Page 542 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Adsorption 497

15.5.2.3.3 Effects of Regeneration Solution (gist of the problem)
A decrease in micropore volume occurs with each regenera- 1. Capacity of carbon for two influent concentrations
. Consider TCE on Filtrasorb 300(40) (Love
tion (West, 1971; Cannon et al., 1993); thus, the carbon does
not return to its virgin state. The molasses number of the et al., 1983, p. 12)
regenerated carbon is about the same as that of the virgin K ¼ 28
carbon. The iodine number is lower, as a rule, indicating 1=n ¼ 0.62
. Let C 1 ¼ 0.01 mg=L ! X* ¼ KC 1=n
fewer pores in the 5–14 Å radius range. The net loss of absorp- 0.62 ¼
tion capacity is up to 13% on the first regeneration cycle; this 28 0.01 ¼ 1.61 mg TCE=g carbon
. Let C 2 ¼ 0.02 mg=L !¼ 28 0.02 0.62 ¼ 2.47 mg
loss diminishes with further cycles; the carbon loss for each
TCE=g carbon
successive cycle ranges from 5% to 10%.
2. Mass of carbon needed:
mass of adsorbate to be removed ¼ mass of adsorb-
15.5.2.3.4 Cost of Regeneration
ate adsorbed by adsorbent
Generally, on-site reactivation is economical only for a carbon
use rate >900 kg=day (2000 lb=day); otherwise, off-site reacti-
Q C 0 Dt ¼ V(carbon bed) (1 P) r X*
vation may be more economical (Groeber, 1991, p. 4); others
give the threshold rate >4500 kg=day (10,000 lb=day) (Stenzel,
1993, p. 42). Usually, off-site reactivation is done on a contract 3. The ratio of carbon required for C 2 is (2.47=1.61)
carbon required for C 1
basis, with spent carbon taken away by truck and regenerated
15.3 Mass of GAC from Published Isotherm Plots
carbon delivered at the same time. For such a case, a reactor
Given
column may serve as storage for spent GAC. Typical cost
Obtain isotherm data for two or more organic com-
is $1.75–2.20=kg ($0.80–1.00=lb) for virgin carbon and
pounds that are classified as ‘‘contaminants.’’ Suppose
$1.30–1.75=lb for regenerated carbon (Stenzel, 1993, p. 42).
that these compounds are to be removed by activated
carbon adsorption.
PROBLEMS Required
(a) For assumed levels of contaminants, in the mg=L
15.1 Mass of GAC from Laboratory Isotherm Plots
range, calculate the amount of carbon required to
Given
treat the water for 30 days. Assume the flow of
Consider the experimental isotherm of Figure 15.6 as
water to be treated is 11 mgd.
being applicable for the design of an adsorption
(b) Suppose that the contaminant levels are doubled.
reactor. Let the influent concentration of Rhodamine-B
Calculate the amount of carbon required.
dye be whatever you wish to select (it should be in the
15.4 Scenarios for Velocity of Wave Front Based on
range of your isotherm curve). Assume the adsorbate is
Isotherms
Dowex-50 resin, as in the figure.
Given
Required
Table CD15.A.1 has Fruendlich constants, K, n, for a
(a) For assumed levels of Rhodamine-B dye, in the
list of organic compounds, with some listed in Table
mg=L range, calculate the amount (i.e., mass in kg)
CDprob15.3. For the problem, use the following data
of Dowex-50 resin required to treat the water for
as necessary:
30 days. Assume the flow of water to be treated is
. Adsorbent is granular activated carbon
11 mgd. 3
. r(carbon) 1.4 g=cm
(b) Suppose the hydraulic loading rate for the reactor is . Porosity, P ¼ 0.40
2
5 gpm=ft . Determine the size of the reactor 2
. HLR ¼ 5 gpm=ft
required. If the depth is quite short, select a longer
. Temperature, T ¼ 208C
time so that the depth of the reactor is at least 3–5m.
. Assume the carbon bed should remain in operation
(c) Suppose that the Rhodamine-B dye levels are
for say 3–6 months before it is exhausted. If this
doubled. Calculate the amount of carbon required.
duration results in a very small length of bed then
15.2 Mass of GAC for Two Adsorbate Influent
make it a reasonable length and calculate the time of
Concentrations
operation before exhaustion.
Given . Most carbon beds in practice for water treatment vary
Assume TCE is to be removed by GAC. Assume the in their length dimension within a fairly narrow range.
3
flow of water to be treated is 0.482 m =day (11 mgd) For example, at the Klein Water Treatment Facility
2
and 12.2 m=h (HLR ¼ 5.0 gpm=ft ). the beds are 3–4 m deep. The Denver Reuse Plant
Required columns were perhaps 10 m depth.
Determine the mass of GAC required for two concen- . The GAC may be any brand for which isotherm
trations of TCE (let C 2 ¼ 2 C 1 . data are available.