Water Treatment Methods and Principles: Hard Water Softening

Water softening requires ion exchange, which uses cation exchange resins to exchange sodium ions for calcium and magnesium ions in hard water, thereby reducing the concentration of these ions. The softening reactions are as follows:

Ca²⁺ + 2Na⁻ + EX → Ca⁻ + EX²⁺ + 2Na⁺

Mg²⁺ + 2Na⁻ + