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ON-DEMAND, GRAVITY POWERED, CERAMIC FILTRATION FACILITY: FORGET WHAT YOU KNOW ABOUT HOW A WTP IS SUPPOSED TO OPERATE. Nathan D. Kutil, HDR, 1715 S. Reserve Street, Suite C, Missoula, MT 59801 [email protected], Ph: 406-532-2239 Introduction The City and County of Butte-Silver Bow (BSB) serves the public water supply system from three sources: the Big Hole River, Moulton Reservoir, and Basin Creek Reservoir. The Basin Creek source has historically operated as a surface water source with disinfection and without filtration. An administrative order from the Montana Department of Environmental Quality (DEQ) has resulted in the need to add filtration to the Basin Creek source. Initial efforts focused on site selection, pilot testing, hydraulic analysis, distribution system modeling, and process selection. Based on these initial efforts, a pressurized membrane filtration process was selected and ultimately the membrane equipment was procured by BSB. The new facility has a firm capacity of 7 million gallons per day. The treatment process includes fine straining, chemical injection with hydraulic jet rapid mixing, pressurized hydraulic detention, ceramic membrane filtration, corrosion control capabilities, and finished water disinfection. Residuals handling processes include inclined plate settling, backwash recovery ceramic membrane filtration, sludge holding, and residuals pumping. The facility houses office space for employees, workshop and maintenance areas, a laboratory, and a large conference room for use as a membrane operators training facility. Background One of the primary design objectives was to minimize energy consumption by maximizing the use of gravity. The site for the new water treatment plant (WTP) was selected based on a hydraulic analysis which allowed placement of the membrane equipment to be at the perfect elevation to accomplish that goal. Other water treatment plants have been designed to operate by gravity, what makes Basin Creek unique is that it operates as an on-demand system. As complicated as this was to design, it is quite elegant and very simple to operate. Staff does not make decisions about how much water to treat in order to meet demand, but rather demand itself sets the flow rate through the WTP at any given moment. Flow control is accomplished by the interaction between distribution system water demand and plant hydraulics. If water demand decreases, it causes pressure in the WTP to rise and the filtration rate decreases. When system demand increases a greater differential pressure between the reservoir and the distribution system causes the flow rate through the plant to increase and match the system demand. In this way, the WTP functions as an on demand facility. Downstream from the filtration process, finished water pumping is not required under most scenarios. The hydraulics (dependent on degree of filter fouling, water surface elevation in the reservoir, and flow rate) allow for complete gravity flow through the WTP and into the distribution system. For scenarios when finished water pumping is required, filter effluent (FE) pumps pull suction from a standpipe which retains any residual pressure available following filtration. The FE pumps are sized to meet both minimum and maximum system demand through the use of variable frequency drives (VFDs) and pump control valves. The FE pumps discharge to a standpipe to provide the exact pressure and flow rate required in the distribution system. The project team developed a plan for overcoming the reality of severe winter weather in the region by procuring under ground piping materials so they would be available to Contractors at notice of award. This allowed Contractors to begin building immediately in a rush to beat the winter weather. The buried hydraulic detention pipeline (HDP) is a 600 foot long 48-inch diameter pipe with internal baffles to promote flocculation and providing 10 minutes of detention time for chemical activation. The HDP internal baffles represent a novel approach to promoting chemical pretreatment in a way that has never been done before. Membrane Filtration The Basin Creek WTP uses a cutting edge ceramic membrane filtration system which is one of the first of its kind to be installed in the United States. The ceramic has many benefits over the polymeric membrane filters that have customarily been used in the US including physical strength, chemical resistance, and longer life expectancy. Another benefit of this filter is the higher overall recovery achievable. The Basin Creek WTP includes a recovery skid designed to filter the backwash waste from the process resulting in an overall 99.95% production capability. In the US membrane market for drinking water filtration, we have grown accustomed to required chemical clean in place (CIP) of the filters on a monthly basis. The CIP interval for the ceramic filters installed at Basin Creek is every 6 months. This means that chemical use is decreased, operation staff labor is cut, and energy use associated with heating chemicals and pumping is reduced by 1/6th over conventional systems. The filtration system will operate on a 4 hour backwash interval with a weekly chemically enhanced backwash (CEB). Flow Control From a chemical pretreatment and disinfection stand point, the on-demand aspect is somewhat problematic because the flow rate through the WTP is constantly in flux and dosing would be erratic in response. For this reason, designers conceived a plan to effectively set the demand in the distribution system to result in a stable daily flow rate through the WTP. By placing a pump station in the distribution system that moves treated water from the Basin Creek WTP to other pressure zones in the City the operators can regain the ability to choose how much water to treat on a daily basis similar to how other WTPs operate. A new pump station in the distribution system is included as part of this project. Since the Basin Creek zone is the lowest pressure zone in the system there will be times when operators choose to pump water produced by the new WTP to upper zones. The pump station will be equipped with a flow meter that constantly monitors the flow into the low zone that the WTP is to serve. The pumps will also have a flow meter and will run on their VFD’s to maintain an operator entered daily flow. For instance, if the operators wish to make 7 MGD and the demand in the low zone is 4 MGD then the pumps will run at 3 MGD. In this way they can effectively set the demand for a stable operating condition at the new WTP while moving water to the upper zones. The added benefit of this pump station is that operators can now select the best water source in various seasons and transfer anywhere within the distribution system. For instance, when the water in the Big Hole River is bad (in the spring runoff), but the water in Basin Creek Reservoir is good, operators can turn down the Big Hole WTP and use more water from Basin Creek using the new pump station to transfer to upper zones historically served by the Big Hole WTP. Conclusion Additional features of the Basin Creek WTP will be discussed in the presentation. When it is all added up, it becomes evident that the Basin Creek WTP is the most reliable, energy efficient, innovative, and technologically advanced WTP in the United States today.