Evaluation of heavy metal inhibition of activated sludge by TTC and INT-electron transport system activity tests

TTC and INT-electron transport system activity tests were compared for assessing heavy metal inhibition of activated sludge. The median inhibitory concentrations (IC₅₀s) of CU²⁺, Zn²6+, Cd²⁺, Hg²⁺, Ni²⁺, Pb²⁺ and Ag⁺ measured via TTC test were lower than those measured via INT test, which indicates that the INT-electron transport system activity test was less sensitive to heavy metal toxicity than the TTC test. Tested heavy metals brought about similar decrease in TTC-electron transport system activity and COD removal rate, but less decrease in INT-electron transport system activity than COD removal rate, which suggests that the TTC-electron transport system activity is a better parameter for reflecting heavy metal inhibition of activated sludge than INT-electron transport system activity. The ranking of tested heavy metals in order of decreasing toxicity based on TTC test was Hg²⁺, Cd²⁺, Cu²⁺, Ag⁺, Zn²⁺, Ni²⁺ and Pb²⁺, and the ranking based on INT test was Hg²⁺, Ag⁺, Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺ and Pb²⁺.