As the second tier of tests in OECD standard methods for biodegradability evaluation, OECD 302A-D are designed to determine whether a material is inherently biodegradable. Prolonged exposure of the test substance to microorganisms and a low ratio of test substance to biomass are allowed, which offers a better chance to obtain a positive result compared to tests for ready biodegradability.
Biodegradation percentages above 20% may be regarded as evidence of inherent, primary biodegradability, whereas biodegradation percentages above 70% may be regarded as evidence of inherent, ultimate biodegradability. This indicates that the material is not likely to persist indefinitely in the environment.
If a sample does not pass the 70% threshold indicating inherent primary biodegradability, it may lead to a preliminary conclusion of environmental persistency. The higher tier of tests OECD 303 may be performed to evaluate the degradation under environmentally realistic conditions, or an evaluation of the environmental effects of the degradation products may be performed.
Please refer to the page "Test Method Overview" for more concepts or overview about OECD 302.
OECD 302C Modified MITI Test (II)
Similar to OECD 301D, the OECD 302C Modified MITI Test (II) measures the DO consumption during biodegradation and the degradation percentage is calculated as the ratio of DO consumption to the theoretical DO consumption. This process also involves the calculation of BOD (biochemical oxygen demand).
Compared to other methods in the OECD 302 series, OECD 302C has the top levels of simplicity and applicability.
The formula of the test substance and its purity may be known to calculate the ThOD. If the ThOD cannot be obtained, the chemical oxygen demand (COD) may be experimentally determined and used as a less satisfactory alternative. However, this may lead to falsely high degradation percentages as COD could sometimes be smaller than ThOD if the test substance is not completely oxidized in the COD test.
This method is capable of testing samples that are highly soluble, poorly soluble, volatile, and/or adsorbing. While testing poorly soluble compounds, however, the shaking or stirring of the solutions during incubation might be helpful to improve the dissolution/dispersion.