Classification of biodegradability by growth kinetic parameters.

On the basis of classical Monod equations for microbial growth, kinetic parameter values are assessed to enable the characterization of biodegradability as a substance property. Although growth is a microbial property, the substance selects those species from a mixed inoculum that can profit at most under the prevailing conditions. Hence it is concluded that the combination of observed cell yield Y approximately 0.5 g solids/g COD, maximum growth rate mu max > 0.3/day, and half-saturation constant Ks < 0.6 mg COD/liter provides a suitable set of borderline criteria for classification of chemicals as readily biodegradable. Of these parameters mu max is suitable for differentiating chemicals, whereas Y seems more or less equal for all readily degradable substances and Ks is not easily measurable with existing standard test methods. In this approach the density of specific microbes in the inoculum or during the test is excluded from the substance characterization and thus adaptation phenomena have no influence. From correlation studies and model calculations it can be expected that if the above parameters are estimated on the basis of standard tests for ready biodegradability, these tests will not produce false positives. Provided that the above parameters can be estimated, any other method than just the standard methods can be used to classify a substance as readily biodegradable.