Page 405 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

Looking at the filter box in more detail, its depth depends on Example 12.5 Depth of Filter Box
the under-drain system (e.g., size of under-drain plenum
or support media), media depth, and headloss desired. Problem statement
Headloss varies in the range, 2–3m (6–9 ft), but there is A filtration process will utilize a media depth of 3000 mm
no limit theoretically (except that turbidity breakthrough will (118 in.) of anthracite with d 10 ¼ 2 mm, UC ¼ 1.5. Assume
the backwash system is generic with laterals and gravel
occur eventually). Plenum systems can range 610–915 mm
support. An air wash system is also installed, but requires
(24–36 in.) in depth (Monk, 1987). If self backwashing
no additional depth. Assume that the tailwater weir is at
is used, a deeper filter box is required, as described by
the same elevation as the top of the media and that the
Monk (1987).
headloss permitted is 3048 mm (10 ft) and that the free-
board is 610 mm (24 in.).
Required
Example 12.4 Under-Drain Design
Estimate the depth of the filter box.
Problem statement Solution
2
2
A filter bed area of 120 m (1300 ft ) is proposed and a The depth of filter box is
generic under-drain system is to be used, that is, header
and laterals with orifices. The filtration rate proposed is D(filter box) ¼ D(gravel support) þ D(media)
2
HLR 22.0 m=h (9 gpm=ft ) and the backwash rate deter- þ headloss þ freeboard
2
mined by pilot tests is 61 m=h (25 gpm=ft ).
¼ 381 mm þ 3000 mm þ 3048 mm þ 610 mm
Required
Estimate the difference in orifice flow between the two (¼15 in: þ 118 in: þ 120 in: þ 24 in:)
extremes of the under-drain system, that is, at the first ¼ 7039 mm (23 ft 1in:)
orifice of the first lateral and the last orifice of the last
lateral.
12.4.4 BACKWASH
Solution
The solution depends on the size of header, laterals, and The backwash system has several components: backwash
orifices. Table CDD.2 is set up with an algorithm to cal- flow distribution, disposal of wastewater, surface wash, and
culate the kind of results as illustrated in Figure 12.30. The air wash. These components are described in this section.
first trial might use a header pipe that is say 381 mm
(15 in.) with laterals say 203 mm (8 in.), and orifices say
12.4.4.1 Manifold Principles
6mm(1=4 in.), with spacing say 203 mm (8 in.). After
Figure 12.30 shows a hydraulic profile for a generic under-
comparing flows at the first orifice in the first lateral
and the last orifice in the last lateral, the difference should drain system; the illustration is for the backwash mode.
be no more than say 5% as a criterion. Pipe sizes and Table CDD.2 provides a spreadsheet calculation algorithm
orifices may be revised, depending on results. to calculate the pressure surface for any given manifold

h (header)
L
HGL (header) c

b
a

g HGL (lateral) d
Q(header) Q(lateral 2) Q(orifice) h (lateral) h
L
e
Water surface
h L (orifice)
i Lateral 4

f Lateral 3
Lateral 1
FIGURE 12.30 Three-dimensional perspective drawing of backwash system.