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The city of Healdsburg, Calif. owns and operates a municipal water system in Sonoma County that serves a population of approximately 12,000. Potable water is supplied by the Dry Creek, Fitch and Gauntlett well fields, each with four or five active wells. The Fitch and Gauntlett well fields were constructed in the late 1950s and early 1990s, respectively, and are located along the Russian River.
These two well fields account for more than two-thirds of Healdsburg’s total raw water supply. Raw water from these well fields is categorized as groundwater under the direct influence of surface water. The Dry Creek well field was constructed in 1997-1998 and became operational in 1999. This well field has not been categorized as groundwater under the direct influence of surface water.
Total production capacity of the three well fields exceeds 6 million gal per day (mgd). Peak day demands typically reach 4.5 mgd. Prior to 2005, no treatment other than aquifer filtration and chemical addition (chlorine, ortho-phosphase and fluoride) was provided.
Healdsburg’s water system is topographically challenging with nine to 10 separate water service zones to service its customers. In addition, the infrastructure is aging and was fully depreciated prior to 1998.
Water quality issues
In June 1999, the California Department of Health Services (DHS) reissued Healdsburg’s Domestic Water Supply Permit, which included seasonal restriction on use of the Fitch and Gauntlett well fields. DHS determined these sources failed to consistently and reliably produce water that met turbidity performance standards during times of high river flow. The city was subject to several boil-water notices prior to the commissioning of the Dry Creek well field because of turbidity spikes during storm events.
After commissioning the Dry Creek well field, water produced from two of its highest producing wells was found to contain high levels of manganese (greater than 0.3 mg/L). This was problematic because DHS was expected to change manganese from a secondary to primary standard, which would eventually necessitate manganese treatment at the Dry Creek well field.
Because the Fitch and Gauntlett well fields have been categorized as groundwater under the direct influence of surface water, they are subject to the same regulations typically applied to surface water sources. Water supplied by the Fitch and Gauntlett well fields must conform to requirements of the Surface Water Treatment Rule (SWTR) and Interim Enhanced Surface Water Treatment Rule (IESWTR) during periods of high river flow, and, when adopted by the state, the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). These rules emphasize multi-barrier water treatment that both removes and inactivates waterborne pathogens. Finished water quality treatment standards for turbidity, viruses, Cryptosporidium and Giardia are presented in Table 1.
Water rights considerations
Most of Healdsburg’s established water rights apply to the Fitch and Gauntlett well fields (1949 and 1956 application dates). In 1996, the city certified an environmental impact report and in 1997 applied for additional water rights to support diversions at the Dry Creek well fields. Endangered Species Act listings for coho and chinook salmon and steelhead complicate water rights approvals throughout the Russian River basin. This application is still pending at the State Water Resources Control Board.
After considering water rights and past investments in Russian River water sources, city staff determined that preserving the Fitch and Gauntlett well field source capacity was essential. The approach for preserving this source capacity was to provide additional treatment to meet the SWTR, IESWTR and pending regulations year-round, thereby eliminating seasonal restriction. This approach would allow Healdsburg to maximize use of the Russian River sources to take full advantage of its already established water rights and eliminate the potential for water rights loss through non-use.
Membrane filtration was the only technology considered for this application since it provides a reliable physical barrier against pathogens compared to conventional coagulation/filtration technologies. City staff and HDR conducted a membrane pilot study to evaluate direct membrane filtration using groundwater from the Fitch well fields. Pilot study objectives were to demonstrate compliance for this specific source water application, establish design and operating criteria for full-scale application, demonstrate the use of oxidants for iron and manganese removal, facilitate regulatory approval and provide the opportunity for city operators to become familiar with the technology. Pilot testing occurred between late February and May 2002. Four low-pressure membrane filters, three positive-pressure-driven and one vacuum-driven, were piloted. Pilot testing results demonstrated the following:
Selected systems & bidding strategy
HDR recommended that the city design a full-scale, 2.6-mgd facility that could be expanded to 4.9 mgd by adding additional membrane skids. To minimize energy use and maintenance costs, and provide a system best suited for the selected treatment plant site, the vacuum-driven Zenon Environmental system was eliminated from consideration.
