Lime, primarily in the forms of calcium oxide (quicklime) or calcium hydroxide (hydrated lime), is a widely used chemical agent in water treatment. This cost-effective substance purifies water for drinking and industrial applications. Its utility in improving water quality extends back centuries, with lime softening first employed to treat the Thames River water in 1841.
The Chemical Basis of Lime’s Utility
Lime’s effectiveness in water treatment stems from its inherent chemical properties, particularly its alkalinity. When quicklime (calcium oxide, CaO) is introduced to water, it undergoes an exothermic reaction to form calcium hydroxide (Ca(OH)₂), also known as slaked lime. This reaction is central to its application in water purification processes.
Calcium hydroxide, though slightly soluble, releases hydroxide ions (OH⁻) into water. These hydroxide ions are key to raising the water’s pH, making it more alkaline. The elevated pH then facilitates various chemical reactions, leading to the formation of insoluble precipitates with dissolved substances in the water. These solids are then easily separated.
Controlling pH and Softening Water
A primary application of lime is adjusting water pH. Lime neutralizes acidic water by reacting with hydrogen ions (H⁺), thereby raising the pH to a more desirable range. Maintaining a stable pH is important for preventing corrosion in water systems and optimizing the effectiveness of other treatment processes.
Lime is also widely used for water softening, a process that removes hardness-causing minerals like calcium and magnesium ions. This process increases water pH, causing calcium bicarbonate and magnesium bicarbonate to precipitate as solid calcium carbonate and magnesium hydroxide. Calcium hardness is typically removed as calcium carbonate, while magnesium hardness precipitates as magnesium hydroxide. This method effectively reduces both temporary and, in conjunction with soda ash, permanent hardness from water.
Enhancing Particle Removal and Eliminating Specific Contaminants
Lime acts as a coagulant aid, improving the efficiency of other coagulants like alum or ferric salts. It adjusts the water’s pH to an optimal range for these coagulants, which promotes the clumping together of small, suspended particles into larger, more easily settled flocs. This enhanced flocculation improves the removal of turbidity and suspended solids from the water.
The alkalinity provided by lime is also instrumental in removing various dissolved contaminants, particularly heavy metals. By raising the pH, lime causes heavy metals such as lead, copper, iron, manganese, nickel, and cadmium to precipitate as insoluble hydroxides. These metal hydroxides can then be separated from the water. Lime can also help remove other specific impurities like fluoride, silica, and phosphates through similar precipitation or adsorption mechanisms.
Managing Treatment Byproducts
Beyond treating the water itself, lime is also valuable in managing the solid waste, or sludge, generated during water purification processes. This application is known as “sludge conditioning” or “dewatering.” Adding lime to sludge improves its dewatering characteristics, making it easier to separate the solid material from residual water.
Lime also stabilizes sludge, reducing odors and inactivating pathogens. Raising the sludge’s pH to above 12, often for a period of at least two hours, significantly reduces the presence of bacteria and viruses. This stabilization makes the sludge safer for handling, disposal, or potential beneficial reuse, such as in agriculture.