Invisible Odor Control: Combining Public Athletic Fields with Nutrient Equalization Basins and Hidden Odor Control Systems

Alexandria Renew Enterprises (AlexRenew) operates a large Wastewater Resource Recovery Facility (WRRF) located in an attractive residential area adjacent to the District of Columbia. The treatment plant already included a centralized 200,000 cfm odor control system that successfully captures and treats odors from the existing Main Plant Site processes. This centralized chemical scrubber system is hidden from site (located indoors) and exhausts treated air through elevated stacks at the roof of a multiple story Solids Processing Building. The current site and buildings aesthetically blend in with typical City of Alexandria architecture. All major ductwork is either buried, indoors, or covered with outdoor canopies.AlexRenew began a State-of-the-Art Nitrogen Upgrade Program (SANUP) in 2009, in order to comply with new stringent nitrogen removal requirements as part of an ongoing effort to protect Chesapeake Bay and the Potomac River. The project included a new Nutrient Management Facility (NMF). The NMF was sited on a newly acquired parcel, the West Plant Site, which was a historic landfill. This facility includes 18 million gallons of covered tankage to store primary effluent, allowing for diurnal nutrient peak shaving and a more consistent feed to secondary biological treatment processes. An athletic playing field is provided on top of the NMF structure for public recreation. As a result of such close proximity to the public, the NMF includes an enclosed and integrated odor control system to mitigate odors from the new tanks. The NMF design was coordinated with high-density residential and commercial development directly to the North, to provide connecting green spaces and public access.The approach to odor control for the NMF was to contain and capture odors from the new NMF tanks. This was accomplished by ventilating the headspace continuously at a rate sufficient to outrun the maximum air displacement during filling of a tank, and maintain negative pressure on the tanks by providing air in-rush into any tank openings.The NMF is based on four tanks with a total storage capacity of 18 million gallons. The odorous air load is 60,000 cfm total, distributed equally to each of the four NMF tanks at up to 15,000 cfm per tank. This results in approximately 1.5 air changes per hour (ACH) on nearly empty tanks, 3 ACH on half-full tanks, and approximately 20 ACH on completely full tanks with 300 fpm capture velocity on all makeup air openings.Odor treatment is provided by four 14-foot diameter dual bed activated carbon systems. The odorous air ductwork, fans and carbon systems are located within the NMF, underneath the athletic field, and hidden from view. Each system includes variable speed fans allowing for operational flexibility in response to maintenance requirements and changing odor loads at the source. The treated exhaust air is conveyed away from the NMF site by underground HDPE ductwork and then exhausted through a remote 50 foot tall exhaust stack that provides the added benefit of stack dispersion.This paper will present the design showing how it is hidden from site, providing a productive community benefit (the athletic field) while also allowing for peak storage of up to 18 million gallons of primary effluent. Odor dispersion modeling was used to ensure impacts to the neighbors met the desired goals set at 99.9% compliance with a 5 Dilutions-to-Threshold (D/T) 1-hour odor impact goal. The supporting modeling effort will also be presented. A rendering of the NMF, athletic field, and surrounding developments is shown in the following figure. The facility has been completed, tested, and is in operation.Image provided by FX FOWLE.