Agenda - Public Works Committee - 10/19/2021

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Chapter 6 — Conjunctive Use System (Surface Water Augmented with Groundwater— Approach 3) A hybrid option for the Northwest Metro to utilize some of its groundwater infrastructure is to build a new water treatment facility with a surface water source for conjunctive use with the existing groundwater systems. 6.1 Conjunctive Use Overview All of the Northwest Metro communities utilize groundwater as their source of drinking water. The intent with Approach 3 is that there is conjunctive use of surface water and groundwater. Conjunctive use is using groundwater and treated surface water in the distribution system at the same time. Approach 3 evaluates options for converting a portion of the drinking water for various communities in the study area from groundwater to treated surface water. The surface water treatment plant capacity for the conjunctive use system will be based on the average day demands for the Northwest Metro communities. Groundwater wells will be utilized for peaking. Communities typically only exceed average day demands in the summer(since the annual average takes into account summer months). 6.2 Conjunctive Use Water Quality A previous desktop study was conducted to identify water quality impacts associated with delivering treated surface water to groundwater communities and the possibility of conjunctive use of surface water and groundwater(Feasibility Assessment of Water Sustainability Approaches in the Northeast Metro Area, Metropolitan Council Environmental Services, 2014). The analysis was qualitative in nature. Preliminary conjunctive use water quality findings are as follows: • Communities may need to switch disinfection methods from chlorine to chloramines with a conversion to conjunctive use with surface water. • Mixing groundwater and surface water is predicted to be feasible. • Customers can expect taste and odor properties to be different with conjunctive use of surface water. A public education program would be recommended. • Lead, copper, and iron solution chemistry will be different with a conversion to conjunctive use of surface water. These constituents will need to be monitored closely and practices to control levels may need to be modified, including corrosion control. 6.3 Conjunctive Use Water Treatment Plant The surface WTP for a conjunctive use system is assumed to be located in Ramsey at the location shown on Figure 4-1. The Ramsey location provides a suitable surface water treatment plant location, along with close proximity to 6 of Ramsey's wells that could be used for conjunctive use blending. Approach 3 consists of constructing a 12 MGD surface WTP to meet 2040 demands and a 28 MGD expansion (total of 40 MGD)to meet ultimate demands. The 2040 average day demand for the Northwest Metro is 7.8 MGD and the ultimate average day demand is 29 MGD. The WTP capacities are designed to be larger than the average day demands because filter backwashing and plant downtime needs to be considered over the course of a year. In addition, most unit processes must consider standby capacity with the largest unit out of service. 6.4 Wells Additional wells will be necessary to meet 2040 and ultimate peaking demands in some of the communities. Table 6-1 identifies additional wells that will be needed in each community. Page—28 1 METROPOLITAN COUNCIL