DIGESTED SLUDGE STORAGE AND TRANSFER
Runoff from 24 circular digesters in Primary Treatment is diverted into four pump station wet wells, one of which is the central wet well for transfer of digested sludge to Solids Processing. The central wet well consists of three individual structures, each with a capacity of 822,800 gallons and equipped with two gas blowers that pump digester gas into the wet well to provide mixing. Normally, sludge is pumped using three digested sludge pumps through rotary screens into two centrifuge feed pumping station wet wells. These two centrifuge feed pumping stations house a total of five pumps. The pumps are used to deliver digested sludge to the centrifuges located in two buildings.
The purpose of the centrifuges is to separate water from suspended solids in digested sludge, thereby, creating a dry cake which is hauled off-site. There are currently 25 low-speed and 13 high-speed centrifuges housed in two buildings. Centrifuge Building No. 1 houses 10 low-speed centrifuges. Centrifuge Building No. 2 houses 15 low-speed and 13 high-speed centrifuges.
The centrifuges utilize centrifugal force from a rotating steel bowl to increase gravitational forces to separate suspended solids from water. The high-speed centrifuges are capable of increasing gravity up to a factor of 3,000, while the low-speed centrifuges increase gravity by a factor of approximately 1,000. Sludge is fed into the centrifuges along with diluted polymer for flocculation. The diluted polymer is mixed with the incoming sludge to achieve a desired solids recovery and cake dryness. Dewatered cake drops through a hopper below each centrifuge onto a conveyor belt, while the waste centrate is collected through a second hopper into a centrate drain system.
Cationic polymer is used as a flocculation aid in the dewatering of digested primary sludge, waste activated sludge, and thickened centrate. The resulting combination of polymer and sludge forms larger flocs that are easier to dewater using centrifugation. Cationic polymer is also used for thickening waste activated sludge and centrifuge centrate using dissolved air flotation.
The polymer facilities for Solids Processing handle concentrated liquid cationic polymer that is either water based (Mannich) or oil based (Emulsion). Concentrated polymer is diluted in mixing/batching tanks or in polymer blending units. The diluted polymer is then conveyed to centrifuges and/or centrifuge feed pump stations for sludge dewatering, and to the dissolved air flotation tanks at the Centrate Treatment Station (Solids) and Dissolved Air Flotation Facility (Secondary Treatment).
DEWATERED CAKE STORAGE
Conveyor belts are used throughout the facility to transport dewatered cake, known as biosolids, from the centrifuges to the storage silos and truck loading stations. Eighteen storage silos are used to temporarily store biosolids. Each silo can hold up to 510 tons of biosolids. From the storage silos, biosolids are conveyed to one of three available truck loading stations. Alternatively, biosolids may bypass the silos completely and be directed to one of the three truck loading stations, if necessary.
The centrate from the centrifuges is collected and gravity flows to the Centrate Treatment System Facility. The Centrate Treatment System Facility is designed to concentrate (thicken) the solids using dissolved air flotation to separate the solids from the centrate.
Air is mixed with water in a pressure vessel, causing the air to dissolve into the water. The pressurized air-water mixture is then fed to the flotation tank. Solids are carried to the top of the tanks as they attach to the rising air bubbles. Dilute polymer is also added to aid in the flocculation of solids. The flotable solids are skimmed from the top of the tank, collected and pumped to the centrifuge feed pump stations for further processing. The clarified effluent discharges to a wet well where it gravity flows to the J.O.-A or J.O.-B plant influent sewers.