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ACTIVATED CARBON PROCESSES 14.17

pended solids concentrations with no backwashing or with less frequent backwashing than
is possible with gravity flow. Pressure adsorbers can be less expensive to design and con-
struct because they can be prefabricated and shipped to the site.
For many systems, the decision between pressure or gravity adsorbers is based solely
on initial installed cost. Most existing and many new water treatment plants require that
the total plant flow be pumped to postfiltration GAC facilities because of insufficient hy-
draulic head downstream of the existing or new conventional filters. Pressurized adsor-
bers (commonly called contactors) may be more cost-effective in these installations.

Design of GAC Filtration/Adsorption Beds
For filtration/adsorption, either as a filter cap or full-depth GAC, major design consider-
ations include:
• Filter run lengths
• Means for cleaning the GAC bed as a part of routine operations
• Life expectancy of the GAC
• Chlorine, polymer, and ozone feed points
• Size and depth of GAC as filter media
• Matched GAC and sand barrier layers during backwash (GAC cap)
• Head loss restrictions
• Backwash restrictions
• Flow rate restrictions--inlet and outlet piping and conduits

Retrofitting Existing High-Rate Granular Media Filters to Filter-Adsorbers

One option readily available for most water treatment facilities is to replace conventional
filter media with GAC. Existing filter boxes can be converted to GAC filter-adsorbers
simply by removing a portion of or all the granular media from the filter box and re-
placing them with GAC. Alternatively, an entirely new filter box, underdrain, and back-
washing system for the GAC can be designed and constructed. In these applications, GAC
is used for turbidity and solids removal, biological stabilization, and dissolved organics
removal by adsorption.
Filter-adsorbers must balance the constraints of both adsorption and filtration. They
typically require backwashing at about the same frequency as conventional filters. A
filter-adsorber design can be easily installed as a retrofit for an existing conventional gran-
ular filter, but the designer must recognize and account for the fact that filter-adsorbers
• Have shorter filter run times and must be backwashed more frequently than postfilter
adsorbers, which in some applications do not require backwashing at all
• May incur greater carbon losses than postfilter adsorbers because of more frequent back-
washing
• May cost more to operate than postfilter adsorbers because carbon usage is less effi-
cient

Washwater Trough Considerations. In retrofitting existing filters, modifications may
be necessary to position the washwater collection troughs to allow for proper bed expan-