A chemicals’ ‘Persistence’ is an important attribute in communicating the potential hazard (hazard identification) (ECHA, 2008c) of that chemical as well as in refining its PEC for ERA (EMEA, 2006). Both require data from biodegradation studies to either substantiate potential hazard identification, or to measure degradation kinetics that can be used to refine a PEC. Biodegradation tests may target specific environmental compartments (sewage treatment, aquatic, terrestrial, benthic, and air) and generally fall into 3 tiers of testing, at screening (.ready’), intermediate (.inherent’) and definitive (.simulation’) level, that provide respective answers as to: i) whether a chemical will rapidly and completely biodegrade or not; ii) whether it has the potential to biodegrade; and iii) to what rate and extent does a chemical biodegrade. The three tiers typically progress in complexity and intend to simulate real conditions, specificity of endpoints, time to conduct the study and overall costs (Bowmer and Leopold, 2004; ECHA, 2008a). Overall, one will find a variety of standardised methods available as a result of this dual use of biodegradation data for hazard identification and risk assessment, the potential application of biodegradation testing across a variety of diverse regulatory frameworks (industrial chemicals, biocides/pesticides, fragrances, PCPs, pharmaceuticals), and the 3 levels or tiers of test methods employed. Standardised methods are available from OECD, as well as their equivalents found in OPPTS and MITI.
The next sections provide a high level overview of the OECD test guidelines typically used, whilst noting where certain test methods may be preferred for ionisable substances to what is currently recommended by the industry sector or regulatory framework. The authors acknowledge that, while this is a current topic of interest and debate, it is not the intention to capture all of the perspectives and nuances that may be associated with a given test, class of chemical or regulatory framework.
Standard methods for the measurement of biodegradation (ready: OECD 301; inherent: OECD 302; and simulation: OECD 303, OECD 314, OECD 308 and OECD 309) have been summarised in Appendix E and evaluated in terms of their applicability to ERA of ionisable compounds and support a recommendation as to which method(s) are most appropriate.