Wastewater from wet lime(stone)-gypsum flue gas desulphufsation (FGD) processes in coal-fired power plants contains nitrate. Where case selective catalytic reduction (SCR) of NOx is applied the wastewater can also contain ammonia. For the removal of both nitrate and ammonia, biological processes are an attractive option. A bottle-neck for application of biological processes might be the high chloride concentration and relatively high temperature of the wastewater. Therefore research work was performed in fluid-bed reactors at pilot-plant scale for both biological nitrification and denitrification. Biological nitrification was studied up to 34 gCl−/l and nitrite was the main product formed Biological denitrification was effective up to at least 45 gCl−/l. Both nitrate and nitrite were removed effectively.