Digested Sludge Storage and Transfer

Runoff from 24 circular digesters in Primary Treatment is diverted into three 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 a centrifuge feed pumping station wet well. There are two centrifuge feed pumping stations that house a total of five pumps. The pumps are used to deliver digested sludge to centrifuges in two buildings.

Centrifuges

The purpose of the centrifuges is to separate water from suspended solids in digested sludge, thereby, creating a dry cake. There are currently 35 low-speed and four high-speed centrifuges housed in two buildings. Centrifuge Building No. 1 houses 10 low-speed centrifuges and rotary screens that remove foreign material that could damage the centrifuges. Centrifuge Building No. 2 houses the remaining 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 elevated centrifuge onto a conveyor belt, while the waste centrate is removed 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 particles that are easier to dewater using centrifugation or dissolved air floatation.

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 Centrifuge Feed Pump Station Nos. 1 and 2 for centrifuge feed and to the Centrate Treatment System Facility.

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 (TLS). Eighteen storage silos are used to temporarily store biosolids. Each silo can hold up to 485 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 air floatation 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 floatation tank. Solids are carried to the top of the tanks as they attach to the rising dissolved air. Dilute polymer is also added to aid in the flocculation of solids. The floatable 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 inlet mains J.O.-A or J.O.-B.