8.3 SR 02-06-2023 - Laserfiche WebLink

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4. SANITARY SEWER SYSTEM ANALYSIS The City's existing sanitary sewer collection system is shown in Figure A17 in Appendix A. The system uses a combination of gravity and pressurized flow to convey wastewaterto the City's wastewatertreatment plant located near the intersection of Trunk Highways 10 and 169. The sanitary sewer serviceability analysis completed for the study areas is summarized in the following sections. 4.1 Existing System Description The City's sanitary sewer collection system includes approximately 81 miles of sewer mains and 23 lift stations. The gravity sewer pipes range in diameter from 8 inches to 24 inches and collect wastewater from approximately 8,000 acres. Prior to commencing this serviceability analysis, City staff identified several lift stations that could be used to convey all, or a portion, of the wastewater generated in the study areas. These lift stations are highlighted in Figure A17, and their design characteristics are summarized in Table 4.1. The existing sanitary sewer system in the northwest study area is shown in Figure A18 in Appendix A. The northeast study area is not currently served by municipal sanitary sewer. Table 4.1 — Existing Sanitary Sewer Lift Station Data Parameter TH 169 Lift Evans Lift Windsor Lift Station�'► Station�2> Station�2� Lift Station Type Submersible Submersible Submersible Upstream Gravity Diameter (inch) 24 24 & 18 8 Number of Pumps 2(space for 4) 3 2 Individual Pump Design Capacity (inch) 1,200 1,150 240 Firm Pumping Capacity (gpm)�3� 1,660 2,300 240 Pumping Head Conditions (feet) 110 53 108 Number of Forcemains 1 1 1 Forcemain Diameter (inch) 14 & 16 14 6 Length of Forcemain (feet) 1,470 (14-inch) 1,850 12,500 275 (16inch) GPM — Gallons per Minute; ft- Feet (1) Individual pump capacity based on drawdown test completed on April 6, 2022. (2) Theoretical pump capacities. Drawdown test not completed. (3) The firm capacity of a lift station is defined as the lift station's capacity with its largest pump out of service. Both the TH 169 and the Evans Lift Stations pump wastewater from the sanitary sewer collection system directly to the City's wastewater treatment plant (WWTP) and could potentially be used to convey wastewater from the study areas. The wastewater plant was originally constructed in the late 1950s and has gone through multiple expansions and rehabilitations with the latest one occurring in 2017. Currently, the plant utilizes an activated sludge treatment train with extended aeration followed by a chemical phosphorus removal process, final clarification, sand filtration, and ultraviolet (UV) disinfection. The biosolids generated throughout the facility are aerobically digested, dewatered with screw presses, and hauled to the Elk River Waste Management Landfill located a few miles north of the wastewater plant. The plant's biological treatment capacity was doubled during the 2017 expansion from 2.2 MGD to 4.5 MGD. Although the plant's biological treatment units can treat up to 4.5 MGD of average flow, the existing headworks infrastructure is not capable of handling that high of volume of wastewater and will require expansion in the future to increase the overall treatment capacity to 4.5 MGD. 4.2 Existing Wastewater Flows Residual capacities were calculated for the existing lift stations shown in Table 4.1 based on wastewater flow data provided by the City to assess their remaining pumping capacity. This capacity study is summarized in Table 4.2. Feasibility Report Northeast & Northwest Urban Service Area Expansion Study City of Elk River, MN WSB Project No. 020010-000 Page 15