Sodium Cycle Cation Exchange: Water Softening Explained
Sodium Cycle Cation Exchange Process
In municipal water softening, the sodium cycle cation exchange process softens a portion of the water to zero hardness. This softened water is then mixed with raw water to achieve the desired composition. The process utilizes ion exchange resins (zeolites) supported by a bed of gravel and sand. The type and quantity of resin depend on the water’s hardness.
Water passes through the resin bed, where calcium and magnesium cations are retained by the resins. In exchange, an equivalent amount of sodium ions is released:
H2O + Ca + Mg + R-Na → H2O + Na + R-(Mg or Ca)
Regeneration Process
At the end of each cycle, the unit is taken out of service for regeneration. This involves removing calcium and magnesium ions from the resins and replacing them with sodium ions to restart the cycle. The regeneration process consists of three stages:
- Backwash: Water is passed in the opposite direction of normal flow.
- Regeneration: A salt solution is passed through the softener. Sodium chloride exchanges calcium and magnesium ions for sodium, restoring the resin.
- Washing: Calcium chloride, magnesium, and excess lime are removed by passing soft water through the softener.
Seawater is often used as a regenerator, especially in coastal towns.
Two-Step Softening Process
The two-step process involves:
- Lime Softening: Reduction of calcium bicarbonate and total bicarbonate hardness using calcium hydroxide and coagulation.
- Sodium Cation Exchange: The effluent is filtered and further softened through sodium cation exchange. The two effluents are then mixed to achieve the desired water quality.
Reverse Osmosis
Reverse osmosis occurs when a semipermeable membrane separates two bodies of water with different salt concentrations. The membrane’s pores are larger than water molecules but smaller than salt molecules.
Water flows from the lower concentration to the higher concentration to equalize salt concentrations. Applying pressure to the concentrated solution can stop this flow. The pressure required to stop the flow is called osmotic pressure. Applying more pressure reverses the flow, moving water from the higher concentration to the lower concentration.
Practical systems use pressure five to 50 times the osmotic pressure. Modern membranes are made of aromatic polyamides, offering better mechanical and chemical properties than earlier cellulose acetate designs.
Because membranes aren’t fully efficient, the produced water isn’t completely demineralized. Dissolved gases, like carbon dioxide, pass through, causing a pH imbalance. Reverse osmosis is a high-performing technology for water conversion compared to ion exchange and electrodialysis.