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8.40 CHAPTER EIGHT

controlled manner, creating a visible boiling action at the top of the fluidized media. If
the support gravel is disturbed, it requires manual regradation after the overlying filter
media are removed.
Improper specifications for the gradation and thickness of gravel layers can also cre-
ate support gravel upset. Refer to AWWA Standard BlOO--Filtering Material and consult
with filter media and filter underdrain suppliers for guidance on support gravel for vari-
ous filter media and underdrain combinations. Design errors relating to the washwater
supply or rate controller, improper programming of an automatic filter wash sequence,
and improper installation of the gravel layers can also lead to support gravel upset.
Air Binding. Filter influent waters, particularly from surface water sources, typically
contain significant concentrations of dissolved gases. Depending on water temperature,
the dissolved gas concentration may reach saturation point. Surface water supplies typi-
cally reach saturation point during algal blooms, during seasonal changes when temper-
atures increase, or where there is significant cascading and aeration of source water.
When head loss exceeds the available head at some elevation within the filter media,
pressure falls below atmospheric and air escapes from solution. Air binding occurs when
the accumulation of air bubbles blocks the water's path. An excessive effective filtration
rate and significantly increased head loss result. Air binding most often occurs a few
inches into a single-medium filter or just below the anthracite-sand or GAC-sand inter-
face in a dual-media filter. Air escapes from the media upon closure of the filtered water
rate control valve, and the resulting agitation can cause loss of media. By design, air bind-
ing does not occur in pressure, declining-rate, or self-backwashing filters.
Recommended Standards for Water Works (Great Lakes, 2003) recommends a mini-
mum water depth of 3 ft (1 m) above filter media in gravity plants. Modem plants that
operate at relatively high filtration rates typically require a water depth of 5 ft (1.5 m) or
more to prevent air binding within the media. The designer should consider the rate of
head loss development along with the desired filter run length to develop design criteria
for the filter media, filter box depth, and freeboard.

Restart after Shutdown. Water treatment plants sometimes operate with individual fil-
ters removed from service during low-demand periods. Additionally, some plants shut
down at night or cycle the filters on and off based on distribution storage tank levels.
Some plants place individual filters or entire plants on standby for periods of time and
even drain the filters when they have sufficient redundancy or more than one plant. Prefer-
ably, all filters should be operated continuously at a reduced filtration rate. Continuous
operation prevents formation of undesirable biological activity caused by water standing
in the filter box and avoids loss of ripened filter media in a drained filter. In any case, in-
active filters require special measures for restart.
Filters that are shut down with standing water require a brief washing or filter-to-waste
period to ensure adequate filter performance before filtered water flows to the clearwell.
The restart wash or filter-to-waste period should be programmed into the centralized con-
trol system for the plant.
Drained filters require manual restart with close supervision, including gradual refill-
ing with washwater to prevent gravel upset or media separation, a complete backwash,
and a longer than normal period of filter-to-waste to ensure adequate performance before
opening the filtered water control valve. Filters should always be filled from the bottom
to prevent air entrainment.

Filter Media Replacement. Rapid sand filters with dual and mixed media can lose 5%
to 7% of the media per year from air binding, excessive washing, air scour, or surface
wash. The loss can be particularly serious when there is cold water, mismatched dual or