Pima County’s Roger Road wastewater treatment plant has a daily flow of 36 MGD, and some of the flow must travel 28 miles before reaching the plant. The plant also receives discharges from a State prison and a Federal prison. In an effort to control odors and septicity, the collection system was treated with three products – Bioxide, sodium hypochlorite and magnesium hydroxide. Even though the arriving flow was being treated with three products to control odors, air phase H2S levels at the head of the plant consistently measured between 50-150 PPM. 

In November 2005, BioCOPE began adding ERI to the head of the plant for local odor control, since much of the collection system is either operated under other departmental budgets or controlled by other cities. Since treatment with ERI began, John Sherlock, the head plant operator, reports air phase H2S levels of approximately 2 PPM in the primary clarifier effluent. 

Several times since treatment began in 2005, Pima County has had one month or longer gaps in which the plant was left untreated. Each time, the odor returned to the same previously high levels and odor complaints resulted. Each time ERI was restarted, the odor was greatly reduced within days. The settling of the clarifiers are much improved with increased water clarity reported by Tucson Water, which reuses 22 MGD for recycled water.