NEW PUMP STATION AND 16 MG EQUALIZATION TANK FOR PEAK FLOW SHAVING AT THE NOMAN M. COLE, JR. POLLUTION CONTROL PLANT FAIRFAX COUNTY, VIRGINIA

This case history describes Task Order 4 South Site Improvements at the Noman M. Cole, Jr. Pollution Control Plant owned and operated by the Wastewater Treatment Division of the County of Fairfax Department of Public Works and Environmental Services. This project included a new Raw Wastewater/Equalization Tank Pump Station, a hydraulic surge tank, Flow Equalization Tank with four compartments (4MG each or 16 MG total capacity), and several thousand feet of 36”, 42” and 54” piping all of which were placed into operation in late 1998. The new Pump Station consists of three 185 hp wet pit submersible pumps which convey screened raw wastewater to the flash mix tanks/primary settling tanks and four 200 hp wet pit submersible pumps which convey primary effluent to the new Equalization Tanks for peak flow shaving when necessary. Each of the pumps is equipped with an adjustable frequency drive unit.A PLC controls the number of pumps in operation and the speed of the pumps so as to maintain preset liquid levels in the wet wells. The PLC and a diversion flow metering system control the rate of gravity flow from the primary settling tank effluent channel to the equalization pump wet well for peak shaving. The diversion flow metering system is comprised of three parallel ultrasonic flow meters each equipped with an electric butterfly valve. The system automatically controls the rate of flow to be diverted to the wet well for pumping in order not to exceed the peak process design flow to the Biological Nutrient Removal Activated Sludge Tanks. The diverted flow is pumped through a 36” pipeline to the concrete Equalization Tank which includes four 4 MG compartments. These compartments are automatically filled one at a time in sequence. When influent plant flow declines below a setpoint flow rate, the PLC will automatically drain each Equalization Tank compartment in sequence utilizing a gravity return flow system comprised of two parallel ultrasonic flow meters each equipped with an electric butterfly valve. Flow is returned by gravity through the same 36” pipeline to the primary settling tank effluent channel.Each 4 MG compartment is equipped with an innovative self cleaning ridge and furrow precast concrete bottom which concentrates the settled solids into a series of troughs called furrows. The furrows are semi-circular and slope at 4% from the end walls of each tank to the center. When each compartment is drained, settled solids are scoured down the furrows to a 1 MGD submersible washwater return pump located in the center of each compartment. A high pressure plant effluent washwater system automatically flushes the furrows clean after each compartment has been drained in sequence. Chlorine solution is also sprayed on the tank walls to prevent slime growth and control odors. The Equalization Tank design is unique in that no other mechanical equipment is located inside the tanks for aeration, mixing or solids removal. The Pump Station and Equalization Tank operation is automatically controlled by the PLC with innovative computer screen graphics which allow the plant operators to monitor system operation and make control adjustments, if necessary, on the computer screen by clicking a mouse. This facility can also be monitored from other remote work stations which are part of the plant SCADA system.