ISO 9509:2006 specifies a method for assessing the short-term inhibitory effect of waters, waste waters or test substances on nitrifying bacteria in activated sludge. The inhibitory effect is estimated over an exposure period of usually 3 h or up to 24 h with weakly nitrifying sludge.

The method is applicable to nitrifying activated sludge derived from domestic and synthetic sewage and also to sludges from industrial and mixed domestic and industrial waste waters.

The nitrifying activity of the sludge is verified by testing in the presence and absence of a specific inhibitor. If the nitrification rate is within a suitable range for the test, i.e. 2 mg of nitrogen per gram of suspended solid and hour to 6,5 mg of nitrogen per gram of suspended solids and hour, the sludge may be used directly. If not, adjustments are necessary.

The method is applicable to water-soluble, non-volatile chemicals, and to waste waters