Odor Control Management for Combined Sewer Overflow Structures

The provision of enhanced protection against wet weather flow events while improving our ability to minimize nuisance concerns continues to become a more pressing concern. This issue will become more prominent and visible as communities continue to expand and utilize areas surrounding treatment facilities and their related appurtenances. An area that many utilities are increasing their efforts is combating odors related to combined sewer overflow (CSO) events. In an attempt to reduce combined sewer overflows into receiving waters, utilities are building tunnels and storage tanks for these normally high intensity, short duration flow events. These events result in large volumes of runoff water mixed with residential, commercial, and industrial sewage. This combination water during wet weather events poses potential odor concerns due to its storage and transmission. The transfer of water to and from the storage structure combined with the storage phase during a wet weather event can liberate hydrogen sulfide gas from the liquid phase and into the air phase. Presented are case studies from two cities outlining their approaches to odor control management within their CSO systems.The City of Lafayette, Indiana, is in the process of designing a new diversion structure composed of a 5.9 million, underground, CSO Storage Tank. The approximately 300-foot by 150-foot rectangular CSO Storage Tank was designed to best utilize the general site layout constraints and account for the geotechnical and structural requirements deemed appropriate for the site. As the area surrounding the location of the CSO Storage Tank is undergoing various exciting stages of development, a decision was similarly made to adjust the odor control strategy in complimentary stages. In the near-term, the decision was to employ Air Ventilation and Dispersion using a ventilation strategy with strobic fans to evacuate the nuisance odors from the tank and the surrounding area. As the area becomes further developed and a greater population surrounds the CSO Storage Tank, a proposed enhancement of the odor control strategy is the implementation of a targeted, granular activated carbon unit.The District of Columbia Water and Sewer Authority (DC Water) is implementing a $2.6 billion Long Term Control Plan (DC Clean Rivers Project). A primary concern for this system is that many of the related infrastructure is located in residential areas of DC. The system is anticipated to release tunnel air at the upstream termini adjacent to historic row homes when the air would be displaced by rapid tunnel filling and long-term stack effects. Specific focus was placed on the system when it is empty and full to properly size odor control units. The technology selected was radial flow granular activated carbon units to meet the odor treatment needs. Additional design considerations included providing vents to prevent nuisance noise and employing gravity dampers to inhibit fugitive emission when the tunnel is empty.The selection of an odor control management strategy is dependent on the size of a given treatment and collection system, space and budgetary constraints, and the level of odor treatment desired. With a fluid nature of land development that effectively maximizes our land use capability, it is imperative we prioritize our management of CSO flows and related odors to provide the best communities for all.