Possible use of waste activated sludge (WAS) fermentation liquid as carbon and energy source for the growth and activity of sulfide-oxidizing bacteria (SOB) and nitrate-reducing bacteria (NRB) for the treatment of enriched sulfide and nitrate under microaerobic conditions was studied for the first time. The present bioreactor simultaneously converted 100% of sulfide to sulfur and 95% of nitrate to nitrogen while removing 92% of COD, with loading of 8.52 kg-S.m(-3) d(-1), 2.53 kg-N.m(-3) d(-1) and 25.13 kg-COD.m(-3) d(-1). This outcome was much better than those reported previously in literature. This work demonstrated that WAS fermentation liquid not only was a potential alternative to traditional SOB and NRB substrates but also had direct implication for sustainable operation of SOB and NRB bioreactors for treating enriched sulfide and nitrate. The results of polymerase chain reaction of 16S rDNA analysis confirmed the sulfide oxidation, denitrification and chemical oxygen demand (COD) removal in the micro-aerobic reactor and showed that Sulfurospirillum sp., Thauera sp., Azoarcus sp., Arcobacter sp. and Clostridium sp. were the dominant bacteria in the bacterial community. (C) 2015 Elsevier B.V. All rights reserved.