A PHYSICALLY-BASED MODEL FOR COMPUTING NATURAL VENTILATION RATE IN SANITARY SEWER ATMOSPHERE

Air flow rate in the headspace of wastewater collection conduit is recognized as a primary input for either equilibrium-or kinetic-based mass transfer modeling. Calculation of the air flow rate also allows the estimation of the volume of air capable of being forced out through openings such as manholes which in turn allows the determination of the size and cost of odorous air treatment alternatives. However, sewer ventilation models currently employed in the emission estimates are noted to overestimate the air flow rate due to the incorrect representation of the driving forces and inappropriate formulation of the physics of the flow phenomena. In this paper, the natural driving forces of wind speed, barometric pressure and wastewater drag are considered. Generalized curves and formulae for computing the air flow rate based on the results of computational fluid dynamics (CFD) modeling are presented. The governing equations are those of the momentum and continuity equations. Turbulence closure is achieved with the use of a generalized eddy viscosity mixing length model and semi-empirical relations. The effects of wind speed and barometric pressure are modeled as dynamic pressure gradient whereas that of wastewater is treated as a Dirichlet boundary condition. The resulting formulations are numerically integrated in a finite element framework.