Evaluation of TTC-Electron Transport System Activity and OUR Tests for Heavy Metal Inhibition of Activated Sludge

TTC-electron transport system activity and OUR tests were compared for assessing heavy metal inhibition of activated sludge. The median inhibitory concentrations (IC₅₀s) of Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, Pb²⁺ and Ag⁺ measured via the two tests were similar, which indicates that the TTC-electron transport system activity was similarly sensitive to heavy metal toxicity to OUR. Both TTC-electron transport system activity and OUR assessed favourably the changes in COD removal rate under heavy metal inhibition, which suggests that TTC-electron transport system activity and OUR are better parameters for reflecting heavy metal inhibition of activated sludge. The ranking of tested heavy metals in order of decreasing toxicity based on TTC test was Hg²⁺, Ag⁺, Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺ and Pb²⁺, and the ranking based on OUR test was Hg²⁺, Cu²⁺, Ag⁺, Cd²⁺, Zn²⁺, Ni²⁺ and Pb²⁺. Classification analysis via the IC50s of heavy metals based on TTC-electron transport system activity and OUR tests was employed to classify tested heavy metals according to their toxicity. The seven tested heavy metals could be classified into three types: (i) Cu²⁺, Ag⁺, Hg²⁺ and Cd²⁺, (ii) Zn²⁺ and Ni²⁺, (iii) Pb²⁺.