The partitioning of polycyclic aromatic hydrocarbons (PAHs) between the particulate and gaseous phases resulting from the combustion of polystyrene was studied. A vertical tubular flow furnace was used to incinerate polystyrene spheres (100-300 µm) at different combustion temperatures (800-1200 °C) to determine the effect of temperature and polystyrene feed size on the particulate and gaseous emissions and their chemical composition. The furnace reactor exhaust was sampled using real-time instruments (differential mobility particle sizer and/or optical particle counter) to determine the particle size distribution. For chemical composition analyses, the particles were either collected on Teflon filters or split into eight size fractions using a cascade impactor with filter media substrates, while the gaseous products were collected on XAD-2 adsorbent. Gas chromatography/mass spectroscopy (GC/MS) was used to identify and quantify the specific PAH species, their partitioning between the gas and particulate phases, and their distribution as a function of emission particle size. The total mass and number of PAH species in both the particulate and gas phases were found to decrease with increasing incineration temperature and decreasing polystyrene feed size, while the mean diameter of the particles increases with increasing incineration temperature and decreasing feed size. In addition, the PAH species in the particulate phase were found to be concentrated in the smaller aerosol sizes. The experimental results have been analyzed to elucidate the formation mechanisms of PAHs and particles during polystyrene combustion. The implications of these results are also discussed with respect to the control of PAH emissions from municipal wasteto-energy incineration systems.

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Abstract-The environmental fate of octamethylcyclotetrasiloxane (D 4 ) and decamethylcyclopentasiloxane (D 5 ) 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 Ϯ 3.9% for D 4 and 95.8 Ϯ 1.5% for D 5 (Ϯ1 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 D 4 and D 5 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.

A steady-state, advanced, general fate model developed to study the fate of organic compounds in primary and activated-sludge systems. This model considers adsorption, biodegradation from the dissolved and adsorbed phases, bubble volatilization, and surface volatilization as removal mechanisms. A series of modeling experiments was performed to identify the key trends of these removal mechanisms for compounds with a range of molecular properties. With typical municipal wastewater treatment conditions, the results from the modeling experiments show that co-metabolic and primary utilization mechanisms give very different trends in biodegradation for the compounds tested. For co-metabolism, the effluent concentration increases when the influent concentration increases, while the effluent concentration remains unchanged when primary utilization occurs, For a highly hydrophobic compound (partition coefficient Kd > 0.01 m 3 /g VSS), the fraction of compound removed from adsorption onto primary sludge can be very important, and the direct biodegradation of compound sorbed to the activated sludge greatly increases its biodegradation and reduces its discharge with the waste activated sludge. Volatilization from the surface of the primary and secondary systems is important for compounds with moderate to high volatilities (Henry's law constant H, = 0.001 to 0.1 m 3 water/m 3 air), especially when these compounds are not biodegradable. Finally, bubble volatilization can be a major removal mechanism for highly volatile compounds (He> 0.8 m 3 water/m 3 air), even when they are highly biodegradable. Water Environ. Res., 70, 1118Res., 70, (1998.