Zahn-Wellens Test

OECD 302 B, Regulation (EC) No 440/2008, C.9, DIN EN ISO 9888, German Wastewater Ordinance No. 406, 407, 408

A) Single substance test

The “Zahn-Wellens test” is a classical test to determine the inherent (in principle possible) degradability of water-soluble, non-volatile organic substances. The test substance is incubated in a mineral medium with relatively high amounts of activated sludge (0.2 to 1.0 g/l) as inoculum during 28 days as a batch test. The test concentration varies between 50 and 400 mg/l DOC or 100 and 1000 mg/L COD, respectively. Samples are taken in regular intervals and the decrease of DOC or COD in the filtrated samples is measured over time. The portion adsorbed to the sludge is estimated from a sample taken after 3 hours. It will be used also as a reference value for the calculation of the biological degradability. At the same time, the blank value of the activated sludge, the degradability of a reference substance and the abiotic elimination of the test substance (intoxicated sample without activated sludge) are each determined in at least one additional test vessel.

B) Wastewater evaluation

Concerning wastewaters, the „Zahn-Wellens test“ is used to determine the refractory (slowly degradable) COD or DOC. The test design corresponds to that used for the single substance test. Elimination extents are related to the (calculated) COD or DOC values, respectively, at test begin while considering the stripped part (as determined in the abiotic control). Due to the fact that it is not possible to distinguish between elimination caused by biological degradation and adsorption-induced elimination, the test result is defined as "elimination extent”. Two different test versions are used:

No. 407, test duration corresponding to the period required to reach the elimination degree of the total wastewater.

No. 408, test duration of 7 days.

The degree of elimination for municipal wastewater is usually obtained by test simulation after 2-3 days, but has just a limited explanatory power due to the portion adsorbed (for municipal wastewater between 40-60%). For this reason, the test duration in our laboratory usually is fixed to 7 days.