Lime is a widely used alkaline chemical additive in municipal and industrial water purification. This compound is employed in various treatment processes to manipulate water chemistry. Lime promotes chemical reactions that change the state of dissolved substances, enabling their subsequent removal. These applications range from reducing mineral content to controlling corrosivity in the distribution system.
The Forms of Lime Utilized
Water treatment facilities primarily utilize two forms of lime: quicklime and hydrated lime. Quicklime, or calcium oxide (\(text{CaO}\)), is a highly reactive compound produced by heating limestone. When quicklime is mixed with water, it converts into hydrated lime through an exothermic reaction. Hydrated lime, or calcium hydroxide (\(text{Ca}(text{OH})_2\)), is a fine, dry powder created by this controlled process, known as slaking.
Hydrated lime is often preferred in smaller treatment plants because it is easier to handle and does not require specialized equipment. Large-scale facilities frequently opt for quicklime because it is less expensive per unit of available calcium oxide and its higher density reduces transportation and storage costs. Quicklime must first be converted into a hydrated lime slurry before being dosed into the water flow.
Primary Application: Water Hardness Reduction
The primary application of lime is in water softening, often called lime softening. This process reduces the concentration of dissolved calcium (\(text{Ca}^{2+}\)) and magnesium (\(text{Mg}^{2+}\)) ions that cause water hardness. High levels of these minerals lead to scaling in pipes and appliances, reducing efficiency.
Lime softening works by adding calcium hydroxide to raise the water’s \(text{pH}\), shifting the chemical equilibrium of carbonate species. The lime first reacts with dissolved carbon dioxide (\(text{CO}_2\)), producing a calcium carbonate precipitate. The added lime then reacts with calcium bicarbonate, a common form of hardness, to produce insoluble calcium carbonate (\(text{CaCO}_3\)) which falls out of solution.
Removing magnesium hardness requires a higher \(text{pH}\) level, typically 10.0 to 10.5, compared to the 9.0 to 9.5 needed for calcium removal. At this elevated \(text{pH}\), magnesium ions convert into the insoluble compound magnesium hydroxide (\(text{Mg}(text{OH})_2\)), which precipitates. For water containing non-carbonate hardness, an auxiliary chemical like soda ash (\(text{Na}_2text{CO}_3\)) is introduced alongside the lime. This combined lime-soda ash process converts dissolved ions into solid particles that are easily removed through sedimentation and filtration.
Essential Role in pH and Alkalinity Management
Lime is also applied to manage water’s \(text{pH}\) and alkalinity, aiding other treatment goals. Adding hydrated lime releases hydroxide ions (\(text{OH}^{-}\)) into the water, neutralizing acidity and raising the \(text{pH}\) level. This adjustment is used for corrosion control within the distribution network.
Raising the \(text{pH}\) to a slightly alkaline level makes the water less corrosive, preventing it from dissolving metals like lead and copper from pipes. Maintaining a stable \(text{pH}\) is also necessary for optimizing coagulation and flocculation, where suspended solids are gathered for removal. Lime acts as a coagulant aid by providing the proper \(text{pH}\) conditions for metal-based coagulants, such as aluminum sulfate, to work efficiently.
Chemical Reactions and Sludge Generation
Lime’s use depends on its ability to trigger specific chemical precipitation reactions. When calcium hydroxide is introduced, it dissociates into calcium ions (\(text{Ca}^{2+}\)) and hydroxide ions (\(text{OH}^{-}\)). The resulting rise in \(text{pH}\) causes dissolved hardness ions and other impurities to combine with available carbonate and hydroxide ions, forming insoluble solids.
These precipitation reactions generate a substantial byproduct known as lime sludge. This finely divided white precipitate is composed mainly of calcium carbonate (\(text{CaCO}_3\)) and magnesium hydroxide (\(text{Mg}(text{OH})_2\)), along with flocculated organic matter. Water treatment plants generate millions of tons of this sludge annually, presenting an operational and financial challenge.
Treatment facilities are increasingly exploring beneficial reuse options due to the sludge’s high calcium carbonate content. The sludge can potentially be used as a substitute for limestone in applications like flue gas desulfurization or as a fill material in construction projects, offering a sustainable alternative to disposal.