Modeling TCE Dynamics in Water Distribution Tanks

Abstract

The paper presents a method to determine the concentration of TCE in a water distribution tank given the influent TCE concentration and the tank fill and drain rates. It then shows how to approximate this solution in the EPANET model. Introduction. Volatile compounds can enter water distribution systems. In typical systems, volatile compounds such as TCE do not significantly degrade either chemically or biologically and the tanks and pipes are dark so there is no photodegradation. The only way to lose a significant amount of these compounds is through volatilization in tanks with a free surface. The purpose of this paper is to determine the amount of TCE (1,1,2-trichloro ethylene, CAS No. 79-01-6) that actually leaves storage tanks and enters the water distribution system given influent concentration and fill and drain rates. This paper will provide the approach for modeling TCE fate in the water distribution system tanks. Details can be found in Walski (1997). First, the equilibrium concentration of TCE will be determined. This will indicate if it is possible for a significant amount of the TCE to leave the liquid phase given an infinite amount of time. Once the equilibrium equations are solved, then a dynamic model of TCE vs. time will be developed first for a tank with a fixed water level to get an appreciation for the rate of volatilizatio n and then for a tank with time varying inflows and outflows which describes the Thomas Road tank. Finally, the detailed results are used to determine an approach for modeling TCE in tanks using the EPANET model. Equilibrium Concentration The key to determining concentration of a volatile compound in a tank is Henry's Law, which states that the equilibrium concentration in the liquid phase is proportional to the equilibrium concentration in the gas phase. The gas phase concentration is usually reported at the partial pressure of the compound in the gas phase. Henry's Law is often written: X = KP