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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Under aerobic conditions, acetate is readily broken down to carbon dioxide and water. It is thus used as a reference compound in OECD guideline 301 (1992) on "Ready Biodegradability" in the form of sodium acetate to check the protocol. Acetate reliably meets the criterium of ready biodegradability (i.e. 70% removal of DOC and 60% of ThOD or ThCO2 production for respirometric methods within a 10-d window within the 28-d period of the test) even when an inoculum is not deliberately added.


The key study of Zahn and Wellens (1980), conducted according to Zahn and Wellens (1974; later on adopted as “Zahn-Wellens-Test” in OECD guideline 302 B “inherent biodegradability”), investigated the biodegradation of acetic acid using 1 g dry weight COD/L of a non-adapted, activated sludge from an organic plant of the company Hoechst, Frankfurt, Germany. After 3 days (including an adaption time of 1 day), acetic acid was degraded by > 90 % with a post-adaption degradation time of 40 % COD/day. The test was valid and sensitive (> 70 % (i.e. 100 %) degradation of diethylene glycol in 14 days, gradual COD and DOC removal, biocidal effects refuted by short adaption time) and confirms that acetic acid is ready biodegradable.

The supporting study of Billen et al. 1980, determined turn-over rates (utilization rates) of acetate by natural microbial populations in estuarine, coastal and seawater after addition of 14C-acetate as a substrate and an adjacent sample incubation at in situ temperature for 4 hours. The results were used to extrapolate mean DT50 values of 0.3, 9.8 and 46 days for estuarine, coastal and seawater, respectively. The results suggest that acetate is ready biodegradable in estuarine and coastal water and that biodegradation is slower in open marine water. However, considering that the data were extrapolated from a 4-h laboratory experiment, the conclusions should not be over-interpreted.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

The key study of Zahn and Wellens (1980) used a method representing a preceding version of the “Zahn-Wellens-Test” in OECD Guideline 302 B “inherent biodegradability”. Therefore, despite its scarce information, the present paper is considered “reliable 2”. According to the most recent version of OECD 302 B (1992) the publication lacks of the following information: temperature, pH, dissolved oxygen concentration, light regime, preparation of inoculum, controls and blank controls, medium, analytical method as well as source and purity of the test compound. Nevertheless, validity and sensitivity were assessable.

The supporting study of Billen et al. 1980 did not follow a specific guideline and used 14C-labbeled acetic acid of an unknown source and purity. It therefore was ranked “reliable 3”.