5.2. ERMUSR 02-12-2008 - Laserfiche WebLink

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4.5.2 Technical Feasibility and Reliability A surface-water intake and treatment system is a proven technology -examples include St. Cloud, Minneapolis, and St. Paul. Reliability issues similar to those associated with existing surface-water works may result during low-flow periods. 4.5.3 Cost The cost of a plant to treat surface water depends on a number of factors, including capacity, whether or not to include softening, availability of land, and the location and type of intake structure. The Kraemer & Sons Inc. quarry in Burnsville is planning construction of a 4 million gallon per day plant to treat quarry dewatering water (which is considered to be under the influence of surface water and requires surface-water treatment). The estimated cost of that plant is approximately $13 million (and does not include softening). A plant in Elk River that is capable of supplying between 8 and 12 million gallons per day would likely be in the range of $20-million to $30-million, including intake structures. As capacity increases, the cost per gallon treated would tend to go down. Thus, there would likely be savings realized with a larger customer base. 4.5.4 Challenges and Other Considerations Capital cost of implementation is likely the biggest obstacle for this alternative. The cost differential for a small treatment plant compared to a large plant is not substantial. Therefore, per capita cost of implementation could be reduced substantially if multiple communities combined resources to build a single facility that would serve a large area. Risks associated with contamination and low flow could be substantially reduced for Elk River with the continued maintenance of the existing well system as a back-up water supply. Softening in the treatment plant would likely improve overall water quality. Mixing of water from two sources (treated surface water and untreated groundwater) could cause water-quality issues through a repetitive cycle of precipitation and dissolution in the piping networks, leading to fluctuating water hardness, iron staining, and scaling. 4.6 Ranney Well and Treatment 4.6.1 Description Ranney wells are horizontal collector wells advanced underneath the bed of river. In this case, one or more collector well laterals would be advanced below the Mississippi River from a central caisson to obtain Mississippi River water that has undergone some filtration through the river-bed bottom. Treatment to surface-water quality standards would still be required because this water would be considered "groundwater under the direct influence of surface water". However, some filtration steps in the treatment process might be eliminated from the treatment plant by using Ranney wells. P:\Mpls\23 MN\71\2371105 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 32