Enhanced Chemical Treatment Calms Perfect Storm of Odor Conditions

The City of Tolleson, Arizona operates a 44.7 cubic meter per hour (m3/hr) [17 million gallon per day (mgd)] capacity, wastewater treatment plant (WWTP). Several residential and commercial developments have been built within ½ kilometer of the Plant and odor complaints have increased. The Plant-wide odor control study was initiated in 2006 to determine the best long-term odor control approach. The initial results of the odor study, plus the urgency of the situation with adjacent neighborhoods, required immediate odor mitigation efforts – specifically by implementing an aggressive chemical addition program utilizing ferric chloride plus hydrogen peroxide. The most significant results and findings of this project were:
• High sulfide concentrations are primarily due to high BOD5 influent in Tolleson interceptor and sludge thickening in the primary clarifiers.
• High dissolved sulfide concentrations in the primary effluent cause high H2S release at the top of the trickling filters.
• Ferric chloride alone provides the majority of sulfide control. Hydrogen peroxide produces an additional 10 to 15 percent dissolved sulfide in the PCs. Hydrogen peroxide provide some additional H2S control at the TFs, however, ferric chloride is the more predominate odor control chemical.
• Chemical dosages are relatively high and will cost the City several hundred thousand dollars per year. Thus, the planned, long-term odor control improvements should be implemented. Faster sludge withdraw and air phase odor control at covered primary clarifiers will greatly minimize the need to add chemical at such high dosages. The cost of operating biological scrubbers will be much less than continued chemical addition.
• Maintaining chemical addition capability is appropriate. The trickling filters cannot be economically covered, and therefore, odor control from the filters must rely on process improvements plus the ability to add chemical as needed.
• This project has clearly demonstrated that chemical addition can be beneficial to TF odors and is a viable means to achieve lower TF odor emissions.                                                                                                                                                                                                                                                                                                                                                                                                                                     Source: WEFTEC 2009 Proceedings