Treatment of exhausted reactive dyebath effluent using anaerobic digestion: Laboratory and full-scale trials

Reactive dyes are difficult to remove from textile waste water due to their solubility and they pass through conventional aerobic biological sewage treatment systems and enter the receiving water body. Investigations into the use of anaerobic digestion to decolourise reactive azo dyes have been successful on a laboratory scale and the investigation was extended to full-scale trials. Exhausted reactive dyebath effluent (3 kl/d) was discharged into a primary digester (1.34 Ml) on weekdays for a 151-d period. On average, 48 kl/d of sludge was fed to the experimental and control digesters. The overflow was monitored for colour, sodium and sulphide concentrations. A laboratory digester was also set up to simulate the full-scale conditions but was operated at twice the exhausted dyebath loading recipe. No visual difference in colour was noted between the overflow of the primary or laboratory digester and the control digester, but elevated levels of sodium and sulphide were obtained due to the high concentration of sodium sulphate used in the reactive dyeing process. The laboratory digester became unstable at sulphide concentrations of 400 mg/l. However, the sulphide concentrations in the primary digester never increased such that it threatened digester stability.