Test No. 224: Determination of the Inhibition of the Activity of Anaerobic Bacteria

This Test Guideline describes a method, which can provide information that is useful in predicting the likely effect of a test substance on gas production in anaerobic digesters. Aliquots of a mixture of anaerobically digesting sludge (20 g/L to 40 g/L total solids) and a degradable substrate solution are incubated alone and simultaneously with a range of concentrations of the test substance in sealed vessels for up to 3 days. At least triplicate sets of bottles for each of a range of concentrations are used. This study includes measurement of pressure (indication about the amount of gas) and pH. The percentage inhibition of gas production brought about by the various concentrations of the test substance is calculated from the amounts produced in the respective test and control bottles (with 3,5-dichlorophenol). The EC50 and other effective concentrations are calculated from plots of percentage inhibition against the concentration of the test chemicals or, more usually, its logarithm.