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7.42 CHAPTER SEVEN

Application
The fact that clarification results from filtration limits the solids levels that can be re-
moved by contact clarification without causing excessive cleaning. This process is most
effective for source water having low turbidity, color, iron, and manganese. On such wa-
ters, it can be an economical alternative to more conventional types of clarification: It is
especially applicable to upgrading overloaded direct filtration systems.

Design Criteria
Contact clarifiers are rated on the basis of surface loading rate and typically operate in
the range of 8 to 10 gpm/ft 2 (20 to 24 m/h). To maintain effective flocculation, units must
also be selected to operate in the range of 50% to 100% of design capacity.

Operation
Source water is chemically treated by the addition of metal salts or polymers in a rapid
mixer, usually of the static mixer type. Chemically treated water is then introduced to the
clarifier in an upflow or downflow mode, depending on the proprietary process selected.
Clarified water is then filtered in a typical granular medium filter.

Inlet Conditions

Proprietary units are designed as upflow units. The Adsorption Clarifier uses a buoyant
medium held in place by a screen. Chemically treated source water is introduced under
the medium through a pipe lateral. Other types of units use a medium heavier than water
that must be supported by a media-retaining support system typical of gravity filters. In
all cases, source water flows up through the medium as it would for any upflow filter.

Outlet Conditions
Clarified water is collected above the medium in an effluent trough similar to backwash
troughs in a filter and then flows to the filter through an outlet valve.

Sludge Removal
Because solids are removed by filtration, they must be removed by backwashing or flush-
ing. The flushing cycle is initiated after a preset head loss has been reached or a preset
time has passed.
The cycle is started by closing the effluent and influent valves. The medium is first
air-scoured. Air is necessary in the adsorption clarifier to allow buoyant media to expand
during the flushing cycle. Air reduces the apparent density of the water, causing media
to lose buoyancy and opening pore spaces so that accumulated material can be flushed
out. Air is also necessary with other types of media to assist water in scouring accumu-
lated solids.