A novel process for anoxic ammonia removal with sulphidogenesis was developed in a 1.75 1 capacity up-flow hybrid reactor in anoxic conditions (ORP = -225 +/- 25 mV) for simultaneous removal of ammonia, organic carbon and sulphate. The process was operated for a total of 240 days. Synthetic feed was used during the first 220 days. After start-up and acclimatization of the process for 120 days, loading rate studies were carried out in continuous operation (at COD/SO42- ratio = 1.3) at HRT of 1 day. The reactor performance decreased at a high organic loading rate of 3.75 kg COD/m(3)/d due to sulphide inhibition. The continuous injection of air at a rate of 0.2 l/min at 40 cm height from the bottom of reactor helped in sulphide oxidation within the anoxic zone and this helped to increase the removals of NH4+, COD and SO42- substantially. Application of the developed process for the treatment of primary treated combined tannery effluent was carried out during the final phase at an organic loading rate of 3.9 kg COD/m(3)/d (COD/SO42- ratio = 1.3) and at an ammonia loading rate of 0.3 kg NH4+/m(3)/d. The percentage removals of NH4+, COD and SO42- were in the ranges of 65.9-89.4, 51.9-70.7 and 70.8-83.1, respectively. The specific rates of sulphate reduction and anoxic ammonia removal of the reactor biomass were 0.36 kg SO42-/kgMLVSS/d and 0.07 kg NH4+/kgMLVSS/d, respectively. (C) 2008 Elsevier Ltd. All rights reserved.