Workshop Report 10 – Workshop on Biodegradation and Persistence.

WR 10 : Workshop on Biodegradation and Persistence. | September 2007

This two-day workshop sponsored and organised by ECETOC and co-sponsored by the Environment Agency for England and Wales, took place at Cranage Hall, Holmes Chapel, UK on the 26-27 June 2007.

The aim of the workshop was to identify the key areas of research that were needed to further progress made in the assessment of biodegradation of chemicals, during the REACH (Registration, Evaluation and Authorisation of Chemicals) RIP (REACH Implementation Projects) Endpoint Working Group on Biodegradation.  The principle areas covered were those that addressed enhanced screening tests and higher tier tests.

The primary outcome of the meeting was agreement on a number of research activities that, if funding was available (e.g. through the Cefic Long-Range Research Initiative) would help improve the current approaches to assessing the evaluation of the persistence of chemicals in the environment.

Screening studies
The workshop agreed that current tests used to assess ready biodegradability (?ready tests?) were not very good at differentiating persistent chemicals and, because they vary widely in their test conditions and contain many historical differences, they also generate very variable data.  More information could be extracted from ready tests (e.g. shape of the degradation curve) and with the implementation of REACH there is an opportunity to consider systematic standardisation of the ready tests (e.g. substrate concentration, inoculum size etc).

There was a broad consensus that an enhanced tier of biodegradation screening studies are required to aid in the prioritisation of PBT (Persistent, Bioaccumulative and Toxic) and vP/vB (very Persistent/very Bioaccumulative) assessments.  Enhancements discussed included extending the test duration, increasing the test volume, enhancing the biomass levels and allowing for acclimation.  Whilst extension of the test duration and conducting studies using higher test volumes posed little concern to regulatory members of the workshop, it was felt that some validation and standardisation was needed with respect to working with higher biomass levels and acclimated inocula.  It was envisaged that such enhanced tests could contribute to a weight of evidence approach to decide if a chemical is persistent.

The group also decided that, in order to prevent confusion over the terms acclimation and adaptation, the terminology ?deliberate pre-exposure of the inoculum to the test chemical? should be used to describe adapted or acclimated inocula.  Some participants recommended the use of highly adapted systems as a positive screen for persistence i.e. chemicals that could not degrade in such systems can be assumed to persist.  The workshop concluded that adaptation should be taken into account in any assessment of the persistence of a chemical (pre-exposure should be at environmentally realistic concentrations).

It was discussed whether there were new analytical and labelling techniques becoming available that may overcome a) the technical limitations associated with conventional endpoints (O2, CO2 and DOC) and b) the requirement for radiolabelled material to study fate, including biodegradation, at low substance concentrations.

The workshop also addressed the possibility of measuring half-lives from a ?battery? of tests, thus generating a distribution of half-lives.  These could be compared to single values and assessed for their usefulness within the regulatory decision making process.  This approach was potentially very powerful when part of a strategy that involved rapid screening tests and addressed microbial diversity.

Higher tiered studies
There was a general consensus that there are no soil, sediment or water biodegradation studies that accurately simulate biodegradation in the ?natural? environment and the phrase ?simulation tests? was misleading.  There are higher tiered biodegradation studies (e.g. OECD 307, 308 and 309) that use environmental media that can describe degradation under conditions that have a greater environmental relevance than the ready biodegradation test.  However, many of these studies do pose considerable problems associated with their interpretation. Problems identified included:

• Selection of the most appropriate compartment for testing;
• how to deal with bound residues;
• how to discriminate between degradation half-lives for persistency assessments and rates of dissipation from water or sediment;
• how to correct for temperature differences and is it really necessary.

Selecting the most appropriate inoculum is important in generating relevant half-life data but this is not as simple as deciding which compartments most of the chemical ends up in (for example >5% in any compartment signifies realistic presence under Canadian legislation).

It was proposed that the OECD 307 and 308 tests could be extended to 180 days when appropriate to match the P criteria.  There was also an agreement that further discussions and more guidance on the interpretation of results from these studies was needed.