CIVL 375: Experiment 7
SOFTENING BY ION EXCHANGE
The most usual ion exchange material employed in water softening is a sulphonated styrene
based resin, supplied by the makers in the sodium form. The resin has a strong affinity for
calcium and magnesium ions and will also remove ferrous ions after nearly the complete
removal of calcium and magnesium.
Softening can be carried out as a batch process. However, it is more convenient to operate a
continuous flow process by passing the water slowly downwards through a column of resin
beads. The exchange reaction takes place rapidly enough for the upper layers of the bed to
approach exhaustion before the lower layers being able to exchange ions.
There is thus, a zone of active exchange which move down the column until the resin at all
depths becomes exhausted. The position at an intermediate stage can be illustrated as shown
Original hard water
Ca2 + in water
Ca2+ & Na+
Na+ in water
When the zone of active exchange reaches at the bottom of the column, the emerging water
starts to show an increasing hardness. This is the breakthrough, when it becomes necessary to
regenerate the resin with a strong sodium chloride solution.
To determine the exchange capacity of a cationic resin in the softening of hard water
1. Take up to 10 liters of hard water (ground water)
2. Determine the hardness of water and place it in the test water reservoir.
3. Make up 200 ml of 10% NaCl solution by dissolving 20 g NaCl in distilled water and then
place this solution in regenerate tank B.
Fill the cation exchanger with cation resin to a depth of 300 mm. Select tank D open valve 3
and 6 and back wash for five minutes. Gradually turn off and measure the final depth.
Select tank B, open valves 2, 12 (and 10 for sampling). Set flow meter to 10 ml/min and
continue flow for 15 minutes.
Select tank C and open valves 2 and 10. Set the flow meter to 50-70 ml/min. Collect samples
at five minutes intervals. Determine the hardness of each sample. Continue until the hardness
reaches more than 50% of the value for the influent (input) water.
Results and Calculations
1. Fill in the following table
Volume of water treated (ml)
Calculate the exchange capacity using .2
Wet volume of re sin bed =
Exchange capacity =
π × (15 × 10 −3 ) 2
× final depth
Volumebreakthrough (liters ) × hardness of original water (mg / l as CaCO3 )
Volume of wet bed
.where Volumebreakthrough is the volume of water treated until the breakthrough point
1. Fill out the provided experiment data sheet and carry out the required calculations.
2. If you are requested to submit a report of this experiment then your report should contain
(a) an introduction that ends with the objective (b) methodology including sample
identification (c) results and discussion (d) conclusion (e) references and (f) an appendix that
contains the filled data sheet.
CIVL 375: Experiment 7 Data Sheet
SOFTENING BY ION EXCHANGE
Initial concentration of
hardness (mg/l as CaCO3)
Column diameter (cm)
Length of packed resin (cm)
Flow rate (ml/min)
1. Plot hardness versus time
2. Calculate the exchange capacity