Probability of Biodegradation, a Novel Concept for Improving Chemical Classification and Risk Assessment

In this article biodegradability is considered as a combination of an inherent substance property, defined as the maximum specific growth rate, μmax and a condition of the environment defined as a specific fraction (ƒs) of the total viable biomass. By proper analysis of test results it is possible to quantify both parameters by one single standard test. Calculations with literature data indicate that for the majority of the degradable substances, μmax may vary between 0.5 and 10 per day. The specific fractions, however, may vary 5 or 6 orders of magnitude and can be as low as 10−8. This concept gives a valuable tool in environmental risk assessment. As the results will be less influenced by test conditions, data will be more reproducible and can be more predictive for a specific environment. The results allow predicting the time needed to achieve adaptation in a treatment plant and, in particular, the behavior under conditions with discontinuous discharge. By using threshold criteria for μmax, ƒs, and percentage mineralization, a new classification scheme with eight different classes for biodegradability is proposed. Compared to the currently used system with two classes, the prediction of biodegradation will be more sophisticated. The proposed system differentiates for six types of inherently degradable substances and identifies those substances with an abnormal growth curve, due to inhibition or toxicity, poor water solubility, or incomplete mineralization. For these classes the proposed equations are not directly applicable and more research will be required to predict their behavior.