Agenda - Public Works Committee - 10/19/2021

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Chapter 7 — Status Quo — Approach 4 - Individual Lime Softening Water Treatment Plants In the absence of a project driver or an incentive to do something different, the Northwest Metro cities will most likely continue to utilize groundwater as their source of drinking water. This section identifies infrastructure that may be necessary in the future with individual community continued reliance on groundwater. 7.1 Lime Softening To provide an equivalent comparison to Approaches 1-3, it is assumed that the Northwest Metro cities will construct lime softening WTPs. A potential driver for selecting a lime softening treatment process for community drinking water systems is a future wastewater discharge limit for chlorides in the Twin Cities metro area receiving waters. The majority of the chloride in wastewater comes from the regeneration process of home water softeners. A sodium chloride solution (salt brine) is used to displace calcium and magnesium (hardness compounds)from ion-exchange softening resin. The waste product is discharged to the sanitary sewer. The only practical way to eliminate chloride from wastewater is to eliminate home water softeners and provide a water supply that is softened at a municipal lime softening water treatment plant. The lime softening process does not add chloride to wastewater. This follows best practices currently recommended by the state to reduce chlorides in wastewater treatment plant discharge. In addition to hardness, the lime softening process and subsequent filtration process would also remove iron and manganese. 7.2 2040 Water Infrastructure The Northwest Metro communities will need to add additional wells to meet 2040 demands. It is also assumed that lime softening water treatment plants are also added. Table 7-1 identifies the water infrastructure needed to meet 2040 demands and estimated costs. Page—31 1 METROPOLITAN COUNCIL