The feasibility of using a single vessel reactor for the biological treatment of recalcitrant compounds, namely reactive azo dyes discharged by the textile finishing industry was investigated. A synthetic effluent was derived from real processes to give reproducibility throughout the experimental period. The literature review found that a sequenced treatment of anaerobic then aerobic redox environments was required for the decolourisation and mineralisation of the dyes. Two biological reactor types were accordingly designed, built and tested, the sequencing batch reactor (SBR) and the hybrid anaerobic baffled reactor (HABR). [Continues.]