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

The exposure assessment portion of an environmental risk assessment involve s developing an understanding of the potential fate of materials in the env ironment and the use of this information to determine a predicted environme ntal concentration (PEC). The aquatic risk assessment for detergents in the Netherlands is based on the comparison of the 90th percentile of PECs-calc ulated at 1,000 m downstream the sewage outfall-to the predicted no-effect concentration (PNEC) for ecosystems or maximum permissible concentration (M PC). The 90th-percentile surfactant concentrations at 1,000 m downstream th e sewage outfall can be calculated using information on release, in-sewer r emoval, treatment efficiency, dilution, and in-stream removal. Predicted av erage removals for linear alkyl benzene sulfonate (LAS), alcohol ethoxylate s (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 enteri ng 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. Avera ge measured removal ranged from 99.1% for soap to 99.8% for alcohol ethoxyl ates. Predicted 90th-percentile surfactant concentrations at 1,000 m downst ream 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.