In connection with the upgrading of the two largest wastewater treatment plants in the Copenhagen area to nutrient removal special attention has been paid to the nitrification process regarding inhibition effects. Inhibitory substances in the wastewater could be identified by simple batch tests, and the long-term effects on the nitrification process were tested in pilot plants or at full-scale. A distinction could be made between effects produced by wastewater from external sources in the catchment area and internally circulated flows in the wastewater treatment plant. Results from programmes monitoring the influent to the Lynetten WWTP and the Damhusåen WWTP and the catchment areas have revealed that discharges from industries are to be considered the most important external sources of inhibition. The load from the external sources has decreased during the investigation period, and since 1993 the nitrification capacity monitored at the pilot plants has been in agreement with the design basis. The recycling of the scrubber water from the cleaning of sludge incineration flue gas was found to be an important internal source of inhibition at the Lynetten WWTP. Investigations show that it is possible to reduce the effect by modifying the existing incineration process and, if necessary, combining it with separate treatment.

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