Evaluation of nitrification-inhibition measurements

A screening method for nitrification-inhibition determination has been eval uated at two laboratories, one in Sweden and one in Spain. Allylthiourea (A TU), methanol, chromium and zinc were used as reference toxicants in combin ation with different kinds of full-scale sludge and sludge produced in the laboratory. Different results were obtained with different combinations of activated sludge and toxicants. It was found that activated sludge often ex posed to small amounts of a variety of toxic substances may build up a gene ral resistance to toxic substances. Specific adaptation to certain substanc es was also observed. Domestic activated sludge responded in the same way t o ATU regardless of whether it originated from Sweden or Spain. Synthetic s ludge and domestic sludge exhibited the same inhibition of nitrification wh en tested with ATU as a toxicant. Synthetic activated sludge from two labor atory-scale plants run in parallel did not exhibit exactly the same sensiti vity to certain toxicants. The variation in repeated tests with the synthet ic sludges was about the same as for repeated tests with full-scale sludge and for other biological toxicity tests. Limits of detection were calculate d to be about 5 % for a single sample with three blanks as references at bo th the laboratories.