Evaluation of Odor Control Improvement Options at South Cary Water Reclamation Facility
- Kate Keenan, P.E., Brian Porter, P.E., Alan Stone, P.E., Jamie Revels, P.E. - Hazen and Sawyer
The 12.8 mgd South Cary Water Reclamation Facility (SCWRF) is a biological removal plant with filtration that treats wastewater collected in southern Cary and Apex. Similar to many communities in North Carolina, the Town of Cary has experienced rapid growth including the growth around the existing SCWRF. Intermittent reports of odors had become more numerous by the Spring of 2007 and the Town contracted Hazen and Sawyer to identify the source and develop a solution.
The objective of this project was to ascertain what compounds were creating the intermittent odor issues and determine the most appropriate and cost effective solution for their removal.
An Odor Abatement Study was performed to determine the location and speciation of the odor causing compounds. Sampling sites were selected based upon odor causing potential and the EPA Screen 3 dispersion model was applied to determine which locations would reach the property line. Following the odor abatement study, an appropriate odor control measure was selected.
The types of odor control measures that were evaluated included:
• Chemical Scrubbing
• Carbon Adsorbers
The modeling results concluded that the following sources have the potential to be detectable beyond the property line:
• Thermal Dryer Exhaust
• Preliminary Treatment Facility Scrubber Exhaust
• Gravity Belt Thickener
The design phase was focused on the best approach to reduce the concentrations of the odor at the preliminary treatment facility to minimal concentrations. A biofilter with a secondary coconut shell based carbon adsorber was selected because of its ability to reduce hydrogen sulfide concentrations as well as reduced sulfur compounds such as mercaptans. Using both types of technologies together minimizes the break through potential and overall removal limitations of the biofilter while reducing the large O&M cost associated with using only a carbon adsorber.
The system was designed with two carbon adsorbers, each rated for the full flowrate of the system to allow for maintenance. The system also includes a biofilter bypass to ensure the odors are treated when the biofilter is down for maintenance. Construction of the biofilter and carbon adsorbers was completed in August 2009 and the performance to date has been effective.
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