Decolorization studies of simulated wastewater containing vat dye (C.I. vat blue 1: indigo) and azo dyes (Reactive blue H3R and Reactive red HE 711) have been carried out under anaerobic conditions using mixed bacterial cultures with long hydraulic retention times (HRT). Laboratory scale semicontinuous reactors were operated using simulated cotton dyeing wastewater at ambient temperatures (24-28 degreesC) by maintaining 10-day HRT for azo dyes and 5-day HRT for indigo dye. Influent dye concentration in wastewater was maintained at 100 mg/l and the reactors were operated for a period of 58 days. The performance of the bioreactors was evaluated by monitoring oxidation-reduction potential (ORP) in the reactor, color and chemical oxygen demand (COD) removal. COD removal of up to 95, 90 and 92% was achieved in a control (no dye), blue and red dye containing reactors. Color removal of 98-99% could be achieved in both the azo dye containing reactors. In the indigo dye containing reactor COD removal of 90% and color removal of up to 95% could be achieved. The results indicated the usefulness of semi-continuous reactors for the degradation of recalcitrant compounds like azo and vat dyes. (C) 2002 Elsevier Science Ltd. All rights reserved.