Sulphate reduction, autotrophic denitrification and nitrification integrated (SANI) process was developed for saline sewage treatment with 90% sludge minimization. This study is further to develop a new SANI, i.e., mixed denitrification (MD)-SANI for beneficial co-treatment of flue gas desulphurization (FGD) wastes with typical sewage through simple wet FGD and energy-efficient sewage treatment. In order to confirm the potential benefits of MD-SANI process, a lab-scale system was operated with mixtures of sulphite, sulphate and synthetic freshwater sewage. Biological co-reduction of sulphite and sulphate in the anaerobic reactor resulted in sufficient sulphide, thiosulphate and residual volatile fatty acids (VFAs) for efficient MD including autotrophic denitrification (AD) on sulphide, AD on thiosulphate, and heterotrophic denitrification (HD) on VFAs in the subsequent anoxic upflow sludge bed (AnUSB). The MD-SANI process increased denitrification rate by seven-time compared with sulphide-based AD in the original SANI. The AD and HD contributed to 72% and 28% of the MD capacity respectively without accumulation of elemental sulphur or reduced sulphur compounds in the final effluent. The MD-SANI process produced sufficient alkalinity not only for its own consumption in aerobic and anoxic reactors but also for potential reuse in SO2 absorption in simple wet FGD practice. The overall sludge yield of MD-SANI process was determined to be 0.03 kgVSS/kgCOD, inheriting the main feature of SANI process in sludge minimization. All these main findings in this study justify simple wet FGD and support co-treatment of its wastes with freshwater sewage to provide potential benefits for both air pollution control and water industries. (C) 2014 Elsevier B.V. All rights reserved.