Flag City Pump Station & Force Main
San Joaquin County, California
West Yost provided planning and design services for the conversion of the Flag City Wastewater Treatment Plant to a lift station and force main system. The project included the design of a 0.73 mgd lift station and 13,200 feet of dual 6-inch diameter force main.
After determining that meeting effluent requirements with the existing treatment facility was economically unpractical, the County elected to convey sewage to the City of Lodi’s White Slough Water Pollution Control Plant, rather than update the Flag City treatment plant. The project included the design of a 0.73 mgd lift station and 13,200 feet of dual 6-inch diameter force main. An in-line grinder was also added, which required structural modifications to the top slab of the wetwell.
The project included several challenges, including developing a control strategy for the dual force main system for variable-flow conditions, to meet the County’s request to size the pump station for both existing and buildout flows. This large range of flows required a dual force main design to satisfy minimum velocity constraints at low-flow conditions. The lift station is essentially two pump stations in one: at low-flow conditions, the lift station uses one constant-speed pump, and at high-flow conditions, it uses one larger pump controlled by a variable-frequency drive. The project required coordinating with the City of Lodi for discharging to the City’s trunk sewer line.