A mathematical model of biodegradability screening tests as an aid to interpretation of observed results

A mathematical model, based on a modification of the Michaelis--Menten and Monod equations describing bacterial growth, has been used to predict the course of removal of the organic substrates in the modified OECD and other screening tests. A range of initial concentrations of bacteria in the inoculum has been selected, using published data on the total bacterial count in secondary sewage effluents and activated sludge, to simulate the wide range of volumes of these materials used in practice. The arbitrary definition of ready biodegradability adopted by the OECD, that is, 5 to the equivalent of 70% DOC removal in not more than 10 days in a total incubation period of 28 days, is expressed in terms of inoculum size and kinetic constants. Modifications are proposed to the screening test with the object of making it more useful and realistic in predicting whether or not a chemical will be removed in sewage treatment.