The selected treatment facility site provides sufficient elevation differential to push the raw feedwater (Gauntlett Reservoir) through the membrane modules without pumps. Because the vacuum-driven system is submerged, controls would be necessary to limit flow from the feedwater tank to avoid overflowing the open tank system. In addition, water would need to be pumped from the membranes to the finished water tank (Panorama Reservoir). At that time, the Aquasource pressure-driven system had not received approval from DHS for drinking water applications, so it was also removed from consideration. Bid documents for pressure membrane systems from the Pall Corp. and US Filter/Memcor were fully developed to allow competitive bidding of these two preferred membrane systems.
A new low-pressure membrane, continuous microfiltration-low pressure (CMF-L), supplied by US Filter/ Memcor, was selected based on bid results. Total construction cost for the new facility was approximately $3.7 million. To date, two membrane chemical clean-in-place (CIP) cycles have been conducted to restore TMP to acceptable levels by removing accumulated scale and organic material from the membrane fibers. The CIP cycle can be automatically requested by each skid, but is normally initiated manually by a city operator. CIP cleaning uses sulfuric acid and sodium hypochlorite solutions, which are recirculated in a closed loop through the membrane modules.
At the time of completion, only 13 facilities in the U.S. used CMF-L membranes in various planning, design, construction and operating stages. Healdsburg’s facility has been in operation since October 2005 and has the largest capacity of all 13 facilities. Compared with standard US Filter/Memcor or similar membranes, advantages of CMF-L membranes include higher capacity, lower average transmembrane pressure (TMP) at comparable water production rates, lower backwash pressure and less frequent chemical cleaning requirements. Operating data indicates that CMF-L membranes reduce energy and chemical costs by approximately 15% compared to CMF or other similar membranes.
The Gauntlett well field is the only well site connected to the CMF-L treatment plant. The Fitch well field will be connected to the treatment plant as part of the Phase II effort, tentatively scheduled for 2008. In the interim, the Fitch well field will continue seasonal operation.
Water produced at the Gauntlett well field is injected with chlorine, sodium fluoride and ortho/polyphosphate solution prior to entering the Gauntlett Reservoir. From the reservoir, feedwater flows through the microfiltration treatment plant, and finished water flows to the Panorama Reservoir. Chlorination, fluoridation and corrosion control/sequestration treatment is now housed and dosed next to the Gauntlett Reservoir. However, the city plans to move this equipment and the dosing point to the microfiltration treatment plant.
The microfiltration treatment plant consists of three CMF-L pressure skids, which utilize a polyvinylidene fluoride hollow fiber with outside-in flow. Each membrane skid has 105 membrane modules. Each module contains approximately 9,600 hollow-fiber filtration membranes. The three units provide a combined capacity of 2.6 mgd at an average flux of 34.6 gal per sq ft per day (gfd) with all three skids in service. CMF-L has been credited with a 4-log removal credit of Giardia and Cryptosporidium cysts, and a 1.5-log removal credit for viruses, with a maximum flux of 52 gfd and maximum TMP of 22 psi. The facility is capable of producing the full 2.6 mgd with two of the three skids in service at a flux of 52 gfd. Because the CMF-L technology is an approved alternative treatment technology, year-round operation of the microfiltration treatment plant is permitted, subject to the compliance provisions specified in the city’s permit.
All equipment is housed within a new building situated above the Panorama Reservoir. Water flows by gravity from the 700,000-gal Gauntlett Reservoir, which serves as a dedicated rate water storage reservoir. The treatment train includes two Amiad 500 micron prefiltration strainers, which primarily provide membrane protection.
The authors would like to thank Paul Delphos, HDR Norfolk, for the contributions and insight he provided during the preliminary design, pilot testing and final design efforts.