Encyclopedia of Environmental Science and Engineering, Volume I and II

982 PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS

providing a highly clarified feed and permitting an extended

operation of carbon beds. Carbon beds were of the upflow

full-countercurrent columns and were usually operated as

packed types since the turbidity of the applied water was

always less than 0.3 JTU. The columns were also operated at

times as expanded beds by drawing 10% of the carbon.

The efficiency of carbon bed treatment depends on:

1. Contact time between the carbon and the
   wastewater


2. The pH of the wastewater (below pH of 9.0,
   the rate of adsorption of organics in wastewater
   increases with decreasing pH)


3. Temperature of the wastewater


4. Quality of the influent.

Carbon-Bed Design The following are the important design

parameters used in the design of activated-carbon beds:

1. Properties of filtering material: The two most
   common sizes of granular carbon normally used
   for wastewater treatment are 8  30 mesh and
   12  14 mesh; 8  30 mesh carbon is preferable,
   although its surface area is less, because it reduces
   losses during regeneration, head loss is less, and
   the operation of the filter becomes easier. Culp and
   Culp (1971) have reported the desirable physical

properties for granular activated carbon for use in

wastewater treatment.

2. Depth of carbon bed: 3 to 10 m.


3. Flow rates: 140 to 700 m^3 /min per square meter
   of column cross-sectional area. For optimum per-
   formance of the bed, the actual depth and rate of
   flow should be determined by a dynamic pilot-
   plant test in the laboratory.


4. Activated carbon contact time: 15 to 35 mm,
   depending on the objective of treatment and the
   impurities to be removed from the wastewater.

A diagram of a carbon-gas-adsorption process is presented

in Figure 3.

In the 1970s, over 30 granular activated-carbon plants

were designed in the United States for use at municipal

wastewater-treatment plants (DeJohn and Edwards, 1981).

Thirteen of the plants are classed as physical-chemical and

15 as tertiary treatment plants, and 4 use carbon to dechlori-

nate. According to the authors, there have been some prob-

lems encountered at certain physical-chemical and tertiary

treatment plants, but in their opinion, granular activated

carbon is a viable treatment alternative when applied under

proper conditions (Grieves et al., 1964).

One of the changes in conventional treatment technology is

the use of the PACT (Powdered Activated Carbon Treatment)

process using powdered carbon in the aeration tank of an

activated sludge system (Meidl, 1981). The development

Backwash effluent

return

Backwash

effluent

sump

Column

1

Column

2

Column

3

Column

From storage^4

reservoir

Raw water

feed pump

Virgin

carbon

makeup

Slurry

mixing

tank

Regenerated

carbon

return

Regenerated

carbon

storage

Utility

water

High-pressure

water

Eductor

Quench

tank

Spent

carbon

tank

Eductor

High-pressure

water

Wet

scrubber

Multiple-

hearth

regen-

eration

furnace

Eductor

High-pressure

water

Utility

water

Final effluent

Effluent

retention

sump

Back-

wash

sump

Backwash

pump

FIGURE 3 Carbon-adsorption process diagram (From Eckenfelder, 2000. Reprinted with permission from

McGraw-Hill.)

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