Dub

copt = optimum volume of carbon bed, lb Q = flow rate, gal

The optimal tradeoff point between a lower carbon usage rate and a smaller carbon bed size can be found through analysis. A typical minimum contact time for gasoline contaminants is fifteen minutes. This contact time corresponds to a liquid loading rate of 2 gpm/ft2 in a standard 20,000-lb and 10-ft-diameter carbon vessel (Noonan and Curtis 1990). Table 9.17.4 lists the contact times as well as carbon usage rates for several organics.

Methods of Construction

GAC is available from a number of suppliers in vessels of different sizes. The vessels are typically open-top, cylindrical steel tanks for gravity systems and closed-top, cylindrical steel tanks for pressure systems. Gravity systems are operated like sand filters and are generally used for high flows, such as at municipal wastewater treatment plants. Pressure systems are generally used for smaller flows and allow higher surface loading rates (5-7 gpm/ft2 compared to 2-4 gpm/ft2 for gravity systems) and pressure discharge to the distribution system, saving pumping costs (Noonan and Curtis 1990).

Activated carbon is commonly made from coal; other materials such as coconut shells, lignite, wood, tires, and pulp residues can also be used. In the formation of GAC, the material is subjected first to a high temperature to remove water and other vapors from it. Then, a superheated steam is released into the material (activation) to enlarge the pores and remove ashes from it (Noonan and Curtis 1990).

Bleedoff valve Sample in

Pump

Backwash inlet -

Flow-control valve

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