In this study the feasibility of preparing bioflocculant from excess biological sludge was investigated. Hydrochloric acid was used to disintegrate sludge to prepare bioflocculant. The effects of acid dosage and flocculating conditions were studied. The optimized disintegration conditions was that acid dosage was 10 mL for 50 mL sludge suspension. Factors such as bioflocculant dosage, pH and temperature of the flocculant system were also tested. The optimal conditions were flocculant concentration 3.0% (v/v) and pH 10.5 of flocculating suspension. Under these conditions, 99.5% of flocculating rate for 4 g/L kaolin clay was achieved. Ethanol and sodium hydroxide were applied to purify the crude sludge bioflocculant together or separately. Results showed that sodium hydroxide could separate the bioflocculant from aqueous solution more effectively than ethanol. Analysis of the purified bioflocculant by Fourier-transform infrared spectrophotometer (FT-IR) and chemical methods indicated that the main component was polysaccharide. Performance test showed that the sludge bioflocculant had moderate thermostability. (C) 2012 Elsevier Ltd. All rights reserved.