Advancing Odor Control to New Levels Gurnee Sewage Treatment Plant North Shore Sanitary District

The North Shore Sanitary District (District) owns and operates a 24-mgd sewage treatment plant located in Gurnee, Illinois. Frequently, odors from the two-stage activated sludge plant's solids handling facilities made their way into the adjacent residential neighborhood. A new two-stage odor control system for the plant's solids handling facility was put on-line in October 1999. This system consisted of three parallel mist scrubber trains, with each train consisting of two sodium hypochlorite scrubbers operating in series. Although this was the “best available technology” at the time, the District continued to receive odor complaints.The primary goal for the District was to eliminate odors from the scrubber system exhaust, thereby eliminating odors in the adjacent neighborhood. Initially, the new two-stage scrubbing system performance was reviewed and optimized. Following this initial optimization, 90% removal of reduced-sulfur compounds and 99% removal of hydrogen sulfide was achieved with the “optimized two-stage mist scrubber”. Although significant odor reduction had been achieved, odors were still present in the adjacent neighborhood.It was determined that to eliminate odors in the adjacent neighborhood, the “optimized two-stage mist scrubber” would have to remove an additional 90% of organic reduced-sulfur compounds. Two innovative solutions were recommended:Adding a surfactant to the raw odorous air to remove nonwater-soluble odors or to allow them to be sorbed into the scrubber's chemical solution.Adding a high performance packed tower scrubber as a third stage to treat the two-stage mist scrubber's exhaust.To validate whether the recommended solutions would improve odor removal, a pilot test was conducted utilizing the existing two-stage scrubbing system with a packed tower pilot unit for a third stage. The pilot test results indicated a high removal of reduced-sulfur compounds, a noticeable reduction in the opacity of the plume, and a reduction in the perceived odor from the third stage exhaust.Based on the pilot test results, a full-scale design was completed and constructed. Based upon operating data since January 2003, hydrogen sulfide removal in the three-stage system is 99.7%, methyl mercaptan removal is 99.1%, and reduced sulfur compound removal is 99.3%. The final odor concentrations (reduced sulfur compounds) exhausted into the atmosphere from the scrubbing system is an average of 36 parts per billion volume (ppbv), with an average influent concentration to the scrubbing system of 9,869 ppbv. To validate the analytical data from a practical standpoint, the District has received less than 2 odor complaints per year for the solids handling facilities over the last three years.