Exposure modeling of detergent surfactants - Prediction of 90th-percentile concentrations in the Netherlands

The exposure assessment portion of an environmental risk assessment involves developing an understanding of the potential fate of materials in the environment and the use of this information to determine a predicted environmental concentration (PEC). The aquatic risk assessment for detergents in the Netherlands is based on the comparison of the 90th percentile of PECs-calculated at 1,000 m downstream the sewage outfall-to the predicted no-effect concentration (PNEC) for ecosystems or maximum permissible concentration (MPC). The 90th-percentile surfactant concentrations at 1,000 m downstream the sewage outfall can be calculated using information on release, in-sewer removal, treatment efficiency, dilution, and in-stream removal. Predicted average removals for linear alkyl benzene sulfonate (LAS), alcohol ethoxylates (AE), alcohol ethoxylated sulfates (AES), and soap were high (98-99%) and not significantly different, using the specified model input. This implies that-within the limitations of the model-only 1 to 2% of total mass entering the wastewater system will be discharged to the receiving surface waters. The monitoring data confirm the effective removal and degradation of the four major detergent surfactants during treatment inactivated sludge plants and indicate that the model predictions on removal are conservative. Average measured removal ranged from 99.1% for soap to 99.8% for alcohol ethoxylates. Predicted 90th-percentile surfactant concentrations at 1,000 m downstream the sewage outfall-based on actual measured raw sewage concentrations, measured mean wastewater removal figures, and predicted in-stream removal rates-ranged from 0.5 mu g/L for AE to 20 mu g/L for soap.