LARGE DIAMETER SEWER AND TUNNEL VENTILATION CHARACTERISTICS AND ODOR CONTROL: RECENT DEVELOPMENTS AND CASE HISTORIES

The construction and operation of large diameter, sometimes deeply buried sewer interceptors over the past 15 years has proved to be a cost-effective way to divert and move wastewater in collection systems. Regionalization of wastewater treatment facilities has required larger and longer interceptors to reach the plants. Large diameter deep sewers allow the use of tunneling methods which avoid utility interferences and make construction less intrusive to the community. In flat topography, large diameter tunnel sewers allow the de-commissioning of costly and troublesome pumping stations and provide flow relief for widespread areas of shallow collection systems.However, with the advent of large diameter and deep interceptor sewers there has been an increased incidence of odor complaints due to unusual ventilation phenomenon. It has long been recognized by many utilities that their large diameter tunnel sewers can often ventilate in a dramatic and forceful manner. When the public has occasion to experience the exhausted air, odor complaints are the result. We have come to recognize the classical “deep tunnel” ventilation dynamics but the engineering community has had little formal education or hard data upon which to base designs for large diameter sewer ventilation correction. The issue of deep sewer ventilation has received more attention in the past 5 years and researchers are starting to develop relationships between flow volume, pipe diameter and slope, hydraulic friction factors, dropstructure configuration and airflow/pressure in a sewer.Recent work performed by the University of Edmonton, Alberta and the City of Los Angeles has shed new light on the issue of large diameter deep sewer ventilation rates. The success and failure of previous sewer ventilation correction designs have yielded specific empirical relationships between dropstructure design, sewer size, flowrate and sewer ventilation. In addition, refinement of the friction drag coefficients for normally flowing gravity sewers and tunnels has led to more accurate prediction of the full-scale ventilation potential. The City of Los Angeles is so confident of the new sewer ventilation criteria that they are using these relationships to predict odor complaint potential and to size odor control facilities during the design phase of a new large diameter tunnel sewer. Accurate prediction of ventilation phenomena and the size of control facilities are essential to prevent unexpected odor complaints. Pro-active incorporation of sewer ventilation and odor control designs into major interceptor and deep tunnel sewer designs prevents public relations problems following the startup of major public works projects. The Northeast Interceptor Sewer in Los Angeles, the Big Walnut/Rickenbacker Interceptor in Columbus Ohio and the Heights/Hilltop Interceptor in Cleveland Ohio are on-going design projects where in-line controls for odor and sewer ventilation are being incorporated into the preliminary design of the facilities.