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Sludge Thickeners

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Sludge Thickeners
The HEI Sludge Thickener is an efficient method to gravity concentrate and decant waste sludge. Liquid sludge pumped from a clarifier will have a solids content of 0.5% from cone bottom clarifiers to 2%-4% from the rake bottom clarifiers.

The sludge thickener must have sufficient capacity to accumulate and thicken the sludge between filter press cycles. To efficiently operate a filter press, the solids concentration in the filter press feed should approach 10% solids by weight.

A typical filter press will produce a filter cake having a solids concentration of 30% by weight. If the press has 20 cubic feet capacity and the concentration of the press feed is 10%, then three volumes of sludge must be p[umped through the press before the solids concentration of the cake reaches 30%.

In this case 10 cubic feet times 3 equals 30 cubic feet or 225 gallons. The total cycle of the press would be less than 2 hours. If the concentration is on 0.5%as from a typical cone bottom clarifier, the volume would be 60 times or 600 cubic feet – 4500 gallons and the cycle time for the press may be 20-30 hours. Utilizing a properly designed sludge thickener will enable the press to be sized smaller, cycled more frequently and produce a better filter cake.

The sludge must be given time for the solids to gravity compact. The net thickening forces is the aggregate weight of the solids not the water. A column of solids 68 feet high will compact most sludge 8-10% by weight. As the rake moves through the sludge it created avenues for the liquid supernatant to move upward as the solids settle downward.

HEI Thickeners are constructed are constructed with flat bottoms and dish bottom with heavy duty rake drive mechanism to move the sludge to the center outlet. The cylindrical thickener provides 3 times the volume of an equivalent cone and hence 3 times the retention time.

The flat bottom thickener is elevated on integrally constructed I-beams. The recessed  center cavity provides a central outlet for the thickened solids.  The HEI thickener’s compact design provides greater storage capacity and more retention time for greater thickening of solids with less height.

A equivalent cone bottom tank would add an additional 10 feet of height. For example, an HEI 8ø x 12 thickener contains 4520 gallons. A typical 8′ diameter cone bottom unit having 4500 gallons storage capacity would be 18′ high.

The heavy duty rake not only thickens the solids but also positively moves the heavy solids to the center drain. The constant movement prohibits the formation of a “rat hole”. Since the supernatant is more fluid than the sludge, when the center outlet is opened, the sludge immediately above the opening is removed.

The sludge immediately above collapses into the hole formed by the solids removal. This in-turn is replaced by the more fluid supernatant, draining the supernatant, hence the formation of a rat hole. Even a 60° cone will not prohibit the formation of the “rat hole”. The motorized rake is the only positive method to move the sludge and prohibit the formation of the “rat hole”.

Larger diameter (greater than 13’10’) are field assembled on a pre cart concrete nopad. The thickener tank cylinder is field erected as a powder coated bolt together tank on the concrete b?  The thickener rake and bridge are prefabricated factory assembled components that are assembled on the side.