Comparative assessment of oxygen uptake rate of activated sludge and Escherichia coli exposed to nanomaterials

"The adverse effects of engineered nanomaterials (ENMs) on bacterial populations found in wastewater treatment plants (WWTPs) or natural systems have been studied for more than a decade, but conflicting evidence on the matter still makes it a subject of considerable concern. In this paper, the short-term exposure impact of titanium dioxide nanoparticles (nTiO2), carboxyl-functionalized multiwall carbon nanotubes (f-MWCNT), and zero-valent iron nanoparticles (nZVI) toward activated sludge and Escherichia coli (E. coli) was investigated through respiration inhibition experiments. Microorganisms were exposed to nanoparticle concentrations of 50, 100 and 200 mg/L (nTiO2, f-MWCNT) and 20, 50 and 100 mg/L (nZVI). The experiments showed that nTiO2 produced no inhibition in activated sludge or E. coli; up to 100 mg/L of nZVI did not inhibit the activated sludge respiration but 50 mg/L inhibited 24 ± 3% the respiration of E. coli and damaged its cell membrane. Activated sludge respiration was inhibited 17 ± 3% with 200 mg/L of f-MWCNT while for E. coli the inhibition was 36 ± 15% and the cell membrane was damaged with a 100 mg/L dose. Transmission electron microscopy (TEM) showed nTiO2-bacteria and nZVI-bacteria surface interaction while bacteria appeared punctured by f-MWCNT. E. coli was more susceptible than activated sludge to the nanomaterials and nZVI was more toxic than f-MWCNT for E. coli. These results demonstrated the absence of acute toxicity effects of the studied nanomaterials at those concentrations expected to occur in activated sludge facilities, and it would only be a concern in case of extremely high inputs, underscoring the resilience of WWTPs biological treatment."