To determine whether chemical additions can be used to reduce sludge production in biological wastewater treatment, 3,3',4',5-tetrachlorosalicylanilide (TCS) was added to activated sludge cultures as a metabolic uncoupler. Batch tests confirmed that TCS is an effective chemical uncoupler in reducing the sludge yield at concentrations greater than 1.0 mg dm(-3); a TCS concentration of 1.0 mg dm(-3) reduced sludge yield by approximately 50%. Substrate removal capability and effluent nitrogen concentration were not affected adversely by the presence of TCS when dosed every other day in a range of 2.0-3.6 mg dm(-3) during the 40-day operation of activated sludge batch cultures. Such sludge growth reduction was associated with the enhancement of microbial activities in terms of the specific oxygen uptake rate and dehydrogenase activity. Sludge settleability of the treated and control samples was qualitatively comparable and not significantly different. Filamentous bacteria continued to grow in sludge flocs only in the control reactor at the end of the 40-day trial. These results suggest that TCS treatment of activated sludge systems may reduce excess sludge yield. (C) 2004 Society of Chemical Industry.