Raking Maximizes Performance in Thickeners and Clarifiers

Raking plays a major role in transporting solids to the underflow discharge point and increasing the solids concentration in a thickener or clarifier. The main purpose of a thickener or clarifier is to separate solids from liquids. A thickener achieves this goal by stimulating the release of as much liquid as possible from solids in the underflow. So a thickener unit's performance is assessed by its efficiency in maximizing the solids concentration in the underflow slurry.

Clarifiers serve a slightly different purpose. These units remove the maximum amount of solids from the overflow to produce a clear liquid with a minimum concentration of suspended solids. Yet in both clarifiers and thickeners, rakes play a vital role. Without rakes to transport settled solids from all points along the radius of the tank to the underflow discharge point, these solids would accumulate in tanks and cause the overflow to become laden with solids. The underflow discharge point is normally in the center of the tank but in a few cases multiple discharge points are located at the outer limits of the raked zone.

Appropriate rake design can also help minimize the length of time that solids stay in the clarifier, which is an especially important consideration for units involved in processing settled activated sludge final tanks. Moving sludge through the unit quickly helps prevent conditions from becoming anoxic or anaerobic accompanied by gassing and floating sludge. Floating sludge also causes a rise in the solids content of the overflow and reduces the performance of the equipment as a clarifier.

In addition to helping transport settled solids to the discharge point, raking sludge causes the material to thicken. The solids in a blanket of settled sludge are normally in compression and further thickening by gravity alone is likely to be limited. But mechanical raking helps break down the bridging between particles in the sludge, releasing trapped liquor from between the particles and increasing the concentration of the sludge. 

Raking may also break down flocs, which can also release liquor and cause additional thickening. Non-flocculated solids are able to pack together more tightly than flocs, which generally have larger and more open structures. But since the solids were flocculated in the first place to aid initial settlement, it is no longer necessary to preserve the floc structure once the material has settled.

When performing sedimentation tests, resultant curves are familiar to most engineers. As an additional feature of this test, the effect of raking settled solids can be approximated by slowly stirring. The increase in settled solids is best recognized in the thickening process. However, the same will also occur, probably to a lesser extent, in a clarifier. It would be very unusual for an increase in underflow solids to be unwelcome in a clarification process.