The Chemistry of Lime-Soda Ash Precipitation Water Softening
The use of lime to soften water originated around 1841 when the Thames River was treated for its bicarbonate hardness. When lime and soda ash are added to water, hardness causing minerals such as calcium and magnesium are precipitated out. Modern day processes for water softening usually treat the water in four steps to make is acceptable for human use.
1 - Pre-Treatment
This step in the process includes aeration and presedimentation. Aeration allows water with high concentrations of dissolved carbon dioxide to be prepared for softening. Presedimentation removes some suspended solids that cause turbidity in the water. A reduction in turbidity allows for the softening reactions to more easily occur.
2 - Softening Treatment
In this stage, lime and soda ash are added to the water to remove the hardness causing minerals.
3 - Recarbonation
After softening treatment is complete, the water is usually devoid of any dissolved carbon dioxide. Recarbonation stabilizes the water's pH, brings the water to a minimum hardness level, and prepares it for filtration.
4 - Filtration
Finally, the treated water is filtered through a membrane or media filter to remove the remaining suspended solids microorganisms.
There are at least 7 different chemical reactions that take place during the Lime-Soda Ash water softening process.
Quick lime (CaO) is added to the water as a pre-treatment. It is typically the first step in softening. This quick lime combines with water to form calcium hydroxide (lime), which then reacts with carbon dioxide to form calcium carbonate. This is done at an optimum pH of 10.3
Reaction: CaO + H2O → Ca(OH)2then see next reaction:
Carbon Dioxide in the water reacts with Lime [Ca(OH)2] to form calcium carbonate, which will precipitate out. If there is a lot of dissolved CO2, the water can be expensive to treat. Because the goal of adding Lime to the water is to soften it, it may be cost effective to aerate the water prior to dosing it with Lime.
Reaction: CO2 + Ca(OH)2 → CaCO3↓ + H2O
Calcium Bicarbonate is commonly removed from water by adding lime. The chemicals combine to form water and calcium carbonate which will precipitate out because it is insoluble.
Reaction: Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓ + 2H2O
Magnesium bicarbonate reacts with lime to form magnesium carbonate. Keep in mind that magnesium carbonate is soluble. You need to add more lime to convert the magnesium to magnesium hydroxide, which is insoluble and will settle out. Magnesium precipitation requires a pH of about 11.0 to 11.3 (caused by adding more lime)
Reaction: Mg(HCO3)2 + Ca(OH)2 → MgCO3 + 2H2O &
MgCO3 + Ca(OH)2 → Mg(OH)2↓+CaCO3↓
If there is magnesium sulfate in the water, you will need two reactions to remove it. Magnesium sulfate reacts with lime to for calcium Sulfate and magnesium hydroxide. The calcium sulfate does not precipitate out, so see the next reaction.
Reaction: MgSO4 + Ca(OH)2 → CaSO4 + Mg(OH)2↓
If the hardness causing chemical is Calcium sulfate, you can add soda ash to form calcium carbonate, which is insoluble, and sodium sulfate, which doesn't contribute to hardness.
Reaction: CaSO4 + Na2CO3 → Na2SO4 + CaCO3↓
Practical Limit of Lime-Soda Ash precipitation are 30 mg/l of calcium as CaCO3 and 10 mg/l of Mg(OH)2 expressed as CaCO3. The following flow chart outlines the various reactions that occur in the water.
Determining How much Chemicals to Add to the Water
How much Lime to add to a water supply:
1) Convert all concentrations of constituents that will react with lime in the water to mg/meq. (The table below will help you).
2) The sum of the mg/meq values is the required amount of lime that you need for the reaction.
3) Convert this value back to mg/l and then to lb/MG to see how many lbs of lime per million gallons of water you need. Multiply mg/l by 8.34 to get lb/MG.
How much Soda Ash to add to a water supply:
1) Convert all concentrations of constituents that will react with soda ash in the water to mg/meq (this will be Magnesium sulfate and/or Calcium sulfate).
2) The sum of the mg/meq values is the required amount of soda ash that you need for the reaction.
3) Convert this value back to mg/l and then to lb/MG to see how many lbs of Soda ash per million gallons of water you need. Multiply mg/l by 8.34 to get lb/MG.
American Water Works Association, and American Society of Civil Engineers. Water Treatment Plant Design. Ed. Edward E. Baruth. Fourth ed. New York: McGraw-Hill Handbooks, 2005.
Corbit, Robert A. Standard Handbook of Environmental Engineering. Second ed. New York: McGraw-Hill Handbooks, 1999.
Perry, Robert H., Don W. Green, and James O. Maloney. Perry's Chemical Engineers Handbook. Seventh ed. New York: McGraw-Hill 1997.
TSC's Water Treatment Engineering Team. Lime Softening. US Department of the Interior, Bureau of Reclamation. September 2009.