Civil Engineering Reference
In-Depth Information
GAC Contactors: Facilities Design
Selection of the general type of carbon con-
tactor to be used for a particular water treatment plant application may be based on
several considerations, including economics and the judgment and experience of the
engineering designer. The choice is generally made from three types of downflow
vessels:
Deep-bed, factory-fabricated, steel pressure vessels up to 17-ft (3.7-m) maximum
diameter provide GAC bed volumes up to 700 ft
3
(20 m
3
)—i.e., 20,000 lb (9,100
kg)—per single contactor. Multiple units are used to build large systems.
•
Reinforced concrete, gravity-filter-type boxes are used for carbon volumes, typi-
cally above 1,000 ft
3
(30 m
3
). Shallow beds could possibly be used only when
short contact times are sufficient or when long service cycles between carbon
regenerations can be expected from pilot plant test results. Shallow beds increase
backwash facility needs. Deep beds are used for most applications.
•
Deep-bed, site-fabricated, 20-30-ft (6-10-m)-diameter, open concrete or steel,
gravity tanks may be used for midsize units.
•
These ranges overlap, and the designer may very well make the final selection
based on local factors other than total capacity that affect efficiency and cost.
As previously mentioned, the current design trend in retrofitting existing water
treatment plants with new GAC adsorption facilities is to provide separate, postfiltra-
tion, downflow contactors. Contactor flow rates are usually 2 to 10 gpm / ft
2
(5 to 25
m / h), and GAC bed depths are normally 2.5 to 15 ft (0.75 to 5 m). A direct linear
relationship between contact time and carbon bed performance has been found in full-
scale plant tests and concurrent small column tests. Carbon performance at a given
contact time has been found to be unaffected by variations in hydraulic loading rates
in the 2 to 10 gpm / ft
2
(5 to 25 m / h) range. Thus, in terms of adsorption only, contact
time is the governing criterion. The surface loading rate is maintained within the ranges
of practicality to limit hydraulic headloss and turbulence.
When the granular carbon bed is functioning as both a turbidity removal unit and
an adsorption unit, there may be reasons to limit the bed depth and flow rate parameters
to remove turbidity effectively and to backwash the filter properly. If GAC is to be
effective in turbidity removal, the GAC particles must be hard enough to withstand
vigorous backwash agitation. At the same time, it should be dense enough to expand
during the backwash cycle and to settle quickly for immediate resumption of filtration.
As discussed earlier, coal-based granular carbon possesses approximately the same
density and filtration characteristics as anthracite and has found increasing use in the
water field.
Particle size of the carbon, in addition to contact time, should be considered care-
fully as a design factor. Reduction of particle size for a given set of flow conditions
is a means of increasing adsorption rates and thereby improving adsorption perform-
ance. Reduction in particle size to improve adsorption must be consistent with other
significant factors such as headloss and backwash expansion. The length of the filter
run in an adsorption-filtration bed would also be a problem if too small a particle size
were chosen.
Where existing rapid sand filters are being converted to adsorption-filtration units,
the permissible depth of the carbon layer will be limited by the freeboard available in
the existing structure. Adequate space between the carbon surface and the backwash
