8.3 SR 02-06-2023 - Laserfiche WebLink

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4.4 System Evaluation and Capacity Analysis for Northeast Areas A 20-year capacity analysis of the TH 169 Lift Station was completed to assess if this lift station can be used to pump wastewater generated in the northeast study area. If the TH 169 Lift Station is used to pump wastewater generated in the northeast study areas, its peak hourly flow is projected to increase to 2,200 and 2,300 gpm when the northeast study area is fully developed. This flow will exceed the lift station's existing firm pumping capacity by 540 to 640 gpm. Although this flow increase would trigger upsizing the TH 169 Lift Station to serve the study area, a capacity increase would not be required initially. The TH 169 Lift Station was originally designed with space to install two (2) additional pumps. Therefore, additional pumps could be installed to increase the lift station's pumping capacity. Installing one (1) additional 1,200 gpm pump would increase the lift station's pumping capacity to 2,860 gpm, which would be sufficient to pump the wastewater generated in the lift station's existing service area and the northeast study area. Installing two (2) additional 1,200 gpm pumps would increase the lift station's firm pumping capacity to 4,060 gpm. Installing a second 1,200 gpm pump could be done if large developments beyond those planned for the study area are expected for the lift station's service area in the future. The TH 169 Lift Station has a 1,745-ft long forcemain. Most of the forcemain (1,470-ft) is 14-inch in diameter and the remaining 275-ft is 16 inches in diameter. Forcemains are typically designed to maintain flow velocities between two (2) and five (5) feet per second (fps) to avoid particle settling. Depending on the application, velocities of up to eight (8) fps could be acceptable. Although higher velocities are sometimes acceptable, they also result in higher pipe friction headloss. Headloss in a forcemain is defined as the reduction in energy of the water as it moves through the pipe. High headloss and velocity indicates a forcemain is undersized for the flow that is conveyed. An undersized forcemain that generates high headlosses will also increase the energy cost to operate the pumps. In order to keep energy costs reasonable, forcemains are usually designed to maintain headlosses below 10-ft per 1,000-ft length of forcemain. The wastewater velocity and headloss through the TH 169 forcemain were evaluated under three different scenarios: existing pumping conditions, addition of one 1,200 pump, and addition of two additional 1,200 gpm pumps. The results of this analysis are summarized in Table 4.8. Table 4.8 — TH 169 Capacity Analysis for Existing 14-inch Forcemain Scenario at TH 169 Total No. Firm Pumping Wastewater Headloss Lift Station of Pumps Capacity (gpm) Velocity (fps) (ft/1,000 ft)�'� Existing Conditions 2 1,660 3.46 3.0 One Additional Pump 3 2,860 5.96 8.3 Two Additional Pumps 4 4,060 8.46 15.9 (1) I he roughness condition ot the existing torcemain is unknown and assumptions were made to calculate the headlosses. As summarized in Table 4.8, the existing 14-inch forcemain could potentially still be used if only one additional 1,200 gpm pump is added. However, serious consideration should be given to installing a parallel forcemain if a second 1,200 gpm pump is added in the future. A more detailed forcemain analysis should be completed in the future to identify if the existing forcemain can be used before installing additional pumps. Upon discussing the use of the TH 169 Lift Station with City staff, it was decided not to use this lift station to convey wastewater generated in the northeast study area. Regardless of whether the existing TH 169 lift station will be used to pump northeast area flows, wastewater flow generated in the northeast study area will need to be treated at the City's VWVfP. Given the plant's existing wastewater flow, it appears that the existing wastewater treatment plant has sufficient capacity to treat wastewater generated in the northeast study area. 4.5 Recommended Sanitary Sewer Infrastructure for the Northeast Areas The recommended sanitary sewer collection system layout and sewershed distribution to serve the northeast study areas are shown in Figures A19 and A20 in AppendixA, respectively. The sanitary sewer layout was preliminary designed based on the following criteria: Feasibility Report Northeast & Northwest Urban Service Area Expansion Study City of Elk River, MN WSB Project No. 020010-000 Page 18