Environmental Engineering Reference
In-Depth Information
10.5.1.2 Flotation Thickening
flotation thickening
is used most efficiently for waste sludges from
suspended-growth biological treatment process, such as the activated
sludge process. Recycled water from the flotation thickener is aerated
under pressure, and the water absorbs more air than it would under
normal pressure. The recycled flow together with chemical additives (if
used) are mixed with the flow. When the mixture enters the flotation
thickener, the excess air is released in the form of fine bubbles. These
bubbles become attached to the solids and lift them toward the surface.
The accumulation of solids on the surface is called the
float cake
. As
more solids are added to the bottom of the float cake, it becomes thicker
and water drains from the upper levels of the cake. The solids are then
moved up an inclined plane by a scraper and discharged. The superna-
tant leaves the tank below the surface of the float solids and is recycled
or returned to the wastestream for treatment. Typically, flotation thick-
ener performance is 3 to 5% solids for waste activated sludge with poly-
mer addition and 2 to 4% solids without polymer addition.
The flotation thickening process requires pressurized air, a vessel
for mixing the air with all or part of the process residual flow, a tank in
which the flotation process can occur, and solids collector mechanisms
to remove the float cake (solids) from the top of the tank and accumu-
lated heavy solids from the bottom of the tank. Because the process nor-
mally requires chemicals to be added to improve separation, chemical
mixing equipment, storage tanks, and metering equipment to dispense
the chemicals at the desired dose are required. The performance of the
dissolved air-thickening process depends on various factors:
•
Bubble size
•
Solids loading
•
Sludge characteristics
•
Chemical selection
•
Chemical dose
10.5.1.3 Solids Concentrators
Solids concentrators
(belt thickeners) usually consist of a mixing
tank, chemical storage and metering equipment, and a moving porous
belt. The process residual flow is chemically treated and then spread
evenly over the surface of the moving porous belt. As the flow is carried
down the belt (similar to a conveyor belt), the solids are mechanically
turned or agitated and water drains through the belt. This process is
primarily used in facilities where space is limited.
10.5.2 operational Considerations
As with other unit treatment processes, proper operation of sludge
thickeners depends on operator observation. The operator must make
routine adjustment of sludge addition and withdrawal rates to achieve
