Filtration - Public Water Systems - page 391

the surface of the unexpanded filter media. In dual- or mixed-media filters, dual-arm
devices have also been used to provide better cleaning at the anthracite-sand interface.
In these systems, a second set of arms is located about 6 inches (150 mm) above the
anthracite-sand interface in a dual-media filter or about 18 inches (450 mm) above
the top of the gravel in a mixed-media filter. The arms are fitted with a series of
nozzles, and revolve because of the water jet reaction. Water pressures of 40 to 100 psi
(275 to 700 kPA) are required for the operation of the rotary surface wash, depending
upon the diameter of the arms. The volume required is about 0.5-1.0 gpm / sq ft (1.2-
2.4 m / h). Surface washers are usually started about 1 minute before the end of the
backwash period. It is recommended that the nozzles be equipped with rubber caps
that act to prevent entry of fine filter media and plugging of the nozzles.
Baylis designed a fixed-jet surface wash system consisting of a grid of distributing
pipes extending to within a couple of inches of the top surface of the bed. Nozzles
with five 1 ⁄ 4 -inch (6-mm) holes are spaced at about 24- to 30-inch (600- to 750-mm)
centers each way. The required flow is about 2 gpm / sq ft (5 m / h) at a head of 20 to
60 feet (6 to 18 m).
Surface wash piping is a direct cross-connection between filtered and unfiltered
water. Normal practice is to bring the surface wash header into the filter over the top
of the filter box and to fit it with a vacuum breaker and a check valve at the high
point to prevent backsiphoning. A single vacuum breaker on the surface wash header
will suffice. An alternate is to use settled water through a separate surface wash pump.
The filter valves must be in the proper position to ensure proper cleaning of the filters.
Table 12-7 shows valve positions for various cycles of filter operation.

Loss of Head Monitoring
The loss of head through a filter provides valuable information about the condition of
the bed and its proper operation. An increase in the initial loss of head for successive
runs over a period of time may indicate clogging of the underdrains or gravel, the
need for auxiliary scour, or insufficient washing of the beds. The rate of headloss
increase during a run yields considerable information concerning the efficiency of both
the pretreatment and the filtration operation.
The determination of headloss through a filter is a very simple matter, involving
only the measurement of the relative water levels on either side of the filter. The
simplest form of headloss device for gravity filters is made up of two transparent tubes

TABLE 12-7. Valve Positions During Various Treatment Operations
Valve Position

Valve Filtering Backwashing Filtering to Waste

Influent Open Closed Open

Effluent Open Closed Closed

Filter-to-waste Closed Closed Open

Washwater supply Closed Open Closed

Washwater drain Closed Open Closed

Surface wash or air scour Closed Open Closed

(From Culp, Gordon, and Williams, Robert, Handbook of Public Water Systems. Copyright 1986 by John

Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)