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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:
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