APPLICATION OF THE INTERCEPTOR WASTEWATER DRAG EQUATION AND DISPERSION MODELING TO DETERMINE ODOR CONTROL DESIGN PARAMETERS FOR A SYSTEM OF JUNCTION BOXES

The Trinity River Authority of Texas (Authority) owns and operates the Central Regional WastewaterSystem (CRWS), a large regional system serving over 1 million people in 20 cities in the north Texas area. The Authority's 200-mile regional collection network delivers wastewater flows to a 162-million gallon per day (MGD) advanced secondary treatment plant located in Grand Prairie.The Authority has begun construction of two, large-diameter, relief interceptors that will serve the CRWS wastewater treatment plant (WWTP). These interceptors, the West Fork Relief Interceptor WF-1 and the Elm Fork Relief Interceptor EF-1, along with four existing interceptors, will serve themain influent pump stations at the WWTP. A new junction box, JB 1E, is being constructed outside the plant levee to serve the WF-1 line. A second box, JB 1F, is being constructed to serve the EF-1line and will be interconnected to JB 1E. Two boxes currently exist, JB 1C (above grade) and JB 1D(below grade and well-sealed). As part of this paper, the remaining capacity of the existing foul air collection system (F.A.C.S.) is presented and compared to the projected airflows from JB 1E and JB 1F. Airflows and odorant concentrations were evaluated for both JB 1F and 1E, and design recommendations were made for odor treatment.Airflows for junction box structures JB 1C, JB 1E, and JB 1F were calculated by determining the airflows expected from the sewer headspace and adding a suitable airflow to provide for a slight vacuum within the box under normal conditions, preventing the escape of odorous emissions. When construction is complete, there will be four junction boxes at the WWTP influent area, each interconnected to the others; therefore, the influent area was approached as one large system. After construction is complete on WF-1, there will be 2 — 102” diameter, 2 — 72” diameter, and 2-108” diameter interceptors carrying flow (and foul air) into the junction boxes. Both wastewater and air will flow co-currently into the junction box system.This paper presents the methods for addressing odor control needs for the new junction box structures currently being constructed. At the completion of construction, tests will be performed to determine if the predicted airflows were adequate to contain volume of air entering the boxes from the interceptors. Additional tests will be performed to determine if the odorant concentration in the foul air stream is within the parameters given during design.Airflows were calculated assuming each pipe flowed approximately 42-percent full and the headspaceair velocity was approximately 37-percent of the liquid flow velocity. Airflow calculations were made using a wastewater drag equation for laminar flow proposed by Edwini- Bonsu and Steffler (2004)Foul air quality was developed using test data collected on the existing JB 1C. A logging hydrogen sulfide data collector was placed inside JB 1C for two weeks (August 3, 2004 through August 17, 2004) to determine approximate H2S levels for design. The logger showed an average H2S concentration of 300 ppm with peaks over 1,000 ppm for the period between August 7 and August 17. Reduced sulfur compounds were not monitored but were expected to be present in concentrations sufficient to be perceptible offsite. For design purposes, hydrogen sulfide concentrations entering the odor treatment unit were expected to average 300 ppm with daily peaks up to 700 ppm and occasional peaks over 1,000 ppm, with seasonal and daily variations. In addition to H2S, the odor treatment unit was designed to treat other, non-H2S odors that would typically be present in raw, septic wastewater. Foul air dispersion modeling was conducted to determine the acceptable level of treatment. Model results indicated that the primary odor impact appears to be due to hydrogen sulfide emissions at the junction box area.To reduce initial capital costs, a treatment system was designed for H2S removal only, with provisions made for future addition of a polishing component for treating other odor causing compounds. A 10,000 cfm bioscrubber was designed to treat odors from the bioscrubber area. The existing biofilter system was utilized to treat an additional 5000 cfm from the system.