More than three years of pilot-plant operation has documented that inhibition of nitrification was found to influence the dimensioning of the largest Danish wastewater treatment plant, which serves a major part of Copenhagen. Hence, a program for investigating the sources of substances inhibitory to nitrification was initiated. Since the number of substances potentially inhibitory to nitrification is very high, the investigations used direct testing of inhibitory effects on nitrification as a tool for the mapping operations.

The overall purpose of the investigation was to determine the types of sources of inhibitory substances and to suggest a program for source control to be implemented by the individual municipalities in the catchment area. This paper describes the strategy for sampling and the results of the first two years of activity. Major conclusions have been that the most important sources of inhibitory substances are to be found among the industries, and that nearly all of the industries investigated exhibited some kind of inhibitory effect Further, is was demonstrated that the toxic unit calculation might be used in the quantification of the sources, and that the observed effects could be explained by the chemical substances in the wastewater from the industries.

Based on the above described results, a strategy for control of the inhibitory effects at the different levels of the catchment area was suggested.