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 screens into a centrifuge feed pumping station wet well. This centrifuge feed pumping station houses a total of three pumps. The pumps are used to deliver digested sludge to the centrifuges.

Centrifuges

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 20 high-speed centrifuges in Centrifuge Building No. 2.

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. 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.

Polymer Station

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 for sludge dewatering, and to the 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.

Centrate Treatment

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. 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.