CE 547

1. CE 547 Softening

2. What is Hardness Hardness is • the ability of the water to consume excessive amounts of soap before foaming • OR the ability of the water to produce scale in water heaters and boilers where water temperature is increased dramatically

3. Causes of Hardness • Water hardness is principally caused by: • Calcium ions • Magnesium ions • Strontium • Iron ions • Manganese ions • Source of calcium and magnesium ions • geological formations

5. Types of Hardness • Carbonate • CO3, HCO3 • Non-carbonate • SO4, Cl, NO3

6. Water Hardness • The maximum level of hardness considered for public supply is 300 to 500 mg/l, though many customers object to water harder than 150 mg/l. • Disadvantages of hardness • excessive soap consumption during laundering • scale-formation in hot water heaters and pipes. • The use of synthetic detergents and pipe linings can overcome those problems.

7. Hardness Ranges

8. Softening • In precipitation softening, lime (CaO) and soda ash (Na2CO3) are used to precipitate calcium and magnesium form water. Lime treatment can also: • kill bacteria • remove iron • help in clarification of surface water (coagulant) • Lime treatment will raise the pH value, so recarbonation, by carbon dioxide, is used to lower the pH by converting the hydroxide and carbonate ions to bicarbonate ion.

9. Lime • Lime is commercially available in the forms of: • quicklime • hydrated lime • Quicklime • available in granular form • contains minimum of 90% CaO • magnesium oxide is the primary impurity • Hydrated Lime • contains about 68% CaO • Slurry lime is written as Ca(OH)2.

10. CO2 • Carbon dioxide is: • gas • colorless • clear • used to recarbonate lime-softened water • produced by burning fuel such as coal, oil, or gas. • applied through diffusers immersed in the treatment tank

11. Reactions

12. Reactions • From the reaction equations it can be seen that: • lime reacts first with free carbon dioxide (eq. 1) • next, lime reacts with calcium bicrarbonate (eq. 2) • lime also reacts with magnesium carbonate and bicrabonate (eqs. 3 and 4) • noncarbonate hardness (magnesium sulphate and chloride) requires the addition of soda ash for precipitation (eq. 5) • noncarbonate hardness (calcium sulphate and chloride) requires the addition of soda ash only for precipitation (eq. 6)

13. Pros and cons of Softening • Advantage of precipitation softening: • the lime added is removed along with the hardness taken out of solution. • TDS of the water are reduced • the chemical reactions can be used to estimate the quantity of sludge produced. • Disadvantage of precipitation softening: • sodium ions, from the addition of soda ash, remain in the finished water

14. Recarbonation • Recrabonation is used to stabilize lime-treated water, thus reducing its scale-forming potential. • Carbon dioxide is used for the recarbonation process. It converts lime to calcium carbonate. Further recarbonation will convert carbonate to bicarbonate.

15. Recarbonation Reactions

16. Excess Lime Softening • Uses of Excess Lime Softening: • to remove Ca and Mg to the practical limit of 40 mg/l • excess lime addition is needed to remove magnesium • In Excess Lime Softening: • after excess lime addition, the water is flocculated and settled to remove CaCO3 and Mg(OH)2 precipitates • After that, recarbonation is carried out in two stages • in the first stage, CO2 is added to lower the pH to 10.3 and converts excess lime to CaCO3. • water is then flocculated and settled • if needed, soda ash is added at this stage to remove noncarbonate hardness • In the second stage, CO2 is added to further lower the pH to the range of 8.5 to 9.5 to convert most of the remaining carbonate ion to bicarbonate ion in order to stabilize the water against scale formation.

18. Selective Calcium Carbonate Removal • If the water to be treated contains low concentration of magnesium (<40 mg/l as caCO3), selective calcium carbonate removal can be used. • Magnesium hardness of more than 40 mg/l as caCO3 is not recommended due to the possible formation of hard magnesium silicate in high temperature waters (180 F) • enough lime is added but not in excess • soda ash may be used depending on the extent of noncarbonate hardness • if precipitation of CaCO3 is not satisfactory, alum or a polymer can be used to aid flocculation • recarbonation is used to reduce scale formation on the filter and to produce stable water

19. Split-Treatment Softening • Split-treatment softening is done by splitting the flow of the raw water into two lines, of different portions, for softening in a two-stage system • the larger portion is given excess lime treatment in the first stage • water is flocculated and settled • treated water is mixed with split flow • excess lime form the first stage reacts with calcium hardness of the split water • soda ash is added to the second stage • in this case, excess lime is used and not wasted, so recarbonation might not be necessary. • Recarbonation is recommended to produce stable water • Advantages: • lime and recarbonation costs are lower than excess lime treatment • possibility of reducing magnesium hardness to less than 40 mg/l

21. Examples