Page 409 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

and do not deteriorate during decades of operation, that is, . A porous plate is available that retains media and
they remain reliable. passes flows of both water and air; headloss is
Since the under-drain system is not accessible, being about 150 mm (6 in.) at a backwash rate of
2
beneath the media, the system cannot be inspected and so 50 m=h (20 gpm=ft ) (Leopold, Product Data
there must be a confidence that the orifices will not clog or Sheet IMS-100, 1995) at 138C (568F).
corrode. It is expected that the media will remain in place for . For proprietary slotted under-drain nozzles that
at least a 20–30 year period with satisfactory performance rest in the media, they must have openings smal-
from the under-drain system. The attributes of an ‘‘ideal’’ ler than the smallest size of the media. Nozzles
under-drain system are outlined below, along with the ‘‘prac- with slotted areas are available with widths
tical’’ aspects. The gravel support is also described. 0.25–9.0 mm (Monk, 1987). An important con-
cern is that the orifice area should be small
1. The ideal: The perfect under-drain technology would enough such that significant headloss occurs, (i.e.,
have the following attributes: the headloss across any given orifice should be
. Backwash water is distributed uniformly over the about the same as every other orifice). This cannot
whole filter area. be true if the orifice headloss is small compared to
. Filtered water is collected uniformly over the header and lateral headlosses; the orifice headloss
whole area. equation is, Q(orifice) ¼ C(orifice)A (orifice)
0.5
. Orifices are not susceptible to clogging, either by [2gDh(orifice)] , which shows that Q (orifice) is
precipitation or by particulates. affected by Dh(orifice).]
. Air and water wash can be done simultaneously.
. Orifices for air and water flows are spaced close Most under-drain systems are available as proprietary equip-
enough that a there are no ‘‘dead-zones’’ for either ment. The classic generic design is to use perforated pipe
flow. laterals with a coarse gravel layer covering the laterals with
. The media can rest on a porous plate and thus graded gravel above. Laterals for air may be added so that a
eliminate the need for a gravel support (which has simultaneous air–water backwash may be used.
potential for displacement and occupies 300–450 Retrofits of existing filter beds often use plastic blocks with
mm (12.18 in.) of depth in the filter box). a porous plate above, which eliminates the need for the gravel
2. The practical: A perfect under-drain system has not layer. Such systems permit a greater depth of media and may
been achieved as yet, but some may have approached provide for simultaneous air–water backwash. Gravel support
the ideal through proprietary innovations. Some real- layers are subject to upset, and with the development of
ities of under-drain design are as follows: suitable porous plates the latter are increasingly favored.
. Headers and laterals will have some friction Thus, the long quest to eliminate the gravel support, with its
headloss and so the differential pressure across inherent risks, is finding suitable alternatives.
orifices will be different between those at the
beginning of the first lateral and the end of the 3. Sizing guidelines: For a generic system, that is, with
last lateral. The larger the conduits the smaller header, laterals, orifices, and gravel support, the fol-
the friction headloss as calculated by the Darcy- lowing guidelines for sizing an under-drain system
2
Weisbach equation, that is, h L ¼ f(L=d)(v =2g). (ASCE et al., 1969, p. 136):
A maldistribution in orifice flow of 10% is
considered acceptable (Beverly, 1995), while sum orifice area 0:0015 0:005
(12:47)
the Leopold (1999a,b,c) states 5% is achieved bed area ¼ 1 1
with their system. Table CDD.2 is a spreadsheet
sum lateral area 2 4
model that computes pressures and orifice (12:48)
sum area of orifices served ¼ 1 1
flows for any assumed sizes of the header pipe,
laterals, and orifices. manifold area 1:5 3
(12:49)
. The requirement for uniform distribution of flow sum lateral area ¼ 1 1
mandates a significant headloss across the ori-
fices, which implies a smaller orifice diameter Other recommendations by Cleasby (1991) taken
(or larger orifice size with increased spacing). from Weber (1972) are as follows: 6 d(orifice)
. To minimize dead zones, the Leopold system uses
13 mm (1=4–1=2 in.); spacing of orifices: 76
a 50 mm (2 in.) spacing between orifices over the x(orifice spacing) 300 mm (3–12 in.); 76
floor area (Leopold, 1999a,b,c). At the same time, x(lateral spacing) 300 mm (3–12 in.); L(lateral)=
the closer the spacing, the smaller the orifices. For d(lateral) < 60.
example the Tetra Ut block technology has ori- The guidelines have been used in practice as a
fices of 6.35 mm (0.25 in.). simple means to size the under-drain system. They
. To minimize surface chemical changes, a ceramic may be used as a starting point for the spreadsheet
or plastic material is preferred to a metal. simulation of an under-drain system as given in