OECD 302B Inherent Biodegradability – Zahn-Wellens Test

    Introduction of OECD 302 B

    The OECD 302 B test, or Zahn-Wellens test, assesses the inherent biodegradability of organic compounds under optimal aerobic conditions. It is not a ready biodegradability test (in the OECD 301 series), which is a measure of rapid decomposition, but rather a measure of whether or not a substance will biodegrade, given enough time and acclimation. It is typically used in regulatory assessments of chemicals that are unlikely to pass the ready biodegradability criteria, but do not pose a meaningful persistence risk.

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    Test Method of OECD 302B

    Test procedure This test evaluates the amount of dissolved organic carbon (DOC) removed or the chemical oxygen demand (COD). The percentage of degradation is calculated from the ratio of DOC removed or COD removed to the original DOC applied, or the initial COD introduced.

    OECD 302B Sample Preparation

    Specimen DetailsA mixture is created that includes the test substance, mineral nutrients, and a considerable quantity of activated sludge. 
    Incubation ConditionThis mixture is stirred and aerated at a temperature range of 20–25°C, either in the absence of light or under indirect lighting, for a period of up to 28 days. 

    OECD 302B Results and Interpretation

    The concentration of DOC or COD is evaluated periodically to determine the extent of biodegradation. The test is considered valid if the reference compound shows a removal rate of at least 70% within 14 days, and the DOC or COD in the test suspension declines steadily over time, signifying biodegradation.

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    Materials Commonly Tested with OECD 302B

    Commonly used for testing chemicals like surfactants, personal care products, and industrial compounds.

    OECD 105 determines the solubility of water.

    Updated on September 17, 2026

    Divakar Shukla
    About Author
    Divakar Shukla
    Divakar Shukla is an Electrical and Electronics Engineer specializing in the convergence of embedded systems architecture, industrial automation, and applied artificial intelligence.
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