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14.28 CHAPTER FOURTEEN

• Number and size of valves required
• Structural considerations

For purposes of determining the optimum number of contactors, estimates of capital
costs should be developed for various contactor configurations. A plot of the relative cap-
ital cost as a function of the number of contactors aids in selecting the best configuration.
Perhaps the most important factor in the design of a GAC adsorption facility is the
type of underdrain system to be used. Underdrains must perform several important
functions:
• Collect water at a uniform rate to ensure GAC retention in the contactor
• Prevent binding or plugging of the collection system
• Distribute water evenly over the entire contactor area
• Provide uniform fluidization of GAC at a low backwash rate during transfer of spent
carbon out of the contactors so that carbon moves to the points of discharge

Common Operating Problems

Operating problems encountered when activated carbon is used as a filter medium or ad-
sorber are similar to those found with most rapid sand or dual-media filters. The most
common difficulties faced in operating a GAC facility that the designer should be aware
of when preparing a design include

• Effectively monitoring the carbon bed depth
• Properly backwashing the carbon, as carbon bed density and depth can change during
backwashing
• Removing carbon from the contactors and storage tanks
• Transporting the carbon
• Regenerating the carbon
• Maintaining a good-quality contactor influent
• Controlling contactor flow rate
• Controlling biological activity in the carbon adsorber

Monitoring the Carbon Bed Depth. A means must be provided for continuously moni-
toring and measuring the levels in all storage vessels, including contactor and storage tanks
(virgin, spent, and regenerated carbon). The designer must recognize that slurry concentra-
tions and flow rates change during the filling process. When carbon is placed in an adsor-
ber, it typically has a density of approximately 90% of the apparent density for random-
packed graded media. When a carbon column is first backwashed, density decreases from
approximately 90% to approximately 83%, resulting in a permanent expansion of 8%.
An 8% expansion in a 24-in. (60-cm) filter is only 1.9 in. (4.8 cm) and does not pre-
sent any major operating problems. However, an 8% expansion in a 15-ft-deep (4.6-m)
carbon bed entails a permanent expansion of 14 in. (35 cm). This can create a major prob-
lem when one is trying to closely control the surface elevation of a deep bed.
The problem of controlling surface elevation of deep bed, graded media is complicated
further by the fact that the bed also compacts during operation. The same 15-ft (4.6-m)
column could see a compaction of 5 to 15 in. (13 to 38 cm). If surface wash sweeps are
used, they must be placed to accommodate this expansion and contraction of the bed. As
noted, conventional surface wash sweeps are not recommended.