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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

The mean biodegradation after 28 days of the test substance was 74%, the 10 day window criterion was not passed. [OECD Guideline 301 F, 2010]

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable but failing 10-day window

Additional information

The biodegradability of the substance tributyl citrate was investigated according to EU Method C.4 -D and OECD Guideline 301 F (2010). The study was conducted in compliance with the Principles of Good Laboratory Practice (GLP). A measured volume of inoculated mineral medium, containing the test item in a concentration of 103 mg/L, which corresponds to an oxygen demand of about 206 mg/L (ThODNH4), was stirred in a closed flask at a temperature of 22 °C for up to 28 days. Aerobic activated sludge, micro organisms from a domestic waste water treatment plant was taken as inoculum. The concentration of the inoculum was 26 mg/L. Analytical grade salts were added to deionised water to prepare the stock solutions. Sodium benzoate (purity of 100 %) was used as reference compound. Also a toxicity control (test item and reference compound mixed) was run in parallel, to ensure, that the chosen test concentration was not inhibitory to microorganisms. The degradation rate of the test item was calculated by the oxygen consumption of the aerobic activated sludge microorganisms after 28 days of incubation. As the test item contains no nitrogen, the evaluation of biodegradation has to be based ThODNH4. The criterion for ready biodegradability under the conditions of a manometric respirometry test is the 10-day window, describing the period between reaching at least 10 % degradation and 60 % degradation. This period should not exceed 10 days. The mean biodegradation after 28 days of the test substance was 74 % (ThODNH4); the 10 day window criterion was not passed (12 % degradation after 3 days, 61% after 21 days) and therefore it is considered to be readily biodegradable, but failing the 10 -day window. The reference item sodium benzoate was sufficiently degraded to 85% after 14 days and to 88 % after 28 days of incubation, thus confirming the suitability of the aerobic activated sludge inoculum used. In the toxicity control containing both, the test item and the reference item sodium benzoate, 45 % biodegradation was noted within 14 days and 59 % biodegradation after 28 days of incubation. Thus, the test item can be assumed to be not inhibitory to the aerobic activated sludge micro organisms and all validity criteria of the test method were met.