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14.22 CHAPTER FOURTEEN

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.~ it~rd a syste 4~ ~ 4 4 •

Sand - 4 to 6 in
Carbon - 3 to 8 in
Sand - 8 to 10 in
Underdrain system - see Figures 14B & 149
FIGURE 14.11 Media arrangement in a GAC sandwich slow sand filter.

Postfilter adsorbers typically use GAC with small effective sizes and large uniformity
coefficients to promote rapid adsorption of organic compounds, better stratification, and
reduction of overall carbon loss. These adsorbers also can provide an improved barrier
against microbial penetration, are more compatible with other advanced treatment pro-
cesses such as ozonation (or ozone and hydrogen peroxide), and use more of the adsorp-
tive capacity of the carbon.

Adsorber Configurations. Various combinations of the three GAC contactor configu-
rations and flow patterns can be made. Some of the most common contactor configura-
tions are shown in Figure 14.12. Table 14.5 provides a summary of each configuration
described below.
Fixed Beds in Series. In this configuration, flow is downward through the carbon bed
for each unit connected in series, with the EBCT divided between the number of adsor-
bers. The first in the series receives the highest contaminant loading and is thus the first
to be exhausted. The last in the series receives the lightest contaminant loading and serves
as a polishing step. When carbon is removed for reactivation, the first adsorber in the se-
ries is removed from service, with the next adsorber in line becoming the lead unit. This
system can be constructed with an extra adsorber on standby to become the first adsor-
ber when the lead adsorber is taken out of service. The configuration can be arranged and
controlled so that the freshest GAC is always the last step in the series.
Capital cost considerations usually limit the number of adsorbers in series to four or
fewer, because the increased cost of piping and valving counters the cost benefit of re-
duced carbon usage. Countercurrent flow provides highly efficient usage of the carbon by
maximizing carbon exhaustion in the lead adsorber before it is removed for reactivation.