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River Islands Development at Lathrop November 2020 Final Phase 2 Sewer Technical Report Rev 1 #6520 2. River Islands Sewer Collection System 2.1 Existing River Islands Sewer Collection System 2.1.1 Interim Sewer Pump Station Construction of River Islands began in the early 2010s. Stages 1A and 1B are nearly completed and occupied and Stages 2A and 2B are currently under construction. Generated sewage from Stage 1A and 1 B is currently being conveyed from each point of connection, to the interim sewer PS located between Lakes 1 and 2 and north of Golden Valley Parkway. Per the City's Master Plan, the interim sewer PS was formally completed in 2017. It includes two 20-horsepower (hp) submersible pumps designed to provide 1,100 gallons per minute (GPM)at a total dynamic head (TDH)of 46.5-feet. The PS was initially designed to pump the raw sewage approximately 8,700 feet east to a manhole under Sadler Oak located within the Mossadale collection area. However, recent force main improvements have allowed for the station to pump directly to the City's CTF for treatment. 2.1.1.1 Capacity Evaluation A sewer pump/lift station firm pumping capacity is required to be equal to the ultimate PWWF of the collection area,with the firm pumping capacity defined as the total pumping capacity of all the pumps minus the capacity of the largest pump (see Section 4.2). The interim sewer PS has firm pumping capacity of 1,100 GPM which is equivalent to an ADWF of 511 GPM when using the City Detail S-1 sanitary sewer peaking factors. As of June 2019, the interim sewer lift station has an average flow rate of 213,500 GPD or 150 GPM which is still below the maximum ADWF the station is rated for. It is recommended that the influent flows are regularly monitored to help accurately predict when the station is approaching its capacity. Sewer Pump Station A2 (Site C) is currently under construction. Before the influent flows exceed the capacity of the interim sewer PS, the new sewer pump station A2 will be brought online to accommodate additional land developments. The City plans on decommissioning the interim sewer PS and transferring the 20-hp submersible pumps to pump station A2. 2.1.2 Existing River Islands Recycled Water Flushing System Per Section 5-4.1 in the City Standards, main sewer lines should account for and be designed to transmit the ultimate buildout sewer flows, including potential tributary areas. As shown in Figure 2-1, Stage 1A will be located at the end of River Islands sewer collection system and will be required to handle flows from several tributary areas. Until full building conditions, the northern trunk line will experience low sewer flows which may lead to sediment/debris accumulation and potential odor and corrosion issues. To avoid these problems, flushing stations have been designed to occasionally add flow from recycled water mains into the sewer system, flushing any accumulated sediment/debris. The original recycled water flushing station, located at the intersection of Lakeside Drive and Somerston Parkway, has been removed as a result of the ongoing expansions in the collection system. A new 1,100 GPM flushing station, located along River Islands Parkway within Stage 2A, has been installed upstream of the original station to flush out the north trunk line of accumulated sediments and reduce odor and corrosion issues. During the initial startup of the flushing station, operators will be able to calibrate approximate flushing time intervals based on observations in the field. 2.2 Phase I and II River Islands Sewer Collection System At full buildout, River Islands sewer collection system is proposed to consist of six sewer collection areas with collection areas Al and A2 located within Phase I and the proposed collection areas C1, C2, B1, and B2 located within Phase Il. Each collection area is/recommended to convey the generated sewage from each POC to the collection area's sewer lift or pump station by gravity. Collection areas C1, C2, B1, B2, 8