The length of the softening cycle ends when 1 to 5 mg/l of hardness is detected in the effluent
(loading rates for synthetic resins are in the area of 10 to 15 gpm/square foot of media surface
Almost all softening units have an alarm on the water meter to indicate when a certain
amount of water has passed through the exchange unit.
Once hardness breaks through, the softener must be regenerated.
In down-flow units, the resin
must first be backwashed to loosen the resin (it becomes compacted by the weight of the water),
and to remove any other material that has been filtered out of the water by the resin.
backwash rate is normally 6 to 8 gpm/square foot of zeolite bed area.
The operator needs to
apply enough backwash water to expand the resin bed by about 50 percent.
The backwash water
is usually discharged to a box containing orifice plates that measure the flow rate.
the top of the unit provide for uniform water distribution and uniform wash-water collection.
Underdrains provide uniform distribution of the backwash water on the bottom of the resin.
Concentrated brine is pumped to the unit from the storage basin.
Brine is diluted through the
injector to a solution containing about 10 percent salt before it is passed through the resin.
time required for regeneration is about 20 to 35 minutes.
The flow rate of brine through the resin
is measured in gallons per minute per cubic foot of media.
The brine needs to be in contact with
the resin long enough to allow for complete exchange of hardness ions on the resin with sodium
ions in the brine.
It is better to allow too much time than to not allow enough.
If the resin is not
totally recharged, the next softening run will be short.
The rinse cycle removes remaining brine from the tank.
The total amount of rinse water needed
is 20 to 35 gallons per cubic foot of resin.
The rinse is started at a slow rate (-2 gpm/square foot
of surface area-) and continues until the chloride concentration of the effluent (which should be
monitored frequently) is quite low.
Disposal of Brine
The volume of brine used during a regeneration cycle, (together with the rinse water that
follows) varies from 1.5 to 7 percent of the amount of water softened by the unit.
concentration in this wastewater could be as high as 35,000 to 45,000 mg/l.
Chlorides will upset
a wastewater treatment plant, and disposal methods have to comply with Minnesota Pollution
Control Agency requirements