Process Study and Ferric Sulfate Conversion to Comply with the Stage 2 D/DBP Rule
- Gary Iversen, Michael Wang, Vinod Korategere - Hazen and Sawyer
- Chris Smith, Jeff Carlisle - Fayetteville PWC
The City of Fayetteville, NC operates two water treatment facilities: the 40 MGD P.O. Hoffer WTF and the 18 mgd Glenville Lake WTF. Both plants were using alum as a coagulant in 2006 when PWC Fayetteville authorized Hazen and Sawyer to conduct a bench-scale study to evaluate Stage 2 D/DBP Rule compliance at the two plants.
Alum and three other coagulants (ferric sulfate, ferric chloride and polyaluminum chloride), and coagulant aid polymer were evaluated in the bench-scale study. Performance was evaluated in terms of TOC removal and 3-day simulated distribution system THM and HAA formation tests.
Figure 1 is a plot of percent TOC removals for the various coagulants evaluated in the bench-scale study. Figure 2 is a plot of 3-day SDS-DBPs for the same coagulants.
Both ferric sulfate and ferric chloride resulted in significantly lower TOCs and lower SDS-DBPs. Ferric sulfate was selected as the new coagulant for both plants due to corrosion concerns (chloride/sulfate ratio) with ferric chloride. The two plants made the full-scale conversion to ferric sulfate during the summer 2008. Hazen and Sawyer staff were present to assist plant staff during the conversion process. Both plants have performed very well since the conversion, producing high quality finished water.
Figure 3 is a plot of Fayetteville’s TTHM system running annual average. The November 2009 TTHM SRAA of 38 µg/L is the lowest since 1983 and both plants are expected to be in compliance with the Stage 2 D/DBP Rule when the Rule becomes effective in 2012.
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