Page 202 -

HIGH-RATE GRANULAR MEDIA FILTRATION 8,21

Depth of Filter Box. A number of factors should be considered in designing a filter box,
including
• Available head
• Depth of water over the filter media surface
• Fixed head losses
• Head losses through the filter media
• Shape of head loss curves
• Rate of filtration
• Elevation of the filtered water effluent control weir (if used)
Available head may be restricted by site conditions and plant layout, by the designed
maximum length of filter run, by optimizing head loss, or by terminating turbidity break-
through. Ideally, allowable head loss is determined from pilot plant studies. Fixed head
losses through piping, venturi meters, and throttling valves can be calculated from man-
ufacturers' literature and by hydraulic analyses. Head losses through filter media are de-
termined by pilot plant studies or are calculated from Darcy's formula.
Once these factors are known, water depth and weir elevation can be established to
achieve a cost-effective design. Objectives in designing a filter box should be to mini-
mize the cost of construction and to avoid the possibility that the filter will develop neg-
ative pressures. Monk (1984) presents the methodology required to determine optimum
filter box depth.

Underdrain Systems
An underdrain system has two purposes: to collect water that passes through the filter me-
dia and to distribute washwater (and air, if used) uniformly across the filter bed. Support
gravel is required when openings in the underdrain system are larger than the filter medium
directly above it. Although the support gravel or other support method does not contrib-
ute to particulate matter removal, it aids in distributing washwater. For this reason, it
should be considered part of the underdrain system. Uneven distribution of washwater can
displace support gravel, eventually requiring removal of the filter media to be regraded
or replaced.
Four basic types of underdrain systems are common: pipe laterals, blocks, false bot-
tom, and porous bottom.
Traditionally, the greatest difficulty in underdrain design has been in providing a bar-
tier to the finest medium that does not clog during filtration or filter washing. Early at-
tempts to use fine screens or strainers were largely unsuccessful, leading to the use of
gravel layers below filter sand. The position of gravel layers may, however, be disrupted
during filter washing. Jet action, which is discussed in greater detail elsewhere (Cleasby,
1972), causes sand and gravel mixtures to be more easily disrupted than gravel alone. If
auxiliary air scour is used, even greater gravel disturbance may occur. Fine gravel, usu-
ally placed at the sand-gravel interface, is most easily dislocated. A possible solution to
this problem is the use of gravel in a coarse-to-fine-to-coarse, or "hourglass," gradation,
which has been shown to be highly stable at high washwater rates (Cleasby, 1972). Fine
media penetrate the upper coarse gravel layer without apparent ill effect.
Mixed-media beds with very fine garnet at the bottom of the bed are generally con-
structed with a layer of coarse garnet on top of the silica support gravel. Coarse garnet
prevents leakage of the fine garnet and also helps stabilize the underlying silica gravel.