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14.16 CHAPTER FOURTEEN

3O (2O)
28 (18)
2s (18)
Sand J GAC (wetted in water)
24 (18)
22 (15)
20 (14)
~E 18(12)
¢=
16(111
I
:~' 14(10)
o o
~>
12 (8)
10(7)
a (8)
6 (4)
4 (3)
2(1)
I I I I I I I
0.4 0.8 1.2 1.8 20 24 2.8
Equivalent partcle d~meter (DgO) -- mm
FIGURE 14.6 Determination of appropriate medium size and backwash rate for a water temperature of
68 ° F (20 ° C) (backwash velocity = 1.3 × Vmf for D90).

depth and corresponding adsorber volume also depends on reactivation frequency and con-
tactor construction cost. The relationship between increased adsorber volume and reduced
reactivation frequency can be compared against a reduced adsorber volume and increased
reactivation frequency to determine the optimum characteristics for design. Together, CUR
and EBCT have the greatest effect on capital and operating costs for GAC processes.
GAC adsorption units can have one of three basic configurations, with single or mul-
tiple adsorbers operated in series or in parallel:

• Downflow fixed bed
• Upflow fixed (packed) or expanded (moving) bed
• Pulsed bed
The upflow configuration is not suitable for application in a potable water treatment pro-
cess unless media filters are located downstream of the adsorption units (Kawamura, 1991).

Type of Contactor Units
Gravity or Pressure Units. Larger installations are likely to be gravity fed. Pressure ad-
sorption units are cost-effective for smaller installations. Pressure flow can be used for
either downflow or upflow beds. Pressure flow achieves higher hydraulic loadings than
would be economically feasible with gravity flow. This higher loading reduces the re-
quired adsorber cross-sectional area. Pressure flow also permits operation at higher sus-