Air flow in sanitary sewer systems: A physically-based approach.

摘要

An accurate modeling of air flow in sanitary sewer systems is a key input for improved understanding of odorous and hazardous pollutant emissions, efficient design of ventilation systems, and improved evaluation of sewer corrosion. However, wastewater collection systems are designed to convey only liquid flow without regard to what happens to the airspaces. As a consequence, the movement of air into, along and out of sewers is for the most part uncontrolled. This causes odour complaints and points of aggressive corrosion that may be unexpected.; Sewer ventilation models currently in use are generally noted to overestimate the air flow field due to incorrect representation of the driving forces and inappropriate formulation of the physics of the flow phenomena. The purpose of this research therefore is to improve and build upon the existing knowledge-base related to the dynamics of air movement in sewers with a view to providing a design protocol for municipal engineers and environmentalists.; In this study, new system framework based on the results of computational fluid dynamics (CFD), the principles of continuity and work-energy, and the validity of system theory is developed. The driving forces considered are wastewater drag and differential pressure (resulting from wind speed, barometric pumping, dropstructures and auxiliary ventilators). The framework conceptually views the sewer headspace air dynamics as Poisseuille-Couette flow and air exchanges via manholes as orifice flow. The modeling approach considers both turbulent and laminar flow regimes in the sewer headspace. The turbulent modeling takes into consideration the turbulence-driven secondary currents associated with the headspace and hence the Reynolds-averaged-Navier-Stokes equations governing the flow are closed with an anisotropic turbulence model which consists of two sub-models: a generalized eddy viscosity-biharmonic mixing length model for the shear stresses and semi-empirical models for the normal stresses. The resulting formulations are numerically integrated in a finite element framework. The predictive performances of the models are in agreement with experimental data reported in the literature. A quadratic characteristic function is proposed to model dropstructure effects in deep sewer systems. Auxiliary ventilators such as scrubbers and blowers are modelled as fans/pumps of known performance curves.; The framework has been used to lay down groundwork for subsequent modeling of air movement within the Kenilworth sewer system in the City of Edmonton as it relates to odour releases.