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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

Key value for chemical safety assessment

Half-life in freshwater sediment:
500 d

Additional information

Ultimate and primary biodegradations of DHTDMAC were assessed in the frame of an OECD 303A test. At least 95% of DHTDMAC are eliminated from waste waters after passing the treatment unit. The fraction of the substance in the sludge decreases continuously. Primary biodegradation is high.

This percentage demonstrates that DHTDMAC is almost completely removed by biodegradation from the wastewater by biodegradation in conventional biological wastewater treatment plants. Following the read-across strategy the same conclusion can be applied toN-benzyl-N-C16-18 (even numbered) -alkyl-N-methyl-C16-18 (even numbered) -alkyl-1-aminium chloride.

Degradation in water

In the European risk assessment, in addition to Boutonnet' 1990 test, reference is made to Hoechst study (Zahn-Wellens-test, OECD 302B with industrial activated sewage sludge), in which DHTDMAC was eliminated to more than 70% after 3 hours. Elimination reached 92 % after 15 days, measured as DOC reduction. A rate of biological degradation could not be determined (Hoechst AG, 1993a.). An OECD-confirmatory test was conducted with DHTDMAC and activated sludge from a domestic wastewater treatment plant (Hoechst AG (1989b). The system was dosed with increasing concentrations of 0.5 -5 mg/L. Based on the concentration of disulfineblue active substance in the effluent of the test system the elimination was higher than 95% after 10 days. Most of the data referring to the elimination in wastewater treatment plants do not distinguish between biodegradation and adsorption. Therefore no degradation constant can be derived but an elimination of 95% is used in the following calculations based on the monitoring data. It can be estimated that about 55% of the elimination are attributable to adsorption.

Degradation in sediment

For degradation in sediments simulation tests are lacking. One test on degradation in river water spiked with sediment. Larson & Vashon (1983) suggests degradation half-lifes in sediment of 80 days or lower. Some experimental details did presumably not represent regular environmental conditions, e.g. sediments were possibly pre-adapted and the concentration of biodegrading microorganisms is regarded to be increased above the normal level.

The available monitoring data reveal that biodegradation in environmental sediments is lower. Hellmann (1995) found an increase of the DHTDMAC concentration at high river flows. As the causes whirling of sediments and rinsing of agricultural soil during strong rainfalls are stated. These results indicate that DHTDMAC adsorbed onto sediments is not or very slowly degraded. A degradation rate cannot be derived from the monitoring data. Therefore, analogously to the degradation in soil, a half-life of 500 d (k = 1.4 . 10-3 d-1) for the aerobic sediment layer is used in the exposure assessment.

There is no hint that DHTDMAC can be degraded under anaerobic conditions and the same assumption is made for N-benzyl-N-C16-18 (even numbered) -alkyl-N-methyl-C16-18 (even numbered) -alkyl-1-aminium chloride.