Towards rationally designed hazard, risk and persistency assessment: putting the “Bio” back into biodegradation testing

Russell Davenport 1 , Andrew Goodhead 1 , Timothy Martin 1,2 , Jason Snape 2 , Jon Ericson 3 , Torben Madsen 4 1 Newcastle University, UK; 2 AstraZeneca, UK; 3 Pfizer, USA; 4 DHI, Denmark

Mitigating the risks that manufactured chemicals pose to the environment and human health is a major global concern and one of the greatest challenges for the 21st Century. Regulatory emphasis has recently shifted to identifying and prioritising chemicals that are persistent, liable to bioaccumulate and are toxic (PBT e.g. REACH). Chemicals with these properties have previously been shown to be those most harmful to human health and the environment. Biodegradation is one of the most important but poorly understood fate processes that determines persistence. It is often measured experimentally by observing the degradation of a chemical substance in the presence of a bacterial inoculum. In reality it should be acknowledged that Ready Biodegradability Tests (RBTs) are notoriously variable. For example, microbial concentrations in inocula can vary by 4 orders of magnitude.

RBTs have been the central foundation for understanding the biodegradation of chemicals in regulatory frameworks for hazard and environmental risk assessments for 2-3 decades. They are highly prescribed, standardised and conservative regulatory tests that measure the relative biodegradability of chemicals (e.g. OECD 301 tests). RBTs rely on the probabilistic inclusion of specific degraders in the test system, but have a high failure rate and are highly variable largely due to the use of low inocula concentrations. Together with their short duration, this makes them unsuitable for persistence assessments.

REACH guidance which advocates the introduction of a new tier of enhanced tests to enable efficient and effective identification of persistent chemicals (ECHA, 2008). Reliable extrapolation from any small-scale systems to predict local and regional environmental impacts depends on incorporating environmental realism into test systems, which includes the nature of the microbial populations present. Enhancements may therefore include increasing inocula to environmentally-equivalent concentrations, and thereby the microbial diversity, to levels likely to be met by a given chemical in the environment.

Our CEFIC/LRI ECO 11 project has been investigating how variations in inocula concentration, community composition, and diversity, relate to biodegradation variation and reliability. In addition, the bias and pragmatism of different methods to concentrate cells in inocula for enhanced tests has been assessed. It was found that enhancement of activated sludge inocula concentrations had a greater effect on reliability than test volume in scaled-up biodegradation tests carried out using GLP (Good Laboratory Practice), but not necessarily for marine inocula. Finally, these enhancements using a set of reference chemicals chosen by CEFIC/LRI ECO 12 project for this purpose were validated.