CE 547 Softening

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

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

Cations Causing Harness Anions Associated with them Ca 2+ HCO 3 - Mg 2+ SO 4 - Sr 2+ CL - Fe 2+ NO 3 - Mn 2+ SiO 3 2-

Types of Hardness Carbonate CO 3, HCO 3 Non-carbonate SO 4, Cl, NO 3

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.

Hardness Ranges Hardness Concentration (mg/l) Degree of Hardness 60 – 120 Moderately Hard 120 – 180 Hard 180 and Over Very Hard

Softening In precipitation softening, lime (CaO) and soda ash (Na 2 CO 3 ) 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.

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.

CO 2 Carbon dioxide is: gascolorlessclear used to recarbonate lime-softened water produced by burning fuel such as coal, oil, or gas. applied through diffusers immersed in the treatment tank

Reactions

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)

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

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.

Recarbonation Reactions

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 CaCO 3 and Mg(OH) 2 precipitates After that, recarbonation is carried out in two stages in the first stage, CO 2 is added to lower the pH to 10.3 and converts excess lime to CaCO 3. water is then flocculated and settled if needed, soda ash is added at this stage to remove noncarbonate hardness In the second stage, CO 2 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.

Selective Calcium Carbonate Removal If the water to be treated contains low concentration of magnesium (<40 mg/l as caCO 3 ), selective calcium carbonate removal can be used. Magnesium hardness of more than 40 mg/l as caCO 3 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 CaCO 3 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

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

Examples