Page 491 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

Mass(pre-coat) ¼ L(DE) A(septum) N(septa) (14:12) 15
10 20 30 Body feed (mg/L) 48
¼ C(pre-coat slurry) V(pre-coat suspension) (14:13) 40
50 60
40
where
Mass(pre-coat) is the mass of diatomite required to provide 80
10
pre-coat charge to a tank of module of several tanks (kg) 32
L(DE) is the mass loading of DE on septum (kg DE=m 2 100
septum area)
24 Headloss (ft)
A(septum) is the septum area for a single element, for 120 mg/L
2
example, two sides for a leaf septum (m )
N(septa) is the number of septa in a given assembly to be 5 16
pre-coated (#)
C(pre-coat slurry) is the concentration of pre-coat slurry
3
(kg=m ) 8
V(pre-coat suspension) is the volume of pre-coat water for
3
diatomite slurry (m ) 0 0
0 200 400 600 800 1000 1200
The quantity to be calculated is V(pre-coat suspension), with Time (min)
C(pre-coat slurry) assumed based upon experiments or upon
FIGURE 14.14 Headloss increase with time. (Adapted from
reported practice, for example, Table 14.4 in which 120
3
C(pre-coat slurry) 240 kg=m . Baumann, E.R. et al., J. Am. Water Works, 70(9), 1109, 1962, p. 111
for 8 mg=LFe at v ¼ 2.44 m=h, 208C.)
14.4.1.2 Body Feed
Selection of DE grade and determination of concentration and The exponential h L versus time curves at lower DE concen-
mixing of slurry are the tasks associated with body feed. trations are due to the particles to be removed dominating the
Usually the grade and concentration are determined as a first cake pores, effectively ‘‘blinding’’ the cake.
estimate during the design phase and refined during operation.
14.4.1.2.4 Body-Feed Charge
14.4.1.2.1 Selection of Grade The body-feed tank is filled with a given amount of water and
The grade of body feed should be the coarsest that results in then given a mass of diatomite that results in the specified
the effluent quality meeting treatment objectives. This may be slurry concentration. Table 14.4 gives slurry concentrations in
3
determined by bench-scale tests. For water treatment, the the range of 12 C(body-feed slurry) 24 kg=m . Equation
coarsest grades are usually those with 18 d(50) 24 mm. 14.10 indicates the variables involved; t(tank) is the associ-
If alum or polymer is required to meet the treatment object- ated time that the slurry volume will last. The mass charge is
ives, then a coarser grade of diatomite may be used; but the calculated as the right-hand side of Equation 14.10.
rate of headloss increase is likely to be higher than desired.
14.4.1.3 Valve and Pump Operation
14.4.1.2.2 Body-Feed Concentration Valves and pumps should be operated by a PLC (program-
The body-feed concentration depends upon the sizes and mable logic controller) or by desktop computer and control
kinds of particles to be removed and upon the grade of DE. software. The entire operating protocol may be programmed
Concentrations in the range of 20–40 mg=L are usual. Ideally, such that ‘‘hands-on’’ control by the operator is not significant
the concentration to be used should be based on a pilot plant in terms of labor required. Rather, the operating labor
study for the particular conditions at hand. Otherwise, the involves, for the most-part, adding the pre-coat and body-
concentration may be adjusted at full scale, starting on the feed charges, monitoring instrument read-outs, servicing
high side and working down to lower concentration. equipment as required, reporting to regulatory authorities, etc.
14.4.1.2.3 Body-Feed Adjustment: Based on Linear
14.4.2 MONITORING
Rate of Headloss Increase
Figure 14.14 illustrates the effect of body-feed concentration Monitoring involves whether the magnitudes of variables are
on the respective headloss versus time curves. As seen, for within specified limits and whether higher-than-expected rates
of change occur. In either case, the computer may take pre-
body-feed concentrations of 10, 20, and 30 mg=L, the h L
versus time curves increase exponentially with time, but at scribed actions should deviations be outside the ranges, for
C(body feed) 40 mg=L, the relationships are linear. The example, shut down the module, or initiate other action such
body feed to be used is the minimum concentration that gives as a telephone call to an operator. The other function is
a linear h L versus time curve, that is, 40 mg=L for the case to provide a documentation of performance to regulatory
shown. The curves shown are unique for the conditions given. authorities.