A screening method for nitrification-inhibition determination has been evaluated at two laboratories, one in Sweden and one in Spain. Allylthiourea (ATU), methanol, chromium and zinc were used as reference toxicants in combination 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 exposed to small amounts of a variety of toxic substances may build up a general resistance to toxic substances. Specific adaptation to certain substances was also observed. Domestic activated sludge responded in the same way to ATU regardless of whether it originated from Sweden or Spain. Synthetic sludge and domestic sludge exhibited the same inhibition of nitrification when tested with ATU as a toxicant. Synthetic activated sludge from two laboratory-scale plants run in parallel did not exhibit exactly the same sensitivity to certain toxicants. The variation in repeated tests with the synthetic sludges was about the same as for repeated tests with full-scale sludge and for other biological toxicity tests. Limits of detection were calculated to be about 5% for a single sample with three blanks as references at both the laboratories.

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