Toxicity of carbon nanotubes to the activated sludge process: Protective ability of extracellular polymeric substances

The discharge of carbon nanotubes (CNTs) from industrial waste or disposal of such materials from commercial and/or domestic use will inevitably occur with increasing production and enter into wastewater treatment facilities with unknown consequences. Therefore, a better knowledge of the toxicity of CNTs to biological processes in wastewater treatment will be critical. The present study examined the toxicity that multi-walled carbon nanotubes (MWCNTs) incur on the microbial communities present in activated sludge. A comparative study using the activated sludge respiration inhibition test was performed on both sheared mixed liquor and unsheared mixed liquor to demonstrate the potential toxicity posed by MWCNTs and to better understand the extent of extracellular polymeric substances (EPS) in protecting the microorganisms from the toxicity of CNTs. Greater respiration inhibition was observed for the sheared mixed liquor compared to the unsheared mixed liquor. The result suggests that EPS did appear to provide protection to the microbial communities in the activated sludge upon exposure to MWCNTs. The toxicity observed by the respiration inhibition test was determined to be dose-dependent the highest concentration of MWCNTs exhibited the highest respiration inhibition. Scanning Electron Microscopy (SEM) images demonstrated direct physical contact between MWCNTs and bacteria/activated sludge flocs.