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

Ambient concentrations of the volatile octamethylcyclotetrasiloxane (OMCTS) in natural waters were predicted using two environmental-fate analyses: an atmospheric model and a sewage treatment plant model. The atmospheric-fate analysis utilizes a completely mixed model of the troposphere where degradation occurs by hydroxyl radical oxidation. The treatment plant analysis incorporates removal mechanisms of biodegradation, volatilization, and adsorption. The treatment train consisted of primary clarifiers followed by secondary treatment using an activated sludge process. Two estimates of influent concentration were used in the model, one based on total consumption data and the other based on measured sludge data. Because of the widely varying estimates, a sampling program was undertaken and the data were used for final effluent estimates. Measured influent OMCTS concentrations of several wastewater treatment plants indicated that less than 5% of the consumable OMCTS is received by treatment plants. From the atmospheric analysis, the predicted OMCTS concentration in the ambient water in equilibrium with the atmosphere is from 10(-5) to 10(-8) mu g/L. Using measured influent concentrations, the sewage treatment plant analysis predicted average effluent concentrations of 0.39 to 0.66 mu g/L. Both analyses predicted concentrations that were less than the lowest chronic no-observed-effect concentration (4.4 mu g/L), thus providing no significant aquatic exposure.