Agenda - Council Work Session - 10/20/2020 - Special

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7.5 76 7,7 softening system treating 3.5 MGD (Ramsey's 2040 average day demand) would use as much as 6 tons of salt every day. The capital cost of adding an ion exchange water softening treatment process to a new water treatment facility would be approximately $5 million. This cost would be in addition to an iron and manganese removal water treatment plant. The operation cost for salt and wasted water for an ion exchange softening process is approximately $500 per million gallons of water treated. This is independent of whether it is done by the City or by a resident. An ion -exchange softening process would add approximately 3 tons of chloride to the wastewater system which is ultimately discharged to the Mississippi River. While the MCES Metro Wastewater Treatment Plant currently meets its discharge limits, chlorides have received more regulatory scrutiny recently. Operating a municipal scale ion exchange softening process may become fess feasible in the future due to chlorides in wastewater. in addition, municipal scale ion exchange softening might not be considered environmentally responsible. Due to the higher operation and maintenance costs, potential future regufations, and environmental responsibility, an ion -exchange softening process is not recommended, Pilot Study Resuils A pilot study was performed by John Thorn of SEH of Rannsey'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 lo filtration, and found no significant difference between the silica sand/anthracite and greensandlanthracite filter media. Because the manganese oxide coating on manganese greensand filters is helpful for manganese removal, greensandianthracite 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 rviGD at 2 gpm/ f12. 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 3200,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. FEASIBILITY STUDY -DRAFT RAMSY 154354 Page 27