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Innovative Approaches for Upgrading Atomized Mist Scrubbers

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This paper presents results and recommendations from a case study at the Union Sanitary District in Union City, California, just east of San Francisco, where several atomized mist scrubbers have been controlling foul air streams from several processes for over 20 years. In recent years, the scrubbers have experienced mechanical issues, mostly associated with the atomized mist nozzles, and odor removal has decreased.

Brown and Caldwell conducted an odor evaluation of the Alvarado Plant and provided recommendations to improve the current gas-phase odor treatment system. An extensive sampling and field/laboratory analysis program was conducted for all scrubbers in August 2008, with the goal of assessing current odor removal efficiencies. Air flow rates through the scrubbers are high, averaging about 15,000 cubic feet per minute (cfm) per scrubber, which is the case because most odorous processes are housed within a building and the entire building is ventilated. The high air flows in combination with high odor levels produces potential for offsite impacts, which was confirmed using dispersion modeling.

A proposed improved technology is to retrofit the vessels, if they are structurally intact, by installing new spray nozzles, packing media, and a media support system to convert the scrubbers to packed tower chemical scrubbers, while using the existing chemical delivery system. The volume in the scrubber vessels is more than adequate, as atomized mist scrubber vessels are designed much larger than packed tower scrubbers treating the same air flow. This retrofit is recommended for sources where hydrogen sulfide (H2S) is the main odorant.

To determine if this retrofit could be successful, the District completed packed tower scrubber pilot testing using 300-cfm air streams. A challenge in employing this retrofit is meeting a District goal to eliminate the use of sodium hydroxide (caustic) at the plant. Accomplishing this with packed tower scrubbers was uncertain, as caustic is typically used in chemical scrubbers primarily targeting H2S in the foul air stream; caustic increases the pH and maintain a strong driving force for mass transfer from the gas to liquid phase. Odor removal with hypochlorite solution only was observed in pilot testing to be acceptable in the headworks, primary clarifiers, and aeration basins, which were deemed to be priority odor sources. This technology was also determined to be the most cost-effective solution.
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Keywords: Atomized mist scrubbers; caustic; hypochlorite; packed towers; retrofit; wet scrubbers

Document Type: Research Article

Publication date: 2010-01-01

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