The focusing on the discharge of nitrogen compounds to aquifers has put pressure on power plants to look into removal of nitrogen compounds from the gas scrubbing liquors (DESOx and DENOx). The scrubber liquor has a high salinity, a high chloride content and a temperature of about 40°. Initial laboratory tests to evaluate the possibility of performing biological nitrification and denitrification showed promise and a pilot-scale test unit was established at one of the Danish coal-fired combined heat and power plant (AVEDØRE 1).

The pilot plant was operated under varying conditions with regard to temperature and chloride concentration. Maximum nitrification and denitrification rates were measured. At 30°C and 20 g Cl/l, maximum nitrification and denitrification rates of 2 and 3 mg N/g VSS·h respectively were measured. In the effluent, concentrations of inorganic nitrogen were below 8 mg N/l.

Twice during the test period, inhibition of the biomass activity was caused by high concentrations of heavy metals. The high content of heavy metals was due to malfunction of pH control in the heavy metal removal plant. The nitrification process was more sensitive to heavy metals than the denitrification process, and during the two toxic events the nitrification rate decreased to zero.

The influence of rapid changes in the chloride and nitrite contents on nitrification was also examined. The results from the performed tests can be applied on other wastewaters with similar characteristics as these scrubber liquors i.e. with salinity. As an example fertilizer and pharmaceutical industries can be mentioned.