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14.24 CHAPTER FOURTEEN

Premature breakthrough of organics may also be reduced. Carbon usage rates under se-
ries operation can approach one-half the rate under parallel operation, reducing operating
costs accordingly. The lead absorber can be backwashed to remove suspended solids that
accumulate in the carbon bed.

Fixed Beds in Parallel The most common GAC application in drinking water treat-
ment is the downflow fixed bed in parallel operation. Downflow operation is appropriate
when the carbon bed is to be used as a suspended solids filter as well as an adsorber. In
this design, carbon contained in the adsorber remains stationary. Flow is divided equally
to each contactor, and each contactor is sized for the design EBCT. Suspended solids are
periodically removed by backwashing in a manner nearly identical to that used for sand
and dual-media (sand and anthracite) filters. When the carbon adsorber is preceded by
conventional filtration, downflow operation can sometimes be used with reduced or no
backwashing at all. In a parallel configuration, each carbon bed receives essentially the
same quantity and quality of flow. Start-up of individual units is staggered so that ex-
haustion of the carbon occurs sequentially. This allows removal of all carbon from each
adsorber, one at a time, for reactivation. For systems operating at full design capacity, a
spare adsorber can be provided to bring online when an adsorber is taken out of service.
In this arrangement, the level of carbon exhaustion when an adsorber is discharged is
not as high as that of fixed beds in series because no adsorber is operated completely to
breakthrough. However, carbon use in a given adsorber can be increased by blending ef-
fluent from all adsorbers. One or more adsorbers can be run slightly beyond the break-
through point while other adsorbers produce water with concentrations below the break-
through point.
Because effluent from each of the units is blended, each unit can be operated until it
is producing a water with an effluent concentration in excess of the treated water goal.
Only the composite flow must meet the effluent quality goal. For example, if 10 adsor-
bers are used in parallel, each adsorber can process 10,000 bed volumes of throughput if
the effluent TOC criterion is 50% of the influent, compared with 5,000 bed volumes if
only a single contactor were used or if all contactors were operated in parallel but were
replaced at the same time. This method of operation may be most appropriate for large
plants.

Upflow Expanded Beds. Upflow expanded (moving) beds are best suited for waters with
high suspended solids concentrations when suspended solids are to be removed by sub-
sequent processes (filtration). For high suspended solids concentrations, upflow beds may
be preferred, because solids accumulation and corresponding head losses would be ex-
cessive in downflow adsorbers. For low suspended solids concentrations, upflow adsor-
bers can be considered, because the carbon bed is not needed as a solids filter.
In an upflow bed, the upward movement of water causes the carbon bed to expand
slightly (approximately 10%). A higher CUR is expected for expanded beds because mix-
ing of the carbon creates a longer mass transfer zone. Mixing may allow the release of
carbon fines into the effluent flow. Expanded beds should not be used where downstream
contamination by suspended solids or carbon fines passing through the bed would be a
problem.

Pulsed Beds. A pulsed bed operates in an upflow mode, with water and carbon flow
moving countercurrent. Pulsed bed adsorbers permit intermittent or continuous removal
of spent carbon from the bottom of the bed while fresh carbon is added at the top with-
out system shutdown. The chief advantage of this system is better carbon use because
only thoroughly exhausted carbon is removed. In contrast with fixed beds, a pulsed bed