Water is called "hard water" if it contains calcium
and magnesium because these substances leave a hard scale on surfaces
in contact with the water. The problem can be minimized by the
use of a water softener, which operates on the principle of ion
exchange.
A water softener tank typically is about 2/3 full with amber-coloured
plastic resin beads. Fresh resin has sodium attached to the resin.
As hard water enters the top of the tank it comes in contact with
the fresh resin. The dissolved calcium and magnesium are attracted
to the resin more strongly than sodium, so the resin lets go of
the sodium and holds onto the hardness ions. The water softener
also takes out iron and manganese, provided they are dissolved
in the water and not suspended. Particulate matter is not exchanged
and is not part of the process.
As water flows through the resin bed, the sodium is given up
from the top of the bed first and the exchange process slowly
works its way downward. Because ion exchange requires time to
occur, there is an area within the bed where water is in the process
of becoming softened. This is called the reaction zone. The depth
of the reaction zone depends on factors such as flow rate, total
dissolved solids, hardness, and resin particle size. When the
resin bed is nearly exhausted, the reaction zone will be at the
bottom of the bed. There will not be enough time for complete
softening to occur, and hardness will enter the service line.
When this begins to occur, the water softener must be regenerated.
The first step in a typical regeneration process is an upflow
backwash to eliminate any particulate matter that may have collected
in the resin bed. The backwash is typically for 10 minutes.
The actual regeneration occurs during the next step, which is
downflow brining. Concentrated brine is drawn from the brine tank
and blended with fresh water. This solution is strong enough to
reverse the softening process. Calcium and magnesium are washed
off of the resin beads as they are inundated by the brine solution.
The hardness ions are rinsed out to waste. The brine eduction
process requires about 10 to 30 minutes, depending upon the amount
of salt used. After all of the brine has been drawn, fresh water
continues to flow down through the resin bed at a low flow rate.
This assures adequate contact time while rinsing out the brine
solution. The rinse cycle lasts about 30 minutes after brine draw.
The last portion of the regeneration cycle is a downflow fast
rinse or purge cycle. This 5 minute rinse at higher flow rate
makes sure that no brine solution remains in the tank before returning
the softener to service. The entire four cycles of regeneration
require about 1-1/2 hours.
Each cubic foot of softening resin has a capacity of 30,000 grains
of ion exchange capacity when dosed with 15 pounds of salt. A
lower salt dosage of 6 pounds will yield a softening capacity
of 20,000 grains. This lower salt dosage is more efficient, but
requires more frequent regenerations. Most softeners are selected
as a balance between equipment expense and operating expense.
Other factors are maximum flow rate, pressure drop, maximum daily
capacity and site requirements.
