Occurrence of toxic compounds in the influent to a wastewater treatment plant with nitrogen removal may disturb the process tremendously. Nitrification inhibition tests act as an important tool in the efforts of isolating sources of toxic compounds. However, it is necessary to evaluate the test methods and to know the importance of variable parameters. We set up a complete factorial experiment for 6 different types of waste water, 3 concentration levels of waste water and three different sludges. Statistical treatment of the results showed that the percentage inhibition of nitrification was significantly depending on the type of waste water, on the concentration of waste water and on the combination of waste water and sludge. It was apparent that the effect of the waste water is related to the type of compound or waste water that usually occurs in the sludge.

An interlaboratory study was performed with the primary object of establishing repeatability and reproducibility for nitrification tests (modified MINNTOX). From the test of 7 types of waste water, we found repeatability between 1.1 and 6.4% inhibition and reproducibility between 7.9 and 91% inhibition. In general, the reproducibility was better when laboratories used the same type of sludge than when different sludges were used.

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