Control of Odor Emissions from a CSO Storage Tunnel in Providence, R.I.

The Narragansett Bay Commission operates a large combined sewer overflow (CSO) storage facility in Providence, RI. The facility consists of a 26-ft diameter, 16,250-ft long tunnel having a wastewater storage capacity of over 65 million gallons. The system includes seven drop shafts and two 26-ft diameter ventilation shafts at the upstream end (Foundry Shaft) and at the downstream end, where there is a pumping station. When the facility came on-line in late 2008, odor complaints were received from residents living near the upper Foundry Shaft.Observations and data collected by Narragansett Bay Commission and Louis Berger Group showed the emissions to be worst during cold ambient temperatures due to natural convection, and during CSO tunnel filling events that displaced the odorous air. Air flow measurements during the winter indicated natural ventilation rates from the Foundry vent shaft up to 50,000 cfm with the potential to approach 100,000 cfm. The air was mildly odorous, with a musty sewage character and low hydrogen sulfide levels of 0 to 50 ppb, with spikes up to 150 ppb.Bowker & Associates was retained to evaluate control strategies to treat this air. Pressure monitoring was conducted to better understand the air flow dynamics in the tunnel ventilation shafts, but the results were inconclusive. Both biofilters and activated carbon adsorbers were evaluated. Due to the high air flow rates, the space requirements for the biofilter were prohibitive. Two 30,000 cfm radial flow activated carbon adsorbers were recommended, which would provide sufficient capacity to treat the vast majority of the air during worst-case conditions. The fans were to be equipped with VFD's to achieve maximum flexibility and minimize power consumption. The proposed carbon media was a lignite-based, high H2S-capacity media with a high tolerance for moisture that was expected in the humid air stream. The conceptual design included grease/mist eliminators upstream of each fan.A request for proposals was prepared for a design-build project and in September of 2009, a contract was awarded to the team of Hart Construction and Wright-Pierce Engineers. Construction began in March, 2010, and was completed in in the fall of 2010.The system has achieved its objectives in controlling odors from the CSO storage tunnel. Performance testing showed H2S and odor reductions of 99 percent, and odor complaints have been nearly eliminated. On some occasions in the winter, the natural ventilation rate of the tunnel exceeds the 60,000 cfm capacity of the carbon systems, and “fugitive odors” can be detected near the vent shaft. However, even in such cases, the volume of untreated odorous air being discharged is reduced by at least 80 percent, minimizing impacts on the community. Activated carbon adsorption is an excellent choice for the control of odors from CSO storage facilities that typically have low odor loadings and intermittent use.