CIVL 375: Experiment 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 below Original hard water Hardness Ca2 + in water Ca2+ & Na+ Na+ in water Soft 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 Objective To determine the exchange capacity of a cationic resin in the softening of hard water Procedure Take up to 10 liters of hard water (ground water) 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 Backwashing Fill the cation exchanger with cation resin to a depth of 300 mm Select tank D open valve and and back wash for five minutes Gradually turn off and measure the final depth Regenerate Select tank B, open valves 2, 12 (and 10 for sampling) Set flow meter to 10 ml/min and continue flow for 15 minutes Softening Select tank C and open valves 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 Fill in the following table Sample Time Final depth Volume of water treated (ml) Hardness mg/l 10 15 20 25 30 Calculate the exchange capacity using Wet volume of re sin bed = Exchange capacity = π × (15 × 10 −3 ) × 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 Requirements: Fill out the provided experiment data sheet and carry out the required calculations 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 Data Sheet SOFTENING BY ION EXCHANGE Name ID Experimental Conditions Experiment date Sample identification Initial concentration of hardness (mg/l as CaCO3) Resin name Column diameter (cm) Length of packed resin (cm) Flow rate (ml/min) Effluent Hardness Time (min) Hardness (mg/l CaCO3) Required Calculations Plot hardness versus time Calculate the exchange capacity Time (min) Hardness (mg/l CaCO3)