Pharmaceutical industries in and around Hyderabad produce a wide variety of products using both organic and inorganic substances as raw materials thereby generating a large quantity of complex toxic organic liquid wastes containing high concentrations of inorganic TDS. These wastes are highly toxic to biological life and are usually characterized by high BOD, COD and a high COD:BOD ratio. In the present study an attempt has been made to evaluate the treatability of a bulk drug pharmaceutical wastewater using a lab-scale activated sludge reactor with acclimatized mixed consortia. The application of chemical coagulation as a pretreatment for the removal of sulphates from the effluent was examined with lime as a coagulant to reduce the TDS levels. A 44-48% removal of sulphates was achieved which helped in increasing the efficiency for achieving better biological treatment. The treatability studies with mixed consortia revealed that the acclimatized bacteria were highly effective in treating the waste by achieving a maximum reduction of 86.6% from as high as 4000 mg/l COD concentration. At an HRT of 4.5 days maximum reduction in the COD concentration is found in all the concentrations studied. The biokinetic parameters like Y, K-s, k, mu(max) and k(d) which will be useful to design a prototype activated sludge reactor are evaluated. (C) 2004 Elsevier Ltd. All rights reserved.