Filtration - Public Water Systems - page 399

Sand Leakage This can be prevented by using the coarse garnet layer between the
fine media and gravel supporting bed as recommended earlier. The garnet layer pre-
vents the downward migration and escape of media fines.
Loss of Media The loss of filter media, particularly anthracite during backwashing,
is one problem for which there is no complete solution. Losses can be reduced by
increasing the distance between the top of the expanded bed during maximum back-
wash flows and the washwater troughs. It also can be helped by cutting off air wash
or auxiliary scour 1 or 2 minutes before the end of the main backwash. A loss of 1-
2 inches per year of anthracite is not unusual, and should be replaced with new media
periodically.
Negative Head and Air Binding These can be avoided in most cases, but there may
be a few extreme situations, usually of short duration, where they cannot be entirely
eliminated. In any case, it is a good idea to provide a water depth of at least 5 feet
(1.5 m) above the surface of the unexpanded filter bed. The more depth the better, at
least as far as negative head and air binding are concerned. The filter should not be
operated to terminal headlosses that are greater than the depth of submergence of the
filter media. in order to minimize the potential for air binding.
When filter influent water contains dissolved oxygen at or near saturation levels,
and when the pressure in the filter media is reduced by siphon action to less than
atmospheric at a point below the surface of the fine media, the oxygen comes out of
solution, and gas bubbles are released. They may accumulate within the bed and tre-
mendously increase the resistance to flow, or headloss. When flow through a filter is
stopped and the water level is lowered in preparation for backwashing, bed pressures
are reduced and more oxygen is released. Even further release of bubbles occurs during
backwash, which may lead to loss of media in the waste backwash water when bubbles
adhere to anthracite or sand particles and carry them into the washwater troughs. More
frequent filter backwashing may alleviate the problem to some extent, as there is then
less time for bubbles to accumulate. However, when the problem is acute—as it may
be in the spring when surface water is warming, and oxygen solubility is decreasing
at the higher temperatures—it may only be endured and not solved. Maintaining max-
imum water depths above the beds and frequent backwashing may help resolve, but
may not completely eliminate, the difficulties.
Air Air can leak into the system by a variety of means. If the filter rate control valve
is located above the hydraulic gradient, air may enter the system through the stuffing
box between the valve and the valve operator. The problem is aggravated by the
aspiration effect of a control valve located in a reduced cross section of pipe. A similar
problem can occur during the filtration cycle through the stuffing boxes on surface
wash systems, where one of the surface wash arms is located in the media. If there is
a negative head in the filter, air may be pulled through the stuffing box into the bed
during the filtration cycle. 50 Such leakage can be prevented by locating the stuffing
box at an elevation that is always submerged during the filter cycle.
DIRECT FILTRATION
The ability of dual- and mixed-media filters and deep, relatively coarse monomedia to
tolerate higher applied turbidities has resulted in several applications where coagulated