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Proper coagulation and filtrationof the Hetch Hetchy supply at the Sunol Valley Water Treatment Plant (SVWTP) has been challenging due to its relatively lowalkalinity (10 to 16 mg/L), low total organic carbon (1.1 to 1.5 mg/L), and high pH levels(9.2 to 9.7). The high pH results from the addition of lime which is San Francisco Public Utilities Commission's (SFPUC's) corrosioncontrol strategy for compliance with the lead and copper rule and for protection of themortar lining in its transmission pipelines.Consequently, SFPUC completed a number of process and design improvements to theSVWTP in 2003 to facilitate treatment and filtration of the normally unfiltered HetchHetchy supply. Chief among these improvements was the ability to inject carbon dioxidefor attaining the proper coagulation pH. SFPUC subsequently conducted a series of pilottests to evaluate strategies for conventionally treating and filtering the Hetch Hetchysupply using aluminum sulfate (alum) as the primary coagulant. Based on the results ofthese pilot tests, SFPUC has, in recent years, successfully treated the Hetch Hetchysupply under full-scale conditions using one of two distinct treatment strategies:"high" alum dose treatment without coagulation pH adjustment; and, "low" alum dosetreatment with coagulation pH adjustment using carbon dioxide.Treatment using the "high" alum dose strategy involves an alum dose of 7.5 mg/L at acoagulation pH of approximately 7.0 whereas treatment using the "low" alum dosestrategy involves an alum dose of 3.0 mg/L at a coagulation pH of approximately 6.5using carbon dioxide. Although both treatment strategies produce treated effluent in fullcompliance with regulatory performance standards and operational objectives, the "low"alum dose strategy has consistently resulted in higher production levels as measured byunit filter run volumes without compromising effluent water quality as measured by turbidity and total particle counts.

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Edition: Vol. - No. Published: 11/01/2009 Number of Pages: 1File Size: 1 file , 850 KB