Odor Control Plan for Large Gravity Sewer Interceptors

The Sub-Regional Operating Group (SROG) is a five-city partnership formed to facilitate the joint ownership and operation of a wastewater conveyance and treatment system in the Phoenix metropolitan area. The cities of Glendale, Mesa, Phoenix, Scottsdale, and Tempe are party to this partnership, which applies to a jointly owned regional wastewater treatment plant (WWTP) and two major interceptors. These two interceptors, Salt River Outfall (SRO) and Southern Avenue Interceptor (SAI), run parallel, east-west collecting wastewater to be ultimately conveyed to the regional 91st Avenue WWTP, located in the southwest corner of the Phoenix area. The SRO and SAI interceptors are each over 20 miles long with pipe sizes ranging from 48 to 90 inches in diameter. As a result of wastewater temperature, travel time, and development, the numbers of odor complaints are rising from residents near the interceptors. SROG conducted odor studies for both interceptors in the year 2003. The studies recommended implementation of odor control facilities to exhaust and treat the headspace foul air at several locations along the interceptors. Odor control measures are in the concept stage and have not yet been implemented.In recent studies, between 2005 and 2007, SROG has identified hydraulic capacity limitations in the SRO and SAI interceptors that may compromise their conveyance efficiency under both dry- and wet-weather conditions. A study is currently being conducted to identify regional municipality specific solutions to manage the interceptor capacity issues. The solution is anticipated to include a combination of measures aimed at reducing inflow and facilities that will permit a controlled discharge of wastewater into the interceptors. As a result, a relatively steady wastewater flow is expected to be achieved in the interceptors. This new flow regime and its impact on pipe headspace air volume are discussed in the paper.Wastewater flow variations in a sewer pipe go accompanied with fluctuations of the headspace air volume. These volume fluctuations increase the potential for sewer headspace pressurization during wastewater level rises, increasing the potential for odor release out of the interceptor (sewer out-gassing). The paper concludes that wastewater and rainfall derived inflow and infiltration (RDII) management programs, typically focused on optimizing the hydraulic capacity of collection systems, should be expanded to include an assessment of their impact on odor release potential along the collection system. Odor generation and headspace air transport modeling should be run in parallel with the system hydraulic model so that appropriate decisions can be made to solve sewer pipe flow capacity and odor release issues in a concerted effort.