Civil Engineering Reference
In-Depth Information
TABLE 12-8. Filter Piping Design Flows and Velocities
Velocity
ft / sec (m / s)
Maximum Flow, gpm / sq ft
(m / h) of Filter Area
Description
Influent
1-4 (0.305-1.22)
8-12 (20-30)
Effluent
3-6 (0.92-1.83)
8-12 (20-30)
Washwater supply
5-10 (1.52-3.05)
15-25 (35-60)
Backwash waste
3-8 (0.92-2.44)
15-25 (35-60)
Filter-to-waste
6-12 (1.83-3.66)
4-8 (10-20)
precaution is to install an influent valve with throttling control for use in slowly re-
filling the beds. The filter-to-waste connection to the filter should have positive air gap
protection against backsiphoning from the drain to the filter bottom. The filter-to-waste,
effluent, and washwater supply lines usually are manifolded for common connection
to the filter underdrain system.
In the design of pipe galleries, reinforced concrete flumes and box conduits and
concrete-encased concrete pipe may be used for washwater drains or other service
when located adjacent to the pipe gallery floor, but should not be installed overhead
because of difficulties with cracks and leaks. Invariably, pipe galleries with overhead
concrete conduits are drippy, damp, unsightly places with a humid atmosphere that
discourages good housekeeping by making it difficult to maintain. Instead, pipe gal-
leries should be provided with positive drainage, good ventilation, sufficient light, and
dehumidification equipment (if required by the prevailing climate). Filter influent and
effluent lines should be provided with sample taps.
The rubber-seated, electrically or pneumatically actuated and operated butterfly
valve has almost entirely replaced the hydraulically actuated and operated gate valves
that were formerly used extensively as filter valves. Of the two types, the butterfly
valve is smaller, lighter, easier to install, and better for throttling services, and it can
be installed and operated in any position. The valves should be factory-equipped with
the desired valve stops, limit switches, and position indicators because field-mounting
of these devices is often unsatisfactory.
Each filter unit, except split beds, should have six valves for its proper operation:
influent, effluent, washwater supply, washwater drain, surface wash or air scour, and
filter-to-waste. The positions of these valves during the three cycles of filter operation
were given in Table 12-7.
Existing Plant Expansion and Conversion
Because multimedia filters operate more efficiently, safely, and reliably at 5 gpm / sq ft
(12 m / h) than do conventional rapid-sand filters at only 2 gpm / sq ft (5 m / h), there
obviously is great potential for expanding the capacity of existing plants at least up
to double with only the nominal expense of replacing sand with dual or mixed media.
This, of course, has been done in a great many instances. Because of the ability of
multimedia filters to remove and store solids from high-turbidity waters, often it is not
necessary to add settling basin capacity in plant expansion. In other cases, this must
be done, or, as an alternative, settling tubes may be installed in existing basins because
this change will allow increasing basin throughput without loss of settling efficiency.
