Gravity Thickening - This process involves the concentration of thin sludges to more dense sludge in special circular tanks designed for this purpose. Its use is largely restricted to the watery excess sludge from the activated sludge process, and in large plants of this type where the sludge is sent direct to digesters instead of to the primary tanks. It may also be used to concentrate sludge to primary tanks or a mixture of primary and excess activated sludge prior to high rate digestion. The thickening tank is equipped with slowly moving vertical paddles built like a picket fence. Sludge is usually pumped continuously from the settling tank to the thickener which has a low overflow rate so that the excess water overflows and the sludge solids concentrate in the bottom. A blanket of sludge is maintained by controlled removal which may be continuous at a low rate. A sludge with a solids content of ten percent or more can be produced by this method. This means that with an original sludge of two percent, about four-fifths of the water has been removed, and one of the objectives in sludge treatment has been attained. Flotation Thickening - Flotation thickening units are becoming increasingly popular at sewage treatment plants, especially for handling waste activated sludges. With activated sludge they have the advantage over gravity thickening tanks of offering higher solids concentrations and lower initial cost for the equipment. Dissolved Air-Pressure Flotation - Although we haven't covered this per se, we should mention that the objective of flotation-thickening is to attach a minute air bubble to suspended solids and cause the solids to separate from the water in an upward direction. This is due to the fact that the solid particles have a specific gravity lower than water when the bubble is attached. Dissolved air flotation depends on the formation of small diameter bubbles resulting from air released from solution after being pressurized to 40 to 60 psi. Since the solubility of air increases with pressure, substantial quantities of air can be dissolved. In current flotation practice, two general approaches to pressurization are used: (1) air charging and pressurization of recycled clarified effluent or some other flow used for dilution, with subsequent addition to the feed sludge; and (2) air charging and pressurization of the combined dilution liquid and feed sludge. Air in excess of the decreased solubility, resulting from the release of the pressurized flow into a chamber at near atmospheric pressures, comes out of of solution to form the minute air bubbles. Sludge solids are floated by the air bubbles that attach themselves to and are enmeshed in the floe particles. The degree of adhesion depends on surface properties of the solids. When released into the separation area of the thickening tank, the buoyed solids rise under hindered conditions analogous to those in gravity settling and can be called hindered separation or flotation. The upward moving particles form a sludge blanket on the surface of the flotation thickener. Parameters: The primary variables for flotation thickening are: (1) pressure, (2) recycle ratio, (3) feed solids concentration, (4) detention period, (5) air-to-solids ratio, (6) type and quality of sludge, (7) solids and hydraulic loading rates, and (8) use of chemical aids. Similar to gravity sedimentation, the type and quality of sludge to be floated affects the unit performance. Flotation thickening is most applicable to activated sludges but higher float concentrations can be achieved by combining primary with activated sludge. Equal or greater concentrations may be achieved by combining sludges in gravity thickening units.
Centrifugation - Centrifugation has been demonstrated to be capable of thickening a variety of wastewater sludges. Centrifuges are a compact, simple, flexible, self-contained unit, and the capital cost is relatively low. They have the disadvantages of high maintenance and power costs and often a poor, solids-capture efficiency if chemicals are not used.
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