Environmental Engineering Reference
In-Depth Information
time. Although screened, the raw water still contains impurities that may be
either dissolved or suspended. The settling basin provides the most conve-
nient way to remove the suspended matter, as it lets the force of gravity do
the work. Within the basin, when flow and turbulence are minimal (quies-
cent conditions), particles more dense than water settle to the bottom of the
tank. This process is called sedimentation , and the layer of accumulated solids
at the bottom of the tank is called sludge (or biosolids in some wastewater
treatment unit processes). The size and density of the suspended particles
have a direct bearing on the speed at which they will settle toward the bot-
tom of the basin. The larger or heavier particles will, of course, settle faster
than smaller or lighter particles. The forces opposing the downward force of
gravity include buoyancy and drag (friction). The particle-settling rate is also
affected by the temperature and viscosity of the water.
Note: The nature of the sedimentation process also varies with the con-
centration of suspended solids and their tendency to interact with one
another.
In the sedimentation process just described, not all suspended solids or
particles can be completely removed from water, even when given very long
detention times. Very small particles called colloids (e.g., bacteria, fine clays,
and silts) will not settle out of suspension by gravity without some help. This
is where coagulants come into play. If we rapidly mix chemical coagulants in
the water and then slowly stir the mixture before allowing sedimentation to
occur, the colloidal particles will settle. Colloids or finer particles must be
chemically coagulated to produce larger floc that is removable in subsequent
settling and filtration.
The coagulation process (along with flocculation) works to neutralize or
reduce the natural repelling electrical force of particles in water, keeping
them apart and in suspension. Particles in water usually carry a negative
electrical charge. Because all these particles carry this same negative electri-
cal charge, they repel each other—in the same way like poles of a magnet
do. The object of coagulation (and subsequently flocculation) is to turn the
small particles into larger flocs, either as precipitates or suspended particles.
These flocs are then conditioned for ready removal in subsequent processes.
Stated another way, in this text we define coagulation as a method to alter
the colloids so they will be able to approach and adhere to each other to form
larger floc particles.
DID YoU KNoW?
Coagulation and flocculation convert nonsettleable turbidity particles
into settleable form for their effective removal by gravity.
 
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