Proactive Planning for Ventilation and Odor Control of a Large Diameter Tunnel System

The criteria by which the public determines the success of a project may vary significantly from the criteria established by those who conceive, design and build the project. The odor impacts of a new project is one example of this disconnect; all too often sewer authorities have found themselves catching up with unexpected odor impacts that cause the public to deem a 'successful' project a failure. The threshold for success in managing odors is often much higher once the public perceives an odor issue. Any major change to a community's sewer system can result in unexpected and negative impacts related to sewer ventilation and odors, and the addition of a large-diameter CSO tunnel is no exception. Akron OCIT The City of Akron's Ohio Canal Interceptor Tunnel (OCIT) is currently under construction and is scheduled for completion in 2020. The proposed OCIT has a 27-foot-ID, mixed face tunnel that will capture and convey dry weather flows. The tunnel receives flow from three (3) shaft locations. In the early stages of the OCIT design, the City chose to take a proactive approach to ventilation and odor control due to the high visibility of the project and the tunnel's planned route through prominent commercial and residential areas. This paper will present a case study of the multiple proactive ventilation and odor control analyses that were performed during the design and construction of the OCIT, which resulted in changes and additions that will help the City of Akron avoid odor-related pitfalls that other cities have encountered. Methods First, a ventilation analysis was performed using field instrumentation to assess odor concentrations and airflow dynamics in the existing system to identify problem areas. This analysis was then expanded to include the proposed tunnel system to determine potential locations that would emit air from the tunnel for the purposes of evaluating odor impact risk. An empirical ventilation model was used to quantify air flow rates within the tunnel as well as near-surface sewers within a range of hydraulic conditions; these results were cross-referenced with hydraulic modeling of the tunnel and sewers in order to estimate frequency, duration and intensity of odorous air emissions at each location. The results of the ventilation analysis and modeling were used to complete a comprehensive facility plan, which provided recommendations for design changes to the tunnel and associated structures to improve control and management of air flow through the system. The facility plan also developed capital costs and footprints for potential odor control facilities. Results The outcomes of these proactive steps were several additions and changes to the design, including: below-grade ductwork at each drop shaft site for the accommodation of future potential odor control facility needs; air curtains to prevent fugitive emissions and the drawing in of additional air; an additional shaft to allow for the release of air at a less sensitive location during wet weather events; modifications to vent vault designs at each drop shaft site; and a ventilation stack at the downtown OCIT-3 site for periodic air emissions expected there. The information provided by these proactive studies allowed the City to be strategic in its planning for managing and, if necessary, treating odorous air flows. In addition to the design changes listed above, the City decided to proactively design an odor control facility at the downstream end of the tunnel due to anticipated constant air emissions at that location. HDR facilitated a technology selection workshop with the City and selected activated carbon as the best technology to meet the City's needs for this site. Conclusion In the absence of the actions taken by the City to understand the ventilation dynamics of its existing and proposed system, it is likely that a difficult odor issue would have developed soon after the OCIT began operations. Instead, through its proactive planning efforts, the City is well-placed to minimize the odor-related impacts of the new OCIT on the Akron community.