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.