Modification of the Zahn-Wellens test: determination of the biodegradability of poorly soluble, adsorbing and volatile substances by measurement of oxygen consumption and carbon dioxide production

Based on the Zahn-Wellens test (OECD 302 B, 1992; DIN EN ISO 9888, 1999), a test system has been developed which enables a continuous and parallel determination of oxygen consumption (pressure measurement) and carbon dioxide production (conductivity measurement). It is a closed test system consisting of a culture flask, a carbon dioxide absorption flask, a pump as well as integrated measuring and control instruments. The air circulating within the test system causes the carbon dioxide present in the test solution to be stripped out completely and directly absorbed by the absorption solution. Avoiding costly thermostatting of the test apparatus, the results of the pressure measurements were temperature-corrected arithmetically. The functional reliability of the measuring apparatus has been demonstrated exemplary in degradation experiments with selected test substances. This new test system also facilitates to test poorly soluble, adsorbing and volatile substances for inherent biodegradability and constitutes an appropriate complement to the standardised Zahn-Wellens test. The Federal Environmental Agency will use it as input to international standardisation activities ongoing within the OECD, as a draft standard.