Exposure calculation under REACH (REACH Reality check)
The exposure calculation using the EUSES default biodegradation rate for readily degradable substances [24/d] predicts for the total AS volume an EU average Clocal effluent = 413 µg/l, whereas the US monitoring data result in an estimated EU average Clocal effluent = 11.8 µg/l. This indicates that the EUSES default biodegradation rates for readily biodegradable substances are very conservative, resulting in this case in a 35-fold overprediction of the local effluent concentrations.
Taking this into account, the default biodegradation rate of [24/d] was not used for the exposure calculation in the REACH registration dossiers. Instead, and based on monitored high elimination rates of AS in WWTP simulation tests of > 99 %, a more realistic biodegradation rate of [192/d] was applied. This inceased biodegradation rate resulted in an EU average Clocal effluent = 51.6 µg/l, which is still by a factor of 4-5 higher (more conservative) than the predicted effluent concentration from monitoring data. The use of realistic removal kinetics based on monitoring data is recommended for use in registration dossiers for AS as well as other surfactant categories. Furthermore, the case study illustrates the conservative nature of exposure assessment with the EUSES model, which is also an important element to consider for mixtures assessments.