A Staged Approach to Odor Control at the Mason Farm Wastewater Treatment Plant

The Mason Farm Wastewater Treatment Plant (WWTP) is a 14.5 MGD facility serving the Carrboro and Chapel Hill communities in southern Orange County, North Carolina. With the WWTP being located adjacent to the North Carolina Botanical Garden, University of North Carolina's Finley Golf Course, and several residential subdivisions (some within 800 feet of the plant boundary), Orange Water and Sewer Authority (OWASA) has developed an aggressive and multi-faceted odor control program consisting of monitoring, community feedback, reporting, analysis, and a phased series of capital improvements.In 2004, OWASA's Board of Directors passed a resolution which formally adopted a goal of eliminating all objectionable odors from leaving the plant site, and between the years 2000 through 2007 OWASA invested approximately 6.7M in odor control improvements. These improvements included the following:A constructed Biofilter to treat odors from the solids thickening buildingElimination of the trickling filtersCovering and chemical scrubbing of the biosolids storage tanksConversion of floating covers to fixed covers for three digesters and a fermenterInstallation of improved sludge gas conveyance systemsConstruction of a new influent pump station and headworks with headspace odors piped to existing chemical scrubberConveyance of the dewatering building odors to the existing chemical scrubberA 2007 study by another consultant identified three additional capital projects which would be needed to meet OWASA's formal odor elimination goal, including providing odor control at: 1) the primary clarifier influent and effluent distribution boxes, two pump station wetwells, and the aeration basin influent channel; 2) the three primary clarifiers; and 3) the four no-air zone and four first-air zone aeration basin cells. A technology assessment determined that these additional odor sources would be treated using activated carbon adsorbers. The distribution boxes and pump station wetwells were covered and odor control equipment was installed in 2008 and the primary clarifier's odor control improvements are currently under construction. The aeration basin improvements have been designed and are slated for construction in a few years.This paper will describe the staged approach to reduction / elimination of odors at the Mason Farm WWTP over the last decade. The rationale for prioritizing odor control projects and lessons learned during this odor elimination program will be presented.