Corrosion Control Study at Fayetteville’s P.O. Hoffer WTF
- Vinod Korategere, Gary Iversen, Michael Wang - Hazen and Sawyer
- Jeff Carlisle, Chris Smith - Fayetteville PWC
During the summer of 2008, the City of Fayetteville’s two water plants (the 40 MGD P.O. Hoffer WTF and the 18 mgd Glenville Lake WTF) switched their primary coagulant from alum to ferric sulfate to comply with the Stage 2 D/DBP Rule. Bothe plants have performed very well since then with lower finished water TOC levels and lower TTHMs and HAA5s in the distribution system. Both plants are expected to be in full compliance with the Rule when it becomes effective in 2012.
The Hoffer WTF uses zinc orthophosphate as the corrosion inhibitor chemical. The change in primary coagulant represents a significant change in finished water quality, and the NC DENR PWSS requires that a corrosion control study be conducted to optimize corrosion control under the new conditions. Fayetteville PWC authorized Hazen and Sawyer to conduct a corrosion control study at the Hoffer WTF.
A comprehensive, nine-month corrosion control study was conducted at the Hoffer WTF from April 2009 to January 2010. A custom-built corrosion pilot plant with five trains (to test five corrosion inhibitors) was used. Train 1 was the control train using zinc orthophosphate at 1 mg/L as phosphate (current dose). The other four trains used zinc orthophosphate at 2 mg/L, orthophosphate without zinc, and two blended phosphates (66/34 poly/ortho and 30/70 poly/ortho). Figure 1 is a photograph of the corrosion pilot plant.
Finished water from Hoffer WTF before the corrosion inhibitor chemical addition was used as feed water to the pilot plant. Lead, copper and mild steel coupons were inserted in each train. The coupons were pulled out after 3, 6 and 9 months to analyze corrosion rates during the course of the study. A copper pipe loop with 40 lead solder joints was part of each pilot train. 8-hour standing samples were collected from the copper pipe loops over the course of the study to evaluate aqueous phase lead and copper levels. The coupon results indicated that the orthophosphate trains had minimal corrosion rates for lead, and minimum to mild corrosion rates for copper and mild steel. The aqueous lead and copper analysis confirmed that the orthophosphate trains (1, 2 and 3) had lower levels and all five trains had low copper levels. The study concluded that Hoffer WTF corrosion control was optimized and recommended that no change was required due to the coagulant conversion from alum to ferric sulfate.
Figure 2 is a plot of the lead coupon results. Figure 3 is a plot of the aqueous lead results.
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