Civil Engineering Reference
In-Depth Information
carbon on a once-through, throwaway basis. Medium-size plants using between 200
and 1,500 lb / d (100 and 700 kg / d) of GAC might also consider central, off-site carbon
reactivation or service contracts with carbon manufacturers to replace and reactivate
spent carbon.
4
Large plants will typically run their own on-site regeneration facilities.
In most instances, spent GAC will be classified as nonhazardous solid waste that can
be taken to a Class D landfill for disposal.
Disinfection By-Product Control
GAC may be used in either of two ways for DBP control. It can be used directly to
remove DBPs, or it can be used indirectly to remove the precursors that react with the
disinfectant to produce DBPs. In either case, good to very good removal is technically
feasible. When the GAC is fresh, removal is nearly complete, but toward exhaustion,
breakthrough begins. Trihalomethanes (THMs) containing bromine are adsorbed better
than chloroform. (Control of DBPs is described in Chapter 19 and in references 5-9.)
Volatile Organics Removal
Volatile organic chemicals occur in both untreated and treated drinking water. Signif-
icant concentrations are more likely to be found in well waters than in surface waters.
The potential health effects and acceptable limits of these substances in drinking water
are of concern. Volatile organics can be removed by aeration or adsorption on GAC
or synthetic resins, or by combinations of these processes, as described in Chapter 9.
Strong oxidants such as ozone and advanced oxidation (see Chapter 16) and reverse
osmosis (see Chapter 15) can also remove volatile organics. Boiling tap water for 5
minutes can also be effective for removing most of these organic compounds.
PRINCIPLES OF CARBON ADSORPTION
Activated carbon removes organic contaminants from water by a process of adsorption
that results from the attraction and accumulation of one substance on the surface of
another. In general, the chemical nature of the carbon surface is of relatively minor
significance in the adsorption of organics from water and is secondary to the magnitude
of the surface area of carbon available. Thus, a high surface area is the prime consid-
eration in adsorption. GACs typically have surface areas of 2.44 to 6.84 million ft
2
/
lb (500 to 1,400 m
2
/ g). Activated carbon has a preference to adsorb organic com-
pounds and, because of this selectivity, is particularly effective in removing organic
compounds that may cause taste and odor problems in water supplies. Because acti-
vated carbon can reduce a wide range of organic compounds to trace concentrations
(low microgram-per-liter level), it is often used as a general method to protect the
consumer against some unknown organic pollutant or to remove DBP precursors.
Activated carbon can be made from a variety of materials, such as coal, wood,
coconut shells, and petroleum coke. Granular carbons made from coal are hard and
dense and can be pumped in water slurry without appreciable deterioration. Hydraulic
handling of coal-derived carbon allows dust-free loading and unloading of filters. These
granular carbons are well suited to water treatment; the carbon wets rapidly and does
not float, but does form a densely packed bed with acceptable pressure drop charac-
