# Testing the biodegradability of difficult compounds: a future challenge for the OECD/ISO standardization

## TL;DR

This paper reviews the limitations of current OECD/ISO biodegradability tests and suggests improvements for testing challenging compounds.

## Contribution

The paper identifies key areas for updating standardized biodegradation tests to better handle difficult compounds.

## Key findings

- UVCBs, hydrophobic compounds, and polymers are particularly challenging for current OECD/ISO biodegradation tests.
- Characterizing inocula more thoroughly can improve the reliability of biodegradation assays.
- The OECD system requires re-examination and technical development for future environmental relevance.

## Abstract

For a period exceeding five decades, industrial and scientific communities, in conjunction with regulators, have utilized a complexified, standardized system (e.g., OECD, Organisation for Economic Co-operation and Development; ISO, International Organization for Standardization; ASTM, American Society for Testing and Materials; CEN, Comité Européen de Normalisation) for the estimation of biodegradability of organic compounds. This system has been adopted in numerous countries worldwide and has also been integrated into European legislation (REACH, registration, authorisation, and restriction of chemicals). In recent years, a number of deficiencies have been identified in the standardized biodegradation test systems. This comprehensive review sets out the fields in which improvements are necessary to set up the next generation of reliable, standardized biodegradation tests. The main focus of the review is the challenges and modifications needed to test difficult-to-test compounds such as volatile, hydrophobic compounds, UVCBs (unknown or variable composition, complex reaction products or biological materials), water-soluble polymers, and plastics. Recent advances in the characterization of inocula for biodegradation tests are also addressed, which offer a valuable opportunity to enhance the reliability and reproducibility of biodegradation assays. Moreover, the potential for predicting biodegradation in the environment is a subject that is discussed in this text.

• It is essential that the OECD system of biodegradability tests be subjected to a thorough re-examination and further technical development.

• It is evident that UVCBs, hydrophobic compounds, and polymers present particular challenges in the context of OECD/ISO biodegradation tests.

• It is necessary that inocula for OECD/ISO-based biodegradation tests are characterized in a much more comprehensive manner.

The online version contains supplementary material available at 10.1007/s00253-026-13798-x.