Civil Engineering Reference
In-Depth Information
of dual- or mixed-media filters. An alternative would replace the alum and nonionic
polymer with the use of a cationic polymer in the raw water entering the rapid mix.
In some cases, preozonation has been reported to enhance the removal of turbidity in
the direct filtration process.
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Potential mechanisms are microflocculation and the pro-
duction of more polar compounds.
Figure 12-14b is a direct filtration scheme utilizing a flocculation basin in which
the chemical dosage alternates are the same as for Figure 12-14a. The flocculation
basin provides additional detention time and mixing to ensure good floc formation on
raw waters that are more difficult to treat.
The Figure 12-14c flow scheme is a direct filtration arrangement with a 1-hour
contact basin between the rapid mix and the filter. The purpose of the contact basin
is to increase the reliability of the process by adding a lead time of 1 hour between
turbidity readings, showing the results of coagulation from the coagulant-control filter
and the entry of water to the filters. In this process, the purpose of the contact basin
is not to provide for settling. There is no flocculation basin, and the contact basin is
not equipped with a sludge collector. The coagulant options are the same as for Figure
12-14a.
Following the proper mixing of the coagulant with raw water, a number of complex
reactions take place with colloidal turbidity and color. These coagulation reactions take
place in less than 1 second. The rapid mixing process for direct filtration usually does
not differ from that for conventional plants.
At this point in the process, the particles formed are very small, and the colloids
are destabilized. When the destabilized particles collide, they stick together, with the
rate of agglomeration of these microscopic destabilized particles to form visible floc
depending principally upon the number of opportunities for contact they are afforded.
In a still body of water, agglomeration takes place at a slow, almost imperceptible,
rate; the rate can be increased by agitation or stirring of the water. In a well-designed
flocculator, agglomeration of all particles might be completed in times varying from
5 to 45 minutes, when enough collisions will have occurred for the floc particles to
become large enough to settle rapidly.
If settling is omitted from the plant flow scheme, as in a direct filtration plant, and
if a properly designed rapid mix is provided, then usually there is no reason to include
flocculation in the direct filtration process. Rather than spending money on floccula-
tion, it may be better to improve the rapid mixing, provide finer filter media, or increase
the depth of the fine filter media. Water containing the destabilized particles can be
taken directly from the rapid-mix basin to a granular filter where contact flocculation
takes place as part of the filtration process. The flocculation rate is greatly accelerated
because of the tremendous number of opportunities for contact afforded in the passage
of the water through the granular bed. The floc particles become attached or absorbed
to the surface of the filter grains. The smaller the filter grains, the greater are the
opportunities for contact, and the more rapid is the removal of particulate matter. Small
filter grains also have a greater surface area per unit volume, which provides more
area for attachment of floc particles to the filter grains than is available with larger
grains.
This contact and the surface attachment or adsorption of particles to filter grains
account for particulate removal beyond that of any simple mechanical straining action
of the fine media. The pores of the filter gradually fill with floc as particles are sheared
off the surfaces of filter grains. As a filter run progresses, the upper pores of the filter
cannot retain any more floc, and the particles move down into the filter to find a
