Water softening methods

Water softening methods

1. Water Softening Methods 12/12/2021 1

2. Water Softening Methods Zeolite (Permutit process) Ion-exchange Mixed bed ion-exchange Reverse Osmosis 12/12/2021 2

3. Permutit or Zeolite Process Zeolite is hydrated sodium aluminium silicate having a general formula, Na 2 OAl 2 O 3 .xSiO 2 .yH 2 O . It exchanges Na + ions for Ca 2+ and Mg 2+ ions. Common Zeolite is Na 2 OAl 2 O 3 .3SiO 2 .2H 2 O known as natrolith. Other gluconites, green sand (iron potassium phyllosilicate with characteristic green colour, a mineral containing Glauconite)etc. are used for water softening. Artificial zeolite used for water softening is Permutit. These are porous, glassy particles having higher softening capacity compared to green sand. They are prepared by heating china clay (hydrated aluminium silicate), feldspar (KAlSi 3 O 8 -NaAlSi 3 O 8 – CaAl 2 Si 2 O 8 ) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the earth’s crust) and soda ash (Na 2 CO 3 ) 12/12/2021 3

4. Natural Zeolites: Natrolite - Na 2 O. Al 2 O 3 4SiO 2 .2H 2 O Laumontite - CaO. Al 2 O 3 4SiO 2 .4H 2 O Harmotome - (BaO.K 2 O). Al 2 O 3 5SiO 2 .5H 2 O - Capable of exchanging its Na ions 12/12/2021 4

5. Permutit or Zeolite Process Method of softening: Na 2 Ze + Ca(HCO 3 ) 2 2 NaHCO 3 +CaZe Na 2 Ze + Mg(HCO 3 ) 2 2 NaHCO 3 + MgZe Na 2 Ze + CaSO 4 2 Na 2 SO 4 +CaZe Na 2 Ze + CaCl 2 2 NaCl+CaZe Regeneration of Zeolite: CaZe (or) MgZe + 2 NaCl Na 2 Ze + CaCl 2 or MgCl 2 Brine solution 12/12/2021 5

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8. Zeolite Process Advantages: Residual hardness of water is about 10 ppm only Equipment is small and easy to handle Time required for softening of water is small No sludge formation and the process is clean Zeolite can be regenerated easily using brine solution Any type of hardness can be removed without any modifications to the process Disadvantages: Coloured water or water containing suspended impurities cannot be used without filtration Water containing acidic pH cannot be used for softening since acid will destroy zeolite. 12/12/2021 8

9. Ion-Exchange Process Ion-exchange resins are cross linked long chain polymers with microporous structure Functional groups present are responsible for ion-exchange properties Acidic functional groups (-COOH, -SO 3 H etc.) exchange H + for cations & Basic functional groups (-NH 2 , =NH etc.) exchange OH - for anions. A. Cation-exchange Resins(RH + ): - Styrene divinyl benzene copolymers - When sulphonated, capable of exchange H + 12/12/2021 9

10. Ion-Exchange Process B. Anion-exchange resins (R’OH): - Styrene divinyl benzene copolymers or amine formaldehyde copolymers with NH 2 , QN + , QP + , QS + , groups. - On alkali treatment, capable of exchange of OH - 12/12/2021 10

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13. Ion-Exchange Process The Process of Ion-exchange is: 2 RH + + Ca 2+ /Mg 2+ R 2 Ca 2+ /R 2 Mg 2+ + 2 H + (Cation exchange) R’OH - + Cl - R’ + Cl - + OH - ( anion exchange ) 2 R’OH- + SO 4 2- R’ 2 SO 4 2- + 2 OH - (anion exchange) 2 R’OH- + CO 3 2- R’2 CO 3 2- + 2 OH - (anion exchange) Finally, H + + OH - H 2 O Regeneration of exhausted resins: Saturated resins are regenerated: R 2 Ca 2+ /R 2 Mg 2+ + 2H + 2 RH + + Ca 2+ /Mg 2+ (cation) (Strong acid) (washings) R’ 2 SO 4 2- + 2 OH - 2 R’OH - + SO 4 2- (Strong base) (washings) 12/12/2021 13

14. Ion-Exchange Process Note: Hard water should be first passed through the cation exchanger and then Anion exchanger to avoid hydroxides of Ca 2+ and Mg 2+ getting formed 12/12/2021 14

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16. Mixed Bed Deionizer 12/12/2021 16

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18. Advantages & Disadvantages of ion-exchange process Advantages: - Can be used for highly acid and highly alkaline water - Residual hardness of water is as low as 2 ppm. - Very good for treating water for high pressure boilers Disadvantages: - Expensive equipment and chemicals - Turbidity of water should be < 10 ppm. Otherwise output will reduce; turbidity needs to be coagulated before treatment. - Needs skilled labour 12/12/2021 18

19. Reverse Osmosis When two solutions of unequal concentrations are separated by a Semipermeable membrane, solvent will flow from lower conc. to higher conc. This phenomenon can be reversed (solvent flow reversed) by applying hydrostatic pressure on the concentrated side In reverse osmosis, pressure of 15-40 kg/cm 2 is applied to sea water. The water gets forced through the semipermeable membrane leaving behind the dissolved solids. 12/12/2021 19

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21. Advantages of Reverse Osmosis Advantage is in removing ionic, non-ionic, colloidal and high molecular wt. organic matter. It removes colloidal silica (which is not removed during demineralisation) Cost is only the replacement cost of membranes (life is 2 years) Membrane replacement is fast and hence uninterrupted water supply can be ensured Because of the above reasons this process is being adopted for converting sea water into potable water and for high pressure boilers. 12/12/2021 21