Lexington’s 22 MG wet weather storage tank is an above grade, pre-stressed concrete structure. It has an overflow weir box and a four-plex submersible pumping station that conveys peak wet weather flows to the tank. The tank has a diameter of 260 feet and a side water depth of 55 feet.
GRW provided design, bidding, and construction phase services for the project.
When flow to the treatment plant exceeds 70 MGD, an electrically actuated modulating gate restricts flow to the treatment facilities. Excess flow diverts by gravity to the wet weather storage facility pump station through another electrically actuated gate. Three variable frequency drive operated pumps work to maintain a constant level in the wetwell, transferring flow into storage.
Once the event has passed and flow to the treatment plant has dropped to 60 MGD, an electrically actuated plug valve initiates gravity return of flow from storage to maintain 70 MGD maximum to the treatment facilities.
The wet weather storage facility is controlled by an Allen Bradley ControlLogix PLC that is networked via multi-mode fiber optic cable to the treatment plant Ethernet-based SCADA system. The PLC at the facility includes an Allen Bradley VersaView OIT for viewing and adjusting the system controls. HMI software is CiTect. Even though the treatment plant is backed up by a redundant power company electrical service, since operation of the flow diversion gates is critical to preventing sanitary sewer overflows, a small (25 KW) natural gas engine-driven generator is provided to power essential controls and gate operators. All local electrical power distribution equipment, motor controls and process controls are housed in a pre-fabricated electrical building adjacent to the storage facility pump station.
This project is considered Phase 1 for wet weather management in this area of Lexington’s sanitary sewer system; ultimately, a 44-million gallon storage facility is planned for the site.