Measuring and Modeling Ventilation in Wastewater Collection Systems

As part of the Water Environment Research Foundation study, Odor and Corrosion in Wastewater Collection Systems – Phase II, headspace ventilation was measured at four locations within the Los Angeles county and King County collection systems. A pulse tracer method was used to measure average air velocity within gravity sewer headspaces of pipes ranging in size from 24 to 96 inches. Additionally, related parameters affecting air movement were simultaneously measured in an effort to fully characterize ventilation within the subject pipes. Measured parameters included:Wastewater depth and velocityWastewater temperatureHeadspace air temperature and relative humidityTemperature, relative humidity and velocity of air entering and leaving at manholesAmbient air temperature, relative humidity and barometric pressure.Ambient wind speedThis work is the first attempt to measure full-scale sewer ventilation simultaneously with all parameters influencing ventilation.Additionally, three ventilation modeling technologies were evaluated and compared based on their accuracy in predicting the measured results. Based on observations of the strengths and weaknesses of each model, a new model was proposed which implements conservation of momentum and potentially accounts for several ventilation mechanisms including:Liquid drag at the air/water interfaceBuoyancy-induced ventilation due to air density differencesEduction caused by wind blowing across openings to the sewer headspacePressurization from connected sewer components.The paper will present the methodology, results, discussion and conclusions documenting the experimental work and model evaluation. An improved ventilation model will be proposed and current work to develop this new model will be discussed.