Anaerobic decolorisation of simulated textile wastewater containing azo dyes B. Manu, Sanjeev Chaudhari * Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India Abstract This study deals with the decolorization of the commercially important azo dyes, Orange II C.I. Acid Orange 7) and Reactive Black 3HN C.I. Reactive Black 8) under anaerobic conditions in wastewater. Laboratory scale semicontinuous studies were conductedusingsimulatedcottondyeingwastewateratambienttemperatures24±28 °C)bymaintainingaHRTof10days.Thedye concentrationinwastewaterwasmaintainedat100mg/l.Thereactorswereoperatedfor58daysandOrangeIIandBlack3HNwere easily decolorized under the experimental conditions employed. The performance of the bioreactors was evaluated by monitoring oxidation±reduction potential ORP) in the reactor, color and Chemical oxygen demand COD) removal. Color removal of >99% wasachievedinboththedye-containingreactors.CODremovalsofupto95%,92%and94%wereachievedincontrol,orange-and black dye-containing reactors, respectively. Eect of dyes and salts present in textile wastewater on methanogenesis was evaluated basedonmaximummethaneproductionandmethanogenicactivity.Basedonthemaximummethaneproductiondata,noinhibition of methanogenesis was observed for dye concentrations of up to 400 mg/l for both the dyes. However from the methanogenic activity data, it was observed that the black dye concentration of 400 mg/l seemed to cause inhibition of methanogenesis. Keywords: Anaerobic decolorisation; Simulated textile wastewater; Azo dyes; Methanogenic activity test 1. Introduction Azo dyes are extensively used for dyeing of cotton and constitute about 60±70% of total dyes produced. They are characterised by their typical AN@N-nature and this is the most common chromophore of reactive dyes. About 1000 mg/l of dye is present in a typical dyebath Ince and Tezcanli, 1999). However, due to the poor exhaustion properties of reactive dyes, as much as 40% of the initial dye remains un®xed and ultimately ends up in the dyebath euent Shah, 1998). Dyes are usually made resistant to biological attack, light, heat and oxidation. In a textile industry about 40±65 l of wastewater is generated per kg of cloth produced. Dyeing, desizing and scouring processes are the major sources of water pollution in a textile industry. Waste- waters from a textile industry are characterized by their highly visible color 3000±4500 ADMI units), Chemical oxygen demand COD) 800±1600 mg/l), al- kalinepH9±11)andtotalsolidsTS)6000±7000mg/l). Color and presence of organics in the wastewater are of concern and are to be reduced before disposal. Color is due to the usage of certain dyes during the dyeing pro- cess. Apart from the aesthetic deterioration and ob- struction for penetration of dissolved oxygen into the natural water bodies caused by the presence of color, some of the dyes, dye precursors and dye degradation products are carcinogenic and mutagenic in nature. Depletion of dissolved oxygen content in water bodies can have a serious eect on aquatic life. Hence various wastewater treatment methods such as physico-chemi- cal, biological methods, usually in a combination, are applied to treat the euent to the discharge limits. Nowadays, due to lack of water source and more stringent euent standards imposed by the regulatory authorities, more ecient and economical treatment technologies are required. Also recycling and reuse of the treated water could be a viable option. Existing physico-chemical methods for decolorisation of dyes, advanced oxidation processes like the use of Fenton's reagent H 2 O 2  Fe 2 , hydrogen peroxide, ozonation, are costly in terms of operation costs. Coagulation±