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.