Civil Engineering Reference
In-Depth Information
short upward journey from the distribution grid through 8 feet (2.5 m) of water to the
water surface, the time for complete reaction between the dissolved CO
2
and hydroxide
and carbonate ions may be as great as 15 minutes in cold water. In the primary phase
of two-stage recarbonation, if the reaction if allowed to go to completion at a pH near
9.3, the calcium carbonate formed is not redissolved in the second phase of recarbon-
ation to a low pH—say, to pH 7.0. Magnesium salts do tend to redissolve under these
conditions. In the recarbonation of water, a floc is formed following first-stage recar-
bonation. This is a rich source of calcium carbonate from which lime (CaO) can be
reclaimed and reused by recalcining at temperatures of about 1,850
F (1,000
C). In
this case, then, it is desirable to allow not only for reaction time (15 minutes) but for
enough time to provide some separation of the calcium carbonate by settling. This will
require a settling basin with at least 30-minute detention at maximum flow rate, and
a basin surface overflow rate of not more than 2,400 gal / sq ft / day (4 m / h) at peak
flow rates. This intermediate settling basin should be fitted with continuous mechanical
sludge removal equipment. Figure 13-8 shows a two-stage recarbonation system with
intermediate reaction and settling.
Single-stage recarbonation systems should be followed by 15 minutes of detention
for completion of the chemical reactions, but no provisions for settling or sludge
collection are required. The light, cloudy floc that may be formed at times with single-
stage recarbonation is removed quite readily by mixed-media filtration with little effect
on filter effluent turbidity, headloss, or length of filter run.
Generally, recarbonated lime-treated water should not be applied directly to beds
of granular activated carbon without filtration. Even at low pH (say 7.0), there can be
10 ft. (3.05 m)
10 ft. (3.05 m)
75 ft. (22.86 m)
41 ft. (12.5 m)
Reaction and
Serttling Basin
Flow In
Flow Out
Secondary
Recarbonation
Basin
Primary
Recarbonation
Basin
CO
2
In
Plan
Carbon Dioxide
Distribution
Grids
Cross
Collector
Sludge
Collector
12 ft. (3.66 m)
12 ft. (3.66 m)
10 ft. (3.05 m)
9 ft. (2.74 m)
Section
Fig. 13-8.
Two-stage recarbonation basin (From Culp, Gordon, and Williams, Robert,
Handbook
of Public Water Systems.
Copyright
1986 by John Wiley & Sons, Inc. Reprinted by permission
of John Wiley & Sons, Inc.)
