Civil Engineering Reference
In-Depth Information
TABLE 18-4. Hardness Conversion Factors
Multiplier to Convert from mg / L as the Ion to grains / gal
(as CaCO
3
)
Ion
Ca
2
0.146
Mg
2
0.240
Fe
2
0.105
Mn
2
0.106
capacity when a 4 percent sodium chloride solution is used instead of a 10 percent
solution. A much more critical factor is the regenerant flow rate, as lower flow rates
give longer contact time between the brine and the resin. Regenerant flow rates in
excess of 1 gpm / cu ft are not recommended, as appreciable capacity reduction is
experienced at higher flow rates. Regeneration is typically in a downflow manner, with
a minimum contact time of 30 minutes.
Rinsing.
The initial rinse flow rate should be the same as that used during regeneration;
rinse water is introduced while the regeneration process is concluding. After one to
two bed volumes of rinse water have been added, the flow rate should be increased to
1.5 gpm / cu ft. This should continue until the sodium concentration in effluent from
the bed is less than 5 mg / L above the rinse water sodium concentration. These two
rinse operations are referred to as the
slow rinse
and the
fast rinse
.
Design Considerations
Exchange Vessels.
Small plants generally use pressure units, as this allows pumping
directly through the exchanger and into the distribution system. Larger plants typically
use open, gravity-flow units, as they are less costly than pressure units per unit volume.
Pressure vessels (Fig. 18-2) may be either vertical or horizontal, with vertical ves-
sels preferred. Disadvantages of horizontal vessels include poor hydraulic distribution
and loss of exchange capacity because resin located in the ''bulges'' at the ends and
side of the pressure vessels is not fully utilized. The vessel interior should be coated
to provide protection against the corrosive sodium chloride regenerant. All piping
should be corrosion-resistant, with plastic pipe most frequently utilized.
Resin Depth.
The minimum resin depth is 2 feet, and the maximum resin depth 5
feet. The maximum resin depth is established by allowable headloss through the bed
and by expanded bed depth during backwashing. Expansion of the resin bed by 50 to
75 percent during backwash should be allowed in the design, without resin reaching
the bottom of the wash troughs. If sufficient contactor volume is not left for resin
expansion, resin will be lost during backwash.
Sizing the Contactors.
Contactor size is a function of several parameters:
Resin volume
•
Maximum raw-water hardness
•
Time interval between regenerations
•
Product water storage
•
