Regulatory trigger values: Appropriateness, assumptions and gaps in science
Biodegradation testing is not necessarily ‘triggered' by any one condition, but rather considered a base set of tests for ERA. Typically it is conducted in a tiered testing strategy, first to rule out any test substance that is ‘readily biodegradable', then advance to more ‘intermediate' and ‘definitive' chemical specific tests as needed for the risk assessment. It is critical to match the design of a particular test method to the use of the product and its main route of discharge into the environment for the data to be relevant for the risk assessment. Type of product, continuous or intermittent use, continuous or intermittent discharge, the level of discharge for a particular substance and the target compartment are all essential factors when considering what test to conduct and under what conditions.
Activated sludge and biofilm biodegradation tests represent the two most commonly used wastewater treatment systems (activated sludge and trickling filter). The two may give different results for wastewater removal for a given chemical substance, therefore it is important to select the most appropriate test for a given application and/or based on what is generally defined as acceptable in a given regulatory framework.
The sediment biodegradation test does not have an available suite of tests for screening, intermediate testing and definitive simulation testing, especially for substances that are discharged into fresh water rivers from wastewater treatment plants. It does not appear that the OECD 308 can adequately represent all of the discharge scenarios and/or use of products. This may potentially lead to either over- or under-estimation of potential environmental risks. In these instances it is recommended that the parent total system half-life be used for classifying the potential hazard, since the sediment half-life cannot be readily determined in most cases where high non-extractible residues are observed. Further research and method development work is needed.
A number of other knowledge gaps in the science still exist today with some fundamental questions about: i) biodegradation at trace levels vs. the levels typically measured in laboratory biodegradation tests; ii) significance of acclimation, especially for continuous discharges at trace levels; and lastly iii) the potential impact of bound residues to their bioavailability and to their ultimate fate in the environment.