Water softening reduces the concentration of dissolved minerals responsible for “hardness.” This treatment is important for both domestic and industrial applications. Among different methods, lime softening, also known as lime-soda ash softening or chemical precipitation, is a widely used and effective technique. It converts hardness-causing minerals into insoluble forms that can then be removed from the water supply.
What Makes Water Hard?
Water hardness is primarily determined by dissolved mineral ions, particularly calcium (Ca²⁺) and magnesium (Mg²⁺). These ions originate from water flowing over geological formations containing minerals like limestone and dolomite, dissolving them. Hardness is typically measured in terms of calcium carbonate equivalent (CaCO₃) concentration.
Water hardness is categorized into two main types: temporary and permanent. Temporary hardness results from bicarbonates of calcium and magnesium, such as calcium bicarbonate (Ca(HCO₃)₂) and magnesium bicarbonate (Mg(HCO₃)₂). This type can be reduced by boiling, which causes bicarbonates to decompose and precipitate as insoluble carbonates.
Permanent hardness, in contrast, is caused by sulfates, chlorides, and nitrates of calcium and magnesium, like calcium sulfate (CaSO₄) and magnesium sulfate (MgSO₄). Unlike temporary hardness, permanent hardness cannot be removed by boiling. Lime softening addresses both temporary and permanent forms of water hardness.
How Lime Softening Works
Lime softening is a chemical precipitation method that converts dissolved hardness minerals into insoluble precipitates by adding specific chemicals. The primary chemical is lime, typically hydrated lime (calcium hydroxide, Ca(OH)₂), and sometimes quicklime (calcium oxide, CaO) which is then slaked. For waters containing non-carbonate hardness, soda ash (sodium carbonate, Na₂CO₃) is also added.
The process begins by adding lime to the raw water, which raises the pH. Dissolved carbon dioxide in the water reacts with added lime to form calcium carbonate (CaCO₃), which precipitates. Following this, lime reacts with calcium bicarbonate, converting it into calcium carbonate, which also precipitates. To remove magnesium bicarbonate, additional lime is required to raise the pH further, typically to between 10.0 and 10.5. This causes magnesium to precipitate as magnesium hydroxide (Mg(OH)₂).
If the water contains permanent hardness, such as calcium sulfate or magnesium sulfate, soda ash is introduced. Soda ash reacts with these non-carbonate hardness compounds to form additional calcium carbonate and magnesium hydroxide precipitates. After chemical reactions occur, the water undergoes rapid mixing to ensure uniform distribution of chemicals, followed by gentle mixing called flocculation, which encourages small precipitates to clump into larger, more easily settleable particles.
These larger particles are then removed through sedimentation in clarifiers, where they settle to the bottom as sludge. Finally, the softened water is filtered to remove any remaining fine suspended solids.
Key Benefits of Lime Softening
Lime softening offers several advantages. It helps prevent scale buildup in pipes, boilers, and appliances, which can reduce efficiency and shorten equipment lifespan.
The process also removes other undesirable impurities often found in water, including iron, manganese, silica, and certain heavy metals like radium and arsenic. The elevated pH levels (typically 10.5-11) achieved during lime softening contribute to disinfection by inhibiting bacterial and viral growth. This high pH also aids in coagulating and removing organic matter, improving water clarity, taste, and odor.
Practical Considerations of Lime Softening
Lime softening involves several practical considerations, particularly regarding waste management and post-treatment. A significant challenge is producing a large volume of sludge, primarily composed of calcium carbonate and magnesium hydroxide precipitates. For every pound of lime used, approximately two pounds of sludge can form, requiring effective disposal methods. This high-pH sludge can add 7-10% to water treatment costs.
Another aspect is the need for post-treatment pH adjustment, commonly known as recarbonation. The softened water’s high pH, necessary for efficient hardness removal, can lead to scaling in the distribution system if not properly managed. Recarbonation involves adding carbon dioxide (CO₂) to lower its pH, typically to a stable range of 8.3 to 8.6, preventing further calcium carbonate precipitation and corrosion. The process demands careful operation and precise chemical dosing to maintain water quality and prevent downstream issues.
Where Lime Softening is Used
Lime softening is widely applied in various settings to improve water quality. Its most common application is in municipal water treatment plants. It provides softened drinking water to communities, enhancing aesthetic qualities and reducing issues like soap scum and appliance scaling. Many cities use lime softening to reduce the demand for individual home water softeners.
Beyond drinking water, lime softening is used extensively in industrial processes where hard water causes significant operational problems. Industries like power generation, textile manufacturing, and chemical processing rely on softened water to prevent scale buildup in heat exchangers, boilers, and cooling towers. This maintains system efficiency, reduces energy consumption, and extends industrial equipment lifespan. The process is also employed in some wastewater treatment facilities to stabilize sludge.