Water Softening - Public Water Systems - page 439

sufficient deposition of calcium carbonate to cause serious problems in the carbon
treatment, which can easily be avoided by prior filtration.
A more recent technology has been introduced which uses carbonic acid (formed
by aspirating CO 2 into a water via a Venturi-type eductor) to form a liquid-to-liquid
transfer of chemical. This system is reported to increase the adsorption efficiency by
10-20 percent and can be diffused into a pipeline, trough, tank, or any location that
has a minimum of 18 inches of submergence. The manufacturer indicates the retention
is also reduced to less than 10 minutes. This can be particularly valuable in reducing
costs associated with plant upgrades, expansions, or new construction.
OPERATION AND CONTROL OF RECARBONATION
The operation and control of recarbonation systems is easy and simple. Automated
control systems ordinarily use a single point of pH measurement following the last
stage of recarbonation as the basis of control. In two-stage recarbonation systems, the
split of total CO 2 flow between the two stages of treatment is fairly constant once it
is established for a given flow and the particular set of pH values desired, and control
based on the final pH alone is satisfactory without readjustment of the valves supplying
the first- and second-stage CO 2 supply headers.
An indirect but more sensitive control of recarbonation is provided by alkalinity
measurements. Continuous reliable automatic monitoring and control equipment is
available for either the pH or the alkalinity method, but the alkalinity measuring equip-
ment is considerably more expensive than the pH equipment. Manual control is also
quite satisfactory, based either on grab sampling and analysis or on observation of
continuous automatic monitoring of pH or alkalinity of the recarbonated water. The
CO 2 demands do not vary rapidly or widely, and manual control of dosage is better
than might be expected.
SAFETY
Under certain conditions carbon dioxide can be dangerous, and there are safety pre-
cautions that must be observed. Prolonged exposure to concentrations of 5 percent or
more CO 2 in air may cause unconsciousness and death. The maximum allowable daily
exposure for a period of 8 hours is 0.5 percent CO 2 in air. Carbon dioxide is 1.5 times
as dense as air, and therefore will tend to accumulate in low, confined areas. Filter-
type gas masks are not useful in atmospheres containing excess CO 2 , and self-
contained breathing apparatus and hose masks must be used. Contact of the skin with
liquid CO 2 can cause frostbite. Recarbonation basins should be located out of doors,
and enclosed structures must not be built above them, because of the danger of ex-
cessive amounts of CO 2 accumulating within the structures. Before repairmen enter
recarbonation basins, the CO 2 supply should be turned off and the space thoroughly
ventilated. In the use of liquid CO 2 there are many other safety considerations too
numerous and detailed to be covered completely here. Complete, published information
can be obtained from liquid CO 2 suppliers.