Waste activated sludge and scum from the Secondary Sedimentation Tanks are pumped to the Sludge Thickener. The purpose of this unit is to thicken the influent sludge from 0.5% – 0.75% solids density to 3.0% – 4.0% solids density. The thickened sludge is then pumped directly to the Digestion Control Building and into the digestion tanks. The Sludge Thickener is the dissolved air flotation type with four basic component parts: The thickener tank (including the mechanism for sludge skimming and raking); the pressurization system (including pressurization pump and air injection system); the sludge and scum pumping; and the polyelectrolyte feed system. The unit has been designed to handle the normal waste activated sludge flows and solids loadings from an average daily plant influent flow of 4.0 million gallons.
Thickening is achieved by continuously returning approximately 325 gallons per minute of air pressurized sludge thickener effluent to the center of the Thickener Tank. The air pressurized effluent is produced in a multi-step process. First, compressed air is injected to the discharge of the second stage of the two-stage pressurization pump or directly into the retention tank. Then this aerated effluent is directed to a retention tank where the air is more fully absorbed into this effluent flow stream. Finally, the retention tank effluent is piped to the center of the Thickener Tank where it mixes with the influent (WAS) to the Sludge Thickener and enters the tank through ports in the center column near the tank’s liquid surface. As the air pressurized effluent enters the center column, its high pressure is reduced to atmospheric pressure conditions. This pressure change releases entrained air from the WAS-recycled effluent mixture. The rising air bubbles suspend or float solids to the tank water surface where the solids are removed by a skimmer. This floating material generally constitutes the thickened sludge, with only nominal amounts settling to the tank bottom. The settled sludge is raked to a hopper in the tank bottom for periodic withdrawal by the Sludge and Sump Pump. Sludge skimmed from the tank surface is collected in a scum sump for periodic withdrawal by the Sludge and Scum Pump. Polymers can be added to the influent sludge piping to aid in coagulating the influent WAS just prior to entrance into the Thickener Tank. Effluent from the Sludge Thickener flows through the tank launder to the effluent recirculation sump, then by pipeline to the Aeration Tank influent. The pressurization pump also draws effluent from the effluent recirculation sump.
The Sludge Thickener is located just north of the Aeration Unit, with the pressurization system, sludge and scum pumping, and polyelectrolyte feed equipment located on a concrete slab-on-grade adjacent to the Thickener Tank. Operation of the Sludge Thickener must be suspended during flooding conditions, as the equipment is not protected from flood waters. During flooding, the Thickener Tank must be full of water to resist damage from buoyant forces. Depending on the extent of flooding, some of the electrical and mechanical equipment may be damaged, since this facility was not designed as a critical facility to be located above projected flood levels.