Pilot plant study to assess the fate of two volatile methyl siloxane compounds during municipal wastewater treatment

The environmental fate ofoctamethylcyclotetrasiloxane(D4) and decamethylcyclopentasiloxane (D5) during municipal wastewater treatment has been investigated in an activated sludge pilot plant by continuously dosing the influent with exagerated amounts of the compounds and measuring their emissions in the off-gas, effluent, and sludge streams. The total removal was found to be 86.4 ± 3.9% for D4 and 95.8 ± 1.5% for D5 (±1 SD statistical uncertainty). However, low overall mass balances were found for both compounds in the pilot system. In order to elucidate the detailed removal mechanisms for these two compounds, the model TOXCHEM was used to simulate their removal and analyze their mass balances in the pilot system. Experimental and model results indicated that the low mass balances were caused by the significant underestimation of the primary sludge removal by the grab sampling method used for sludge analyses. This underestimation may be due to the high variability of the primary influent suspended solids concentration and hence inhomogeneity in the primary sludge underflow. When this underestimation was taken into account, primary sludge removal and volatilization in the aeration basin were found to contribute approximately equally to the removal of D4 and D5 in the pilot plant system. The present results are useful for the environmental risk assessment of these two siloxane compounds in the unlikely event of their point source discharge into municipal wastewater systems.