The use of the tetrazolium salt XTT for the estimation of biological activity of activated sludge cultivated under steady-state and transient regimes

The tetrazolium salt 3'-{1-[(phenylamino)-carbonyl]-3,4-tetrazolium}-bis (4 -methoxy-6-nitro) benzenesulfonic acid hydrate (XTT) was used as a tool for estimating the activity of the electron transport system (ETS) in activate d sludge cultivated under steady-state and transient regimes in chemostat c ulture. Production of formazan by reduction of XTT depended on the initial concentration of the XTT following a saturation law and was proportional to live cell biomass. Addition of cyanide (KCN) to activated sludge gave an i nitial 1.5-fold increase in XTT reduction, while addition of 3,5-dichloroph enol (3,5-DCP) reduced this value drastically. At steady-state and transien t regimes of an activated sludge chemostat, oxygen uptake rate (OUR) and XT T reduction rate were highly correlated and indicated significant variation s depending on the growth conditions. (C) 2000 Elsevier Science B.V. All ri ghts reserved.