Nanotechnology is currently at the forefront of scientific research and technological developments that have resulted in the manufacture of novel consumer products and numerous industrial applications using engineered nanomaterials (ENMs). With the increasing number of applications and uses of ENMs comes an increasing likelihood of nanoscale materials posing potential risks to the environment and engineered technical systems such as wastewater treatment plants (WWTPs). Recent scientific data suggests that ENMs that are useful in, for example, medical applications due to their novel physicochemical properties, may also cause adverse effects to the bacterial populations used in wastewater treatment systems. In this review, the toxicological effects of titanium nanoparticles (nTiO(2)), zinc oxide (nZnO), carbon nanotubes (CNTs), fullerenes (C(60)) and silver nanoparticles (AgNPs) to bacteria were examined. The results suggest that the potential ENMs risks to bacteria are non-uniform (need to be assessed case-by-case), and are dependent on numerous factors (e.g. size, pH, surface area, natural organic matter). Currently available data are therefore insufficient for evaluating the risks that ENMs pose in WWTPs. To fill these knowledge gaps, we recommend scenario specific studies aimed at improving our understanding on: (i) estimated volumes of ENMs in effluents, (ii) the antibacterial sensitivity of cultures within WWTPs towards selected ENMs, and (iii) processes improving the stability of ENMs in solutions. Two factors that merit consideration for elucidating the potential risks systematically are the toxicity mechanisms of ENMs to bacteria, and the influencing factors based on inherent physicochemical properties and environmental factors. Furthermore, the complexity of behaviour and fate of ENMs in real WWTPs requires case studies for assessing the ENMs risks to bacteria in vivo. The current laboratory results derived using simplified exposure media do not reflect actual environmental conditions.

The fate and behaviour assessment of ZnO- and Ag-engineered nanoparticles (ENPs) and bacterial viability in a simulated wastewater treatment plant (WWTP) fed with municipal wastewater was investigated through determination of ENPs stability at varying pH and continuous exposure of ENPs to wastewater, respectively. The ENPs were introduced to a 3-L bioreactor (simulated WWTP) with a hydraulic residence time (HRT) of 6 h at a dose rate of 0.83 mg/min for 240 h. The stability of the ENPs was found to be dependent on their dissolution and aggregation at different pH, where ZnO ENPs exhibited the highest dissolution at low pH compared to Ag ENPs. The results also showed that both ENPs had high affinity for the sewage sludge as they undergo aggregation under typical wastewater conditions. Results of effluent monitored daily showed mean COD removal efficiencies of 71 ± 7% and 74 ± 8% for ZnO and Ag ENPs in test units, respectively. The treated effluent had low mean concentrations of Zn (1.39 ± 0.54 mg/L) and Ag (0.12 ± 0.06 mg/L); however, elevated mean concentrations of Zn (54 ± 39 mg/g dry sludge) and Ag (57 ± 42 mg/g dry sludge) were found in the sludge - suggesting removal of the ENPs from the wastewater by biosorption and biosolid settling mechanisms. Using X-ray diffraction (XRD) and transmission electron microscopy (TEM), the mineral identities of ZnO and Ag ENPs in the sludge from the test units were found comparable to those of commercial ENPs, but larger due to agglomeration. The bacterial viability assessment after exposure to ENPs using the Live/Dead BacLight kit, although not quantitatively assessed, suggested high resilience of the bacteria useful for biodegradation of organic material in the simulated wastewater treatment system.