Agenda - Council Work Session - 09/13/2022

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7.5 7.6 7.7 7.8 7.8.1 Pilot Study Results A pilot study was performed by John Thom of SEH of Ramsey's water in January 2020. The Pilot Study Report is included in Appendix E. The objectives of the pilot study were to evaluate the effectiveness of detention time prior to filtration, and to determine the optimal filter media. The pilot study found no significant difference between direct filtration and utilizing 30 minutes of detention time prior to filtration, and found no significant difference between the silica sand/anthracite and greensand/anthracite filter media. Because the manganese oxide coating on manganese greensand filters is helpful for manganese removal, greensand/anthracite filter media is recommended. Filter Sizing The required filter area is determined by dividing the nominal filtration capacity by a flux rate (filtration rate). Ten States Standards requires sand filtration rates from 2 to 4 gpm / ft2. Because the required filtration capacity is 10 MGD under normal operating conditions, the facility will be designed for 10 MGD at 2 gpm / ft2. Therefore, if 15 MGD is necessary for short periods of time, the filtration rate will still be in the acceptable range. With a capacity of 10 MGD and a filtration rate of 2 gpm / ft2, it is necessary to have 3,200 ft2 of filter media. To have reasonable backwash rates and operational flexibility, this will be broken into eight filters. Operator Input Ramsey operators and City Staff toured existing water treatment plants in Andover and Brooklyn Center as part of this feasibility study. Operator feedback from the tours was gathered and incorporated into the building layouts discussed in the following sections. Treatment Alternative 1 - Gravity Filter Layout General In an iron and manganese gravity filtration system, water to be filtered is pumped, under low pressure, to the treatment facility where it flows by gravity through the various treatment processes. Following the oxidation process, the water flows through the filter cells from top to bottom. As the water passes through the filter media, the insoluble particles of iron and manganese are removed. As more and more water is filtered, the restriction to flow, created by the accumulation of iron and manganese solids on the media, steadily increases. In a gravity facility, this restriction to flow, called head, is measured in feet of water depth in the filter cells. As the solids accumulate, the depth of water in the filter cells increases. Due to the physical nature of a gravity filter, when the depth of water in a cell reaches its maximum designed head (high water level) backwashing is required. Failure to backwash at the proper time could result in the filter overflowing or poor effluent water quality being produced. Gravity filters are typically constructed of concrete or steel. Steel filters are generally found in smaller water systems. Because of the large size of the filters required for Ramsey, steel filters are not being considered. FEASIBILITY STUDY RAMSY 154354 Page 29