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8.12 CHAPTER EIGHT

sometimes at the filter walls. Cracks can cause short-circuiting of the bed during filtra-
tion, with subsequent decline in filtered water quality. Clogged areas also contribute to
channeling of washwater, which can lead to bed upset. The mechanisms by which wash-
ing problems lead to filter failures are discussed in greater detail in Cleasby (1972).
The selection of a washing technique is closely tied to filter media and underdrain se-
lection. In current practice, washing normally includes upflow water flushing. The rate
and duration of water flushing are variable, however, and may be supplemented with air
scour or surface water wash. Operational sequencing of combined washing systems and
the source of the washwater introduce additional variations.

Washwater Source. Washwater source options include the following:
• Flow bled from high-service discharge and used directly for washing or to fill an above-
ground washwater tank that is subsequently used for gravity washing
• Gravity flow from a separate elevated finished-water storage tank
• Direct pumping from a sump or belowground clearwell
Bleeding flow from a high-service discharge main results in energy loss because of
pressure reduction required before washing to avoid media loss. For direct washing, a
pressure-reducing valve or orifice is placed in the washwater supply line. For bleeding
flow to fill a washwater tank, an altitude valve or other level control device is used to
control the water level in the tank. In either case, the washwater supply line is often sized
to restrict the maximum amount of water that can be delivered. Both options avoid pro-
vision of separate washwater pumps. Direct washing also avoids construction of a wash-
water tank but presents greater difficulty in controlling washwater flow. Because of the
large pressure drop often involved in supplying washwater by high-service bleeding, the
potential for cavitation in or following head-dissipating devices in the supply line is
significant.
If elevated finished-water storage is not available to provide head for filter washing,
washwater may be pumped to a separate washwater storage tank or directly to the filters.
Use of a washwater tank permits pumping at a lower rate. Tank storage volume must be
sufficient to permit filter washing at the maximum wash rate while the pump operates at
the minimum run times.
A number of proprietary filters are available that obtain washwater by means other
than those previously listed. One design uses vertical steel tanks divided into upper and
lower compartments. Sufficient filtering head is provided so that following downflow fil-
tration in the lower compartment, filtered water flows through a pipe into the upper tank.
When terminal head loss in the filter bed is reached, washwater flows from the upper tank
back through the filter.
Some filter control systems permit gravity flow washing of a filter using effluent from
the filters remaining in service. Such filters are called self-backwashing filters. They do
not use pumps or piping for backwashing; instead, all the filters discharge into a common
channel. A filtered water weir controls the water level in the channel so that the water
level is always higher than that of the filter washwater troughs or side weir. This differ-
ence in level must be sufficient to provide the head needed to deliver adequate water for
backwashing. Also, there must be a sufficient number of filters in operation to meet the
demand for backwash water. Figure 8.6 shows the configuration of a typical self-back-
washing filter. To provide the required backwash driving head, the filter box must be sub-
stantially deeper than those required for more conventional types of filter backwash sys-
tems. But, because no equipment is involved, the capital cost is the lowest. One
disadvantage of this design is that there is no way to control the backwash rate.