LAUSR

Abstract In-situ sludge reduction is an important process in the biological treatment of dyeing wastewater. In this study, an anaerobic-oxic-settling-anaerobic process (A + OSA) was employed for dyeing wastewater treatment under the condition of sludge return to investigate the efficiency of dyeing wastewater treatment and sludge reduction performance. This was compared with an anaerobic-anaerobic-oxic process (AAO) without sludge return. Results showed that treatment of dyeing wastewater with the A + OSA process gave a 30% reduction in sludge and a 90% decolourisation efficiency, which was more efficient than the AAO process. Interestingly, the effluent chemical oxygen demand from the A + OSA process was higher than that from the AAO process. This was due to the inhibition of microbial activity due to the accumulation of intermediate compounds―aromatic amines―caused by long-term sludge return. Moreover, the returned sludge may have been degraded as a co-metabolism substrate, which resulted in the concentration of extracellular polymeric substances in the A + OSA process was higher than that in the AAO process. Additionally, high-throughput sequencing analysis indicated that sludge reduction performance was correlated with the growth of Saccharibacteria. Furthermore, analysis of the enzyme activity showed that dehydrogenase and catalase content decreased in the A + OSA process. This A + OSA process could be further scaled-up and optimized to serve as a promising and effective technology for the treatment of dyeing wastewater.