Civil Engineering Reference
In-Depth Information
stresses on the accumulated floc. Eventually the total hydraulic headloss may approach
or equal the head necessary to provide the desired flow rate through the filter, or there
may be a leakage or breakthrough of floc particles into the filter effluent. Just before
either of these outcomes can occur, the filter should be removed from service for
cleaning. In the old slow-sand filters, the arrangement of sand particles is fine to coarse
in the direction of filtration (down); most of the impurities removed from the water
collect on the top surface of the bed, which can be cleaned by mechanical scraping
and removal of about
1
⁄
2
inch (10 mm) of sand and floc. In rapid-sand filters, there is
somewhat deeper penetration of particles into the bed because of the coarser media
used and the higher flow rates employed. However, most of the materials are stored
in the top few inches of a rapid-sand filter bed. In dual-media and mixed-media beds,
floc is stored throughout the bed depth to within a few inches of the bottom of the
fine media.
Rapid-sand, dual-media, mixed-media, and deep monomedia, filters are cleaned by
hydraulic backwashing (upflow) with potable water. Backwashing of filters that have
become clogged with particulate matter captured on the grains of fine media and in
the pore spaces between grains can be accomplished in a variety of methods.
Types of Backwash Systems
The traditional backwash system uses treated water
wash, with the water introduced into the bottom of the filter bed through the underdrain
system to obtain full-bed fluidization. Three basic approaches are normally employed
for gravity filter backwashing:
Treated water wash
•
Treated water wash with surface wash
•
Treated water wash with air scour
•
Surface wash and air scour greatly reduce the potential for formation of mud balls
that endanger filter effectiveness.
The surface wash assisted wash water system is the most commonly used backwash
system for gravity filters. Although fixed nozzles can be used, normal practice is to
employ a rotary arm mechanism for the surface wash system, with nozzles located
directly above the filter bed so that they are in the expanded bed during backwash.
Fixed-nozzle assemblies are not normally used, as they restrict access to the filter
media and are costly to maintain. The advantages of a rotary surface wash system are
that the:
System is simple to implement and operate, as a source of high-pressure water is
the only auxiliary system needed.
•
Equipment is easily accessible for maintenance.
•
The disadvantages of a surface wash system are that the:
Rotary arm can stop rotation, preventing proper cleaning of the filter media.
•
Corner areas of the filter chamber are difficult to effectively clean, as the rotating
nozzles cannot pass directly over the filter surface.
•
Surface wash mechanism cleans only at one horizontal plane in the filter, provid-
ing no benefit to cleaning of the bed area further below the mechanism.
•
