FATE OF OCTAMETHYLCYCLOTETRASILOXANE (OMCTS) IN THE ATMOSPHERE AND INSEWAGE-TREATMENT PLANTS AS AN ESTIMATION OF AQUATIC EXPOSURE

Ambient concentrations of the volatile octamethylcyclotetrasiloxane (O MCTS) in natural waters were predicted using two environmental-fate an alyses: an atmospheric model and a sewage treatment plant model. The a tmospheric-fate analysis utilizes a completely mixed model of the trop osphere where degradation occurs by hydroxyl radical oxidation. The tr eatment plant analysis incorporates removal mechanisms of biodegradati on, volatilization, and adsorption. The treatment train consisted of p rimary clarifiers followed by secondary treatment using an activated s ludge process. Two estimates of influent concentration were used in th e model, one based on total consumption data and the other based on me asured sludge data. Because of the widely varying estimates, a samplin g program was undertaken and the data were used for final effluent est imates. Measured influent OMCTS concentrations of several wastewater t reatment plants indicated that less than 5% of the consumable OMCTS is received by treatment plants. From the atmospheric analysis, the pred icted OMCTS concentration in the ambient water in equilibrium with the atmosphere is from 10(-5) to 10(-8) mu g/L. Using measured influent c oncentrations, the sewage treatment plant analysis predicted average e ffluent concentrations of 0.39 to 0.66 mu g/L. Both analyses predicted concentrations that were less than the lowest chronic no-observed-eff ect concentration (4.4 mu g/L), thus providing no significant aquatic exposure.