Water hardness refers to the total concentration of calcium and magnesium ions in water. It is categorized into two types:
- Temporary Hardness: Caused by calcium and magnesium bicarbonates and a small amount of carbonates. It can be removed by long-term heating, as the substances form precipitates that are then removed.
- Permanent Hardness: Caused by calcium and magnesium salts, such as chlorides, sulfates, and nitrates, which cannot be removed by heating.
1. Chemical Softening (Using Agents):
Chemical softening relies on the principle of chemical precipitation, where appropriate chemical agents are added to convert hardness-causing ions into insoluble salts, thereby removing them from water. Common agents include lime, soda ash, caustic soda, sodium tripolyphosphate, and sodium hydrogen phosphate.
- Lime Softening: Suitable for water with high hardness and alkalinity, it removes carbon dioxide and carbonate hardness from water and converts magnesium’s non-carbonate hardness into calcium hardness. This method removes most carbonate hardness, leaving 0.5-1.0 mmol residual hardness and removing about 20% of organic matter.
- Lime-Soda Softening: Suitable for water with high hardness and low alkalinity. It can be carried out through cold, warm, or hot methods, with hot methods typically using temperatures around 98°C. Excessive dosage of agents can generate NaOH and CO2, leading to corrosion in boiler systems and condensate pipelines. Hence, careful control of the dosage is required.
- Lime-Gypsum Softening: This method is suitable for water with high alkalinity and negative hardness. Lime and gypsum are added to the water to remove hardness effectively.
- Phosphate Softening: Phosphates have a lower solubility product, making this method suitable for deep softening, such as boiler feedwater treatment. Agents like sodium tripolyphosphate or sodium hydrogen phosphate are commonly used.
2. Ion Exchange Softening:
Ion exchange involves the exchange of ions between the water and an ion-exchange medium, widely used for water softening, desalination, high-purity water production, industrial wastewater treatment, zero discharge, and heavy metal recovery.
- Sodium Ion Exchange: In this method, calcium and magnesium ions that cause hardness in the water are exchanged with sodium ions from the exchange resin.
- Hydrogen Ion Exchange: This method not only softens hard water but also removes salts and prepares pure or ultrapure water.
3. Membrane Softening:
Membrane softening uses selective permeability of separation membranes to efficiently and automatically remove hardness from water.
- Tube Microfiltration Membrane Softening: Tube microfiltration membranes can achieve efficient solid-liquid separation. Softening agents are added to the reaction tank in the pretreatment system, and tube membranes are used to intercept precipitates. This method achieves filtration in a short time without the need for additional flocculants and provides excellent filtration results.
- Nanofiltration Membrane Softening: Nanofiltration membranes have selective permeability for different solutes, effectively retaining divalent ions while allowing some monovalent ions to pass through. This method is often used for groundwater-based drinking water, as it removes hardness while also removing color, pesticides, soluble organic substances, trihalomethanes, etc., while retaining essential minerals in the water.
- Reverse Osmosis Membrane: Reverse osmosis membranes have a high desalting capacity and can significantly retain calcium and magnesium ions, achieving effective hardness removal. However, when using reverse osmosis for softening, it is important to control anti-scaling agents and recovery rates to prevent scaling issues.