3.3. Observations with activated sludge: The effects of long-term ENP exposure
Longer-term impacts have been assessed using continuously operated bioreactors, usually sequencing batch reactors. These studies offered a dynamic assessment of reactor performance, microbial activity, and phylogenetic structure. When the threshold inhibition concentration is exceeded, a wide range of NPs can cause severe deterioration of AOBs and/or NOBs (Li et al., 2014; Ma et al., 2015; Li et al., 2017; Yazdanbakhsh et al., 2019). Reductions in species populations have been reported. The relative abundance of Nitrospira species was reduced from 20.1% to 6.9% in the presence of 50 mg/L of bimetallic Ag-Fe NPs (Yazdanbakhsh et al., 2019). Ma et al., 2015 reported that 10 mg/L Ag NPs decreased the normalized amoA gene abundances (e.g. amoA gene/16S rRNA gene) by two orders of magnitude. Gradual reductions in the abundances of Nitrosomonas and Nitrospira were observed in the presence of 10 mg/L of MWCNT NPs (Gao et al., 2019). No effects were observed during continuous loading of CeO2 NP (at up to 20 mg/L) and zero-valent Fe NP (at up to 20 mg/L) (Wu et al., 2013, Ma et al., 2015). Interestingly, the literature appears conflicted on whether heterotrophic bacteria have higher (Jeong et al., 2014; Hou et al., 2013) or lower (Xu et al., 2017) AgNP tolerance, compared to that of autotrophic nitrifiers. The differences in the sludge composition, reactor operation, and NP properties may contribute to the apparent conflict.
Recovery of nitrification is of interest during long term ENP exposure studies. Ni et al. (2013) documented the recovery of TN removal as well as AMO and NOR activity during long-term exposure (i.e. 50 days) to 50 mg/L magnetic NPs. They also found that NAR and NIR activities were not affected after short-term exposure but increased by 23% and 27%, respectively after long-term exposure. A 10-day study showed recovery of ammonia removal 8 days after the start of long term exposure to< 5 mg/L graphene oxide (GO) and graphene nanomaterials (Nguyen et al., 2018). They also observed the recovery of nitrification in spite of the loss of several genera associated with nitrification.
Membrane bioreactors (MBRs) include micro- or ultrafiltration modules which are directly submerged into the activated sludge basins, and they are generally operated to maintain higher MLVSS concentrations than those found in conventional activated process. These systems are now possible due to advances in membrane materials, and they were developed to reduce the facility footprint (by replacing the secondary clarifiers) and improve the quality of the treated wastewater. MBRs appear to provide more operational resilience, relative to the effects of ENPs. Zhang et al (2014) reported that long-term (i.e. 60 days) continuous loading with 0.1 mg/L AgNPs did not affect nitrification in the MBR. Yuan et al. ( 2015) also found that 0.1–5 mg/L AgNPs caused no adverse effects on nitrification (or denitrification) in an anaerobic-anoxic-oxic membrane bioreactor system. The higher resilience of MBR-based nitrification to NP stress may be attributed to the higher mixed liquor suspended solids concentrations in these systems.