Dynamic Modeling to Assess Worker Exposure to Gas‐Phase Volatile Organic Compounds in Collector Sewers: Fid-Bau Portal

Dynamic Modeling to Assess Worker Exposure to Gas‐Phase Volatile Organic Compounds in Collector Sewers

Parker, Wayne J. / Yu, Jiyoung

A dynamic model was developed to simulate the behavior of volatile organic compounds (VOCs) in collector sewers when released to the sewer as a slug. The model incorporates liquid–gas mass transfer in sewer reaches and drop structures. The model was configured to reflect the structure and flows of a full‐scale collector sewer that had been previously characterized by a tracer study. Selected release scenarios were simulated to reflect potential spills into the sewer system. In the scenarios, the response of benzene, toluene, and tetrachloroethylene were predicted and the effect of sewer structures and flow conditions on headspace concentrations was assessed. It was found that the presence of inverted siphons can substantially reduce gas‐phase concentrations immediately downstream of the siphon. Tributary sewers that contribute contaminant‐free air to the collector can substantially reduce gas‐phase concentrations. Drop structures that enhance liquid–gas mass transfer can create zones of elevated gas‐phase concentrations of VOCs. Contaminants that enter a sewer in the gas phase can be scrubbed into the aqueous phase, thereby reducing gas‐phase concentrations. Application of the model to selected spill scenarios suggests that spills of gasoline into sewer systems may result in worker exposures to benzene that exceed short‐term exposure limits for this substance. Excessive exposures were observed irrespective of whether the compounds entered the collector sewer at the upstream end that had relatively low flow rates or part way along the collector sewer where flowrates were well established. Spills of other common solvents such as toluene and tetrachloroethylene did not seem to be likely to cause unacceptable exposures.