Response of sOUR and AUR to heavy metal contamination in activated sludge

The aim of this work was to make a contribution to the knowledge of inhibition by heavy metals of both nitrifying and heterotrophic bacteria in the activated sludge, based on measurements of ammonium uptake rate (AUR) and oxygen uptake rate (sOUR) activities. For the inhibition tests, we used different concentrations of the following heavy metals: Cd 2+, Cr 6+, Cu 2+, Pb 2+ and Zn 2+, and the toxic effect of each metal was measured after 1 and 24 h exposure. Tests were performed on activated sludges from a full-scale nitrifying plant. The inhibitory concentrations were evaluated for each ion and in a decreasing order of toxicity two groups were found: Cd>Cu>Zn and Pb>Cr. Nevertheless, experimental results indicated that the response of AUR and sOUR to the tested metals was different. Higher AUR values than sOUR ones were observed in the tests with Cd 2+, Cu 2+ and Pb 2+, indicating a lower sensitivity of nitrifiers to these heavy metals in comparison with heterotrophs. Inhibitory effects of Cr 6+ and Zn 2+, were quite similar on both AUR and sOUR activity. Our results suggest that in nitrifying activated sludge plants the sOUR test can be used better than AUR test to identify possible inhibitory effects of heavy metals from wastewater.