Activated carbon systems can be operated as upflow, ex-panded-bed columns or downflow, fixed-bed columns. Upflow expanded beds can tolerate higher suspended-solids loading than downflow beds. They also make efficient use of the carbon since fully exhausted carbon can be removed from the bottom of the bed while fresh carbon is added to the top. In addition, carbon beds can be operated in parallel (single-stage) or in series (multiple-stage) as shown in Figure 9.17.13. When operated in series, the leading contactor removes the majority of the contamination, while the second contactor removes any residual organics from the water. Furthermore, multiple-stage use allows a contactor to be completely exhausted before regeneration, while effluent quality remains protected by the subsequent contactor. When operated in parallel, contactors should stagger startup to permit bed-by-bed regeneration without reducing effluent quality.
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