mg/L) inhibited nitrifier respiration in activated sludge. Yang et al. (2014) injected 40 mg/L of 35 nm diameter AgNPs, inhibited nitrification, and reduced the relative abundance of Nitrospira sp. from 10 – 12% to <5% during batch tests. Similar results have been reported for SiO2 NPs (Li et al., 2017). Influent pulses of some types of ENPs can damage AOB in mixed culture.