A method of biologically decolorizing wastewater from fiber reactive dyeing of cotton by sequential anoxic/aerobic treatment steps using a single biomass has been developed and evaluated for chemical oxygen demand and color removal. A viable biomass that effectively removes both color and chemical oxygen demand in a sequential anoxic/aerobic treatment process has been developed. This method may be retrofitted to existing aerobic wastewater treatment systems that are typically used to treat dyeing wastewater. (c) 2006 Society of Chemical Industry