Methods for Softening Drinking Water - Lime-Enhanced Coagulation Softening Method

The chemical softening process for water involves adding appropriate chemicals to the water as needed, based on the principle of volumetric volume. These chemicals react with calcium and magnesium ions to form insoluble precipitates, CaCO3 and (OH)2. Commonly used chemicals include lime, soda ash, caustic soda, and trisodium phosphate, with lime being the most frequently used.

After slaking, lime is made into lime slurry and added to the raw water. Under high pH conditions, it reacts with bicarbonates in the following ways:

Ca(HCO3) + Ca(OH)2 = 2CaCO3↓ + 2H2O

Mg(HCO3)2 + Ca(OH)2 = CaCO3↓ + MgCO3 + 2H2O

MgCO3 + Ca(OH)2 = Mg(OH)2↓ + CaCO3↓

Among these, CaCO3 and Mg(OH)2 are precipitates. The precipitates formed by the divalent calcium and magnesium ions act as coagulants during settling, with CaCO3 exhibiting excellent flocculation properties. Various precipitates are flocculated in the reaction tank and removed in the sedimentation tank and filter. For groundwater, the turbidity is generally not high, and no other coagulants are needed. pH value has a significant impact on the coagulation effect. When calculating the lime-softening ratio, local water quality conditions should be considered, and beaker tests should be conducted to observe the removal effect at different pH values, while also considering the economics of chemical dosing to determine the optimal pH value. In production trials, Ca(OH)₂ is added to adjust the aqueous solution to the lowest possible pH value. In this method, the pH value of the water and the dosage of the chemical are crucial. These are usually determined through beaker tests, model tests, and production trials. To enhance the coagulation effect, polyacrylamide can be added as a coagulant aid. After adding lime, the pH value of the effluent will be higher; acid neutralization should be performed in the effluent to adjust the pH value to meet drinking water quality standards. In many areas, water sources often exceed hardness standards along with total dissolved solids (TDS), iron, and manganese. Enhanced coagulation and lime-chemical methods can also remove certain amounts of iron, manganese, and TDS. Because lime is inexpensive and widely available, it is suitable for raw water with high carbonate hardness and low non-carbonate hardness. For drinking water that does not require deep softening, current research indicates that this method produces effluent hardness that fully meets drinking water quality standards, making lime-enhanced coagulation an economical and effective method. Adding lime can remove most carbonate hardness from water, but not non-carbonate hardness. If the raw water has high non-carbonate hardness, the lime-soda method can be used. With increasingly severe water pollution, the concentration of organic matter in water sources is generally increasing, and Cryptosporidium and other pathogens have appeared in the water. The United States has conducted extensive research on the removal of precursors for disinfection byproducts using lime-enhanced coagulation. This research used water samples from different sources with varying water qualities and calcium-magnesium ion concentrations to investigate the relationship between lime softening and the removal of precursors for disinfection byproducts. To remove natural organic matter from water, anion exchange resin can be added after lime softening treatment. This method is effective in removing organic matter and color, reducing the color from 17 degrees to less than 3 degrees. Generally, anion exchange resin can effectively remove organic matter with a molecular weight of more than 1000, but it may not be effective for those with a molecular weight of less than 1000.