Continued development of a mass balance model of chemical fate in a sewage treatment plant.

Chemicals that pass through a sewage treatment plant (STP) and into receiving waters lead to exposure of human and ecological receptors. Most countries require that such chemicals and especially those that are new to commerce be assessed for their treatability in STPs using a screening-level model. The STP model has been widely used for such assessments in Canada, the US and elsewhere. It is important for both industry and regulators that such a model be simple, accurate and applicable even with the limited data available for most chemicals. The STP model has been upgraded to include the capability to handle ionizing chemicals, and a variety of treatment plant configurations commonly used in Canada and elsewhere around the world. A scheme for obtaining appropriate biodegradation half-lives for the different treatment options from available aqueous biodegradation half-lives or standard biodegradability tests is suggested. Model simulations show good agreement with pilot-scale experimental data from literature for 20 organic chemicals with widely varying physico-chemical properties.