The HEI sludge thickener is an efficient method to gravity concentrate and decant waste sludges.
Liquid sludges pumped from a clarifier will have a solids content of 0.5% from cone bottom clarifiers to 2% - 4% from rake bottom clarifiers.
The sludge thickener must have sufficient capacity to accumulate and thicken the sludges 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 filter press has 20 cubic feet capacity and the concentration of the filter press feed is 10%, then three volumes of sludge must be pumped 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 - 4,500 gallons and the cycle time for the press may be 12-24 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 sludges 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 6 - 8 feet high will compact most sludges to 8 - 10% by weight.
The rake, as it moves through the sludges, provides avenues for the liquid supernatant to move upward as the solids settle downward.
HEI thickeners are constructed with flat bottoms and heavy duty rake drive mechanisms to move the sludge to the center outlet. The flat bottom 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 channels. 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. An equivalent cone bottom tank would add an additional 10 feet of height. For example, an HEI 8’Ø x 12' high thickener contains 4,520 gallons. A typical 8’ diameter cone bottom unit having 4,500 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, and creating 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’.
Hoffland environmental constructs its factory assembled sludge thickener in readily transportable sizes. Available sizes are: