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Lino Lakes WTP <br />January 19, 2024 <br />Page 2 <br />K:\021336-000\Admin\Docs\Design Memos\MEMO Lino Lakes WTP Capacity Sensitivity Analysis.docx <br />As shown in Table 1, two additional aerators and one additional high service pump would need to <br />be added in the future to increase the WTP capacity from 6,000 gpm to 10,000 gpm. With some <br />adjustment, space can be reserved in the WTP for this equipment. Provisions would need to be <br />made to position a crane to lift the additional aerators into place through a removable wall or roof <br />at the southwest corner of the building. <br /> <br />The filters, detention tank, clearwell, and backwash tank volumes would remain unchanged with <br />the increase in capacity. This will result in a decrease of detention time in the detention tank and <br />residence time in the clearwell when operating at 10,000 gpm, but the resulting times are <br />acceptable for biological filtration. The filter backwash sequence and backwash waste volumes <br />will remain the same, so the backwash tank volume does not need to change. <br /> <br />The increased capacity would also require improvements to the water distribution system as <br />summarized in Table 2, to transmit the higher flow rate from the central WTP out to the <br />northwestern and eastern parts of the system (and to Tower No. 1 in particular). These <br />improvements were determined using an extended period simulation (EPS) in the City’s water <br />model during maximum day demand, and they are the minimum needed to maintain tower levels <br />and AWWA-recommended hydraulic conditions in the distribution system. <br /> <br />Table 2 – Water Distribution System Comparison <br />Status 6,000 gpm Sizing 10,000 gpm Sizing <br />Completed Pheasant Run 12” Trunk WM Pheasant Run 12” Trunk WM <br />Planned Black Duck 12” Trunk WM Black Duck 12” Trunk WM <br />Additional - <br />Lake Drive 12” Trunk WM Connections <br />Hodson Road Parallel 16” Trunk WM <br />North 16” Trunk WM Loop <br /> <br />As shown in Table 2, the higher flow rate would require more trunk watermain improvements than <br />previously planned to convey flow from the central WTP location outward to the rest of the <br />system. Although the North 16” Trunk WM Loop would be challenging to construct through the <br />wetlands and highway corridor in the northern part of the City, it would provide benefits of looping <br />and redundancy in addition to the transmission needed under the 10,000 gpm scenario. <br /> <br />Long-term (post-2040), the alternative to the biological 10,000 gpm WTP capacity would be to <br />construct about four new wells and a second WTP in the northern portion of the system. This <br />would eliminate the need for the additional trunk watermain improvements since the water supply <br />would be decentralized or dispersed with a supply point in the north. However, the wells and WTP <br />would also be costly. The order of magnitude cost difference between these two scenarios is <br />summarized in Table 3. <br /> <br />Table 3 – Order of Magnitude Cost Comparison <br />Description 6,000 gpm WTP and Future <br />Well Field and WTP Biological 10,000 gpm WTP <br />Additional Facilities 4 Wells and 1 WTP Trunk Watermain Improvements <br />(see Table 2) <br />Order of Magnitude Cost $40 million $15 million <br /> <br />Table 3 suggests that the additional trunk watermain improvements are more cost -effective than <br />the new well field and WTP. Therefore, we recommend proceeding with the adjustments to the <br />WTP design to allow for the higher biological capacity and the addition of the two aerators and <br />one HSP in the future. Given the long-term nature of these projections, there will be ample <br />opportunity to further study and evaluate the City’s water supply system. <br />54