Page 469 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

424 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

Influent flow-containing body feed and particles -to-be removed Filter cake Pre-coat (3–5 mm) Effluent-particles are smaller than pore size of filter cake at the start of the filter run (McIndoe, 1969a, p. 51).
pre-coat also acts as an initial filtering media, which is effective
filtered particles
Body feed and
14.1.1.3.2
Body Feed
Upon completion of the pre-coat, body feed is started, as
illustrated in Figure 14.2b. The body-feed tank is charged
with a mass of DE sufficient for operation for the expected
number of hours (i.e., before the headloss reaches a desig-
nated maximum). The slurry concentration should be high
Septum enough that the tank size is not excessively large and low
Diatoms enough that the DE may be maintained in suspension. As in
(bodyfeed)
the pre-coat tank, the DE in the body-feed tank is maintained
Influent Effluent as a slurry by means of a mixer (not high speed).
The transition from the pre-coat phase to the body-feed
Particles-to- phase must be ‘‘smooth,’’ that is, without pressure shock to
be removed
the pre-coat that has just been deposited. To attain the
smooth transition, the raw-water valve is opened slowly
while the valve from the pre-coat line is closed slowly; at
High pressure side Low pressure side the same time, the body feed is started and the effluent
valve from the tank-septum assembly is opened slowly.
FIGURE 14.1 Septum, pre-coat, body-feed cake, particles-to-be- A filter-to-waste period is advisable until the effluent turbidity
removed, water flow. (Adapted from McIndoe, R.W., Water and reaches the objective level.
Wastes Engineering, 50, 1969a.)
14.1.1.3.3 Cleaning
body feed. A body-feed concentration of about 25 mg=L, after Figure 14.2c illustrates the cleaning cycle. As illustrated, clean
mixing with the raw water, would be representative. water is returned to the pressurized tank in the form of (1) high
pressure jets that impinge on the cake that has formed on each
14.1.1.2.5 Filter Cake
septum surface, and=or (2) as a backflow across the septum
The accumulation of body feed on the septum, added to the surfaces. Drainage is at the bottom of the tank through pipes
pre-coat, builds in thickness as a filter cake. The thickness of large enough that permit an easy flow of spent DE slurry and
the cake depends on the headloss permitted and the clearance removed particles. The pressurized tank that contains the
needed between filter cakes deposited on adjacent septa. Hav- septum assembly should be at atmospheric pressure during
ing adequate space between the cakes minimizes the probabil- the cleaning phase through an open valve at the top of the
ity of the cake being dislodged by hydraulic shear. tank. The high pressure jet method may be facilitated, in a leaf
system, by a provision to rotate the leaf elements such that the
14.1.1.3 Phases of Operation entire septum area is subject to the cleansing action of the jet.
Figure 14.2 illustrates the three phases of DE operation. That Other variations in cleaning are provided by equipment manu-
is, pre-coat, body feed, and cleaning. These three phases are facturers, for example, ‘‘dry-cake’’ discharge.
explained in the paragraphs of this section.
14.1.1.4 Process Description
14.1.1.3.1 Pre-Coat The filter media provides a rigid porous structure that passes
In the pre-coat phase, the septum is prepared for operation the water being treated, and retains particles. The body feed
with a deposit of DE as illustrated in Figure 14.2a. As shown, entrains and ‘‘embeds’’ the particles, that is, blocks their
a pre-coat tank has a ‘‘charge’’ of DE adequate for a specified movement, leaving the pore channels substantially intact
2
2
deposit, for example, 0.5–1.0 kg=m (0.1–0.2 lb=ft ). The DE within the filter cake for the flow of fluid (Cummins, 1942,
charge in the pre-coat tank is maintained in suspension as a p. 403). The correct body-feed concentration occurs when the
slurry and is pumped, in a closed loop, through the septum. DE structure dominates the pore structure rather than the
Circulation is continued until the slurry becomes clear. The particles being removed. If the particles being removed dom-
pre-coat is distributed uniformly since any nonuniform areas inate the pores, then additional diatomite is needed. Without
of deposit carry a higher velocity and thus a higher DE mass body feed, the particles to be removed will be retained on the
flux density to the deficient area. surface of the pre-coat and will ‘‘blind off’’ the filter, that is,
During the circulation, a bridging of media particles occurs causing an inordinately high rate of headloss increase.
such that they are retained on the septum fabric. The openings
of the fabric may be larger than the DE particles but not so 14.1.1.5 DE Selection
large as to not permit bridging and retention. In water treatment, the general rule is to select the coarsest DE
The purpose of the pre-coat is to protect the septum from grade that provides the requisite effluent quality. If the run
the particles to be removed and to support the filter cake. The length is too short, then DE grade is probably not a good fit