Waste water from flue gas desulphurization by the wet lime-gypsum process is characterized by high contents of nitrate (150–300 mg/l N) and chloride (10–30 g/l Cl) and high temperature (40–50°C). Continuous and batch experiments with biological denitrification were performed with suspended cultures in lab-scale reactors fed with synthetic waste water with chloride concentrations up to 30 g/l Cl. Process temperatures in the range of 30–50°C were investigated. Acetate was added as carbon source. The results of the experiments show that biological denitrification was feasible at the extreme environmental conditions prevailing in wet lime–gypsum flue gas desulphurization waste water. Stable continuous denitrification was performed at chloride concentrations up to 30 g/l and at temperatures up to 45°C. A temperature optimum of 40°C was found for nitrate removal. At 50°C the denitrification had ceased in the reactors. In batch experiments an increased tendency to intermediate nitrite accumulation at increased temperatures and increased chloride levels was observed. This indicates that efforts should be made to equal out load variations in high chloride and high temperature biological denitrification in order to avoid periodical nitrite accumulation.

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