Civil Engineering Reference
In-Depth Information
The required resin volume is calculated according to the formula:
QHT
100
V
E
24%
OP
QHT
V
(metric)
240
E
%
OP
where
V
required resin volume, cu ft (m
3
)
Q
flow, gal / day (L / d)
H
raw-water hardness, kilograins / gal as CaCO
3
(mg/L)
T
regeneration frequency, hours (h)
E
resin exchange capacity, kilograins as CaCO
3
/cu ft (g/m
3
)
%
OP
average percent of time that contactors are operating, as %
Underdrains.
Similar to a sand filter, the underdrain collects water during operation,
distributes backwash water, and collects brine and rinse water during and following
regeneration. Good flow distribution is essential if each of the four phases of the cation
exchange process is to be performed efficiently. The total area of the holes in the
underdrain should be 0.16 to 0.18 percent of the surface area of the exchanger. Porous
plates can be used only with relatively clear water, as suspended material will clog
the plates. A number of proprietary underdrains are available.
Gravel Layer.
To prevent resin from being washed out of the underdrain system, as
well as to achieve good flow distribution during backwash, a layer of graded gravel
is used. Porous plate underdrains do not require a gravel layer. A total gravel depth
of 15 to 18 inches using three or more layers of gravel graded from
1
⁄
8
inch to 1 inch
is typical.
Wash Troughs.
Wash troughs are located at an elevation above the highest level
reached by the resin during backwash. Thus, wash trough elevation must account for
gravel depth and media depth during backwash.
Brine and Rinse Water Distribution.
Assuming downflow regeneration, the brine dis-
tribution manifold is placed immediately above the softener bed. Prior to brine intro-
duction, water in the contactor is drawn down to slightly above the top of the resin.
This allows brine to be introduced with minimal disruption of the resin. Rinse water,
assuming downflow rinse, is introduced either through the brine distribution system or
by flooding the bed, if this can be done without disrupting the resin surface.
Brine and Salt Storage.
Salt storage and brine production are usually accomplished
in the same basin. Salt is added to the basin in excess of the quantity that can dissolve.
Thus, as concentrated brine (approximately 26 percent at saturation) is withdrawn from
the bottom of the basin, fresh water introduced at the top of the basin dissolves ad-
ditional salt. Undissolved salt should always be present.
Tank volume is established by the quantity of salt that must be stored, which is a
function of the proximity of the plant to a reliable source of salt and the method of
