Environmental Engineering Reference
In-Depth Information
Sedimentation, in the conventional water treatment process, is typically
the step between flocculation and filtration. Design criteria are based on
empirical data from the performance of full-scale sedimentation tanks. The
common criteria for sizing settling basins are detention time (typically from
1 to 10 hr), overflow rate, weir loading, and, with rectangular tanks, horizon-
tal velocity.
In water treatment, the majority of settling basins are essentially upflow
clarifiers where the water rises vertically for discharge through effluent chan-
nels. More specifically, in the idealized sedimentation tank, water flows hori-
zontally through the basin and then rises vertically, overflowing the weir of
a discharge channel at the tank surface. Floc settles downward, opposite the
upflow of water, and is removed from the bottom by a continuous mechani-
cal sludge removal apparatus. The particles with a settling velocity greater
than the overflow rate are removed (settled) while lighter flocs are carried
out in the effluent. The effluent is then filtered.
Note:
Sedimentation tanks, either circular or rectangular, are designed
for slow, uniform water movement with a minimum of short-circuiting.
DID YoU KNoW?
Short-circulating causes a short detention time, resulting in an inef-
ficient sedimentation.
Filtration
Even after chemical coagulation and sedimentation by gravity, not all of the
suspended solids or impurities are removed from water. Nonsettleable floc
particles (about 5% of the suspended solids) may still remain in the water, and
with only that small percentage left we might ask, “Isn't this good enough?”
No, it isn't. This remaining floc would cause problems (including noticeable
turbidity), and particles shield microorganisms from the subsequent disin-
fection processes. The goal of water treatment is to produce potable water
that is perceptually crystal clear and that satisfies the Safe Drinking Water
Act (SDWA) requirement of 0.5 NTU for turbidity. To accomplish this, an
additional treatment step is required that follows coagulation, flocculation,
and sedimentation.
Filtration (sometimes called a
polishing process
) involves the removal of sus-
pended particles from water by passing it through a layer or
bed
of a porous
granular material—sand, for example. As water flows through the filter bed,
the suspended particles become trapped within the pore spaces of the filter
