Science of Water : Concepts and Applications

310 The Science of Water: Concepts and Applications
FIGURE 9.6 Sedimentation.
Proper fl occulation requires from 15 to 45 min. The time is based on water chemistry, water
temperature, and mixing intensity. Temperature is the key component in determining the amount of
time required for fl oc formation.
Polymers are often added to increase the speed of fl oc formation and the strength and weight
of the fl oc.
SEDIMENTATION
After raw water and chemicals have been mixed and the fl oc formed, the water containing the fl oc
(because it has a higher specifi c gravity than water) fl ows to the sedimentation or settling basin (see
Figure 9.6).
Sedimentation is also called clarifi cation. Sedimentation removes settleable solids by gravity.
Water moves slowly though the sedimentation tank/basin with a minimum of turbulence at entry
and exit points with minimum short-circuiting. Sludge accumulates at the bottom of the tank/basin.
Typical tanks or basins used in sedimentation include conventional rectangular basins, conventional
center-feed basins, peripheral-feed basins, and spiral-fl ow basins.
In conventional treatment plants, the amount of detention time required for settling can vary
from 2 to 6 h. Detention time should be based on the total fi lter capacity when the fi lters are passing
2 gpm/ft 2 of superfi cial sand area. For plants with higher fi lter rates, the detention time is based on a
fi lter rate of 3–4 gpm/ft 2 of sand area. The time requirement is dependent on the weight of the fl oc,
the temperature of the water, and how quiescent (still) the basin.
A number of conditions affect sedimentation: (1) uniformity of fl ow of water through the basin;
(2) stratifi cation of water due to difference in temperature between water entering and water already
in the basin; (3) release of gases that may collect in small bubbles on suspended solids, causing them
to rise and fl oat as scum rather than settle as sludge; (4) disintegration of previously formed fl oc; and
(5) size and density of the fl oc.
FILTRATION
Pretreatment
stage
Addition of
coagulant
Water Mixing Flocculation Settling
supply tank basin
tank
Screening
Sludge
processing
In the conventional water treatment process, fi ltration usually follows coagulation, fl occulation, and
sedimentation (see Figure 9.7). At present, fi ltration is not always used in small water systems. However,
recent regulatory requirements under the USEPA’s Interim Enhanced Surface Water Treatment
(IESWT) rules may make water fi ltering necessary at most water supply systems.
Water fi ltration is a physical process of separating suspended and colloidal particles from water
by passing water through a granular material. The process of fi ltration involves straining, settling,
and adsorption. As fl oc passes into the fi lter, the spaces between the fi lter grains become clogged,
reducing this opening and increasing removal. Some material is removed merely because it settles
on a media grain. One of the most important processes is adsorption of the fl oc onto the surface of
individual fi lter grains. This helps collect the fl oc and reduces the size of the openings between the
fi lter media grains.