“RECIRCULATING COOLING WATER SYSTEM FOR AN ALUMINUM ROLLING MILL REPLACES A SINGLE PASS SYSTEM”

A recirculating cooling water system (RCWS) was designed and constructed to provide process cooling water of sufficient quantity and quality to meet the production needs of Alcan Aluminum Corporation's Oswego Works Plant (Alcan) in Scriba, NY. The new facility replaces the previous single-pass cooling water system. The single pass system pumped water from Lake Ontario to the plant for contact and non-contact cooling needs and flowed through a series of wetlands prior to discharging to Lake Ontario.The recirculating cooling water system, placed into service in late 2001, consists of a low lift pumping station, gravity type oil water separators, cooling towers, blowdown structure, and control building. The current capacity of the new treatment system is 10 MGD, with a design capacity of 16.4 MGD for future expansion.Recirculation of the cooling water results in the concentration of contaminants within the water stream. Treatment of recirculating cooling water consists of the following:Removal of oil & grease by gravity type oil/water separatorEvaporative cooling via cooling towers for temperature controlAutomatic blowdown (discharge) and lake water make-up to maintain cooling water qualityChemical addition to control scaling, pH and biological growth in the systemAlcan's decision to replace the single pass cooling water system with the recirculation cooling water system was based on the environmental benefit of reduction in discharged and withdrawal rates from Lake Ontario while delivering a more consistent water quality to the plant. The critical requirement was the ability to control and provide a consistent contact cooling water quality to the aluminum casting process and meet discharge limits to Lake Ontario.The recirculating cooling water system was dependent upon the implementation of four major elements:Separation of the plant's stormwater drain system from the return cooling water, diversion of the return cooling water from the single pass system, and management of the separated storm water runoff.Construction of the recirculating cooling water pumping, conveyance, treatment and water quality control systems.Meet effluent requirements for discharge of blowdown to Lake Ontario.Implementation of the construction sequencing to start-up the system without disruption to existing plant production.The recirculating cooling water pumping and treatment system consists of the following:A diversion structure to redirect the return cooling water from the existing single pass system to the new recirculating cooling water system.A low lift pumping station consists of three Archimedes type enclosed screw pumps to reduce turbulence and minimize emulsification of oil and grease present in the plant effluent. Each pump is variable capacity, rated at 4,200 gpm (max.), with a total pumping capacity of 16.4 mgd.Oil/water separators consisting of four trains of gravity units in a reinforced concrete structure.Cooling is performed by cross-flow, induced draft type cooling towers. The tower media is a splash-type polymer fill. The cooling tower fans are provided with variable frequency drives. The four vertical turbine pumps at the cooling tower hot well are cycled on and off, and the fan speeds modulated to maintain a setpoint cooling water effluent temperature.Water quality is controlled primarily by blowdown water being discharged from the system. The blowdown structure consists of a reinforced concrete vault housing flow meters and control valves, varying the blowdown and lake water make-up volumes.The control building is located adjacent to the recirculating cooling water system, and houses a PLC station, the power distribution equipment, an emergency generator, chemical feed area, and skimmed oil removal station. The system is fully automated through a central PLC controller. The major control variables are recirculating cooling water temperature, pH, and conductivity.The recirculating cooling water system, through its first year of operation has reduced the mill's discharge to Lake Ontario and provided consistent water quality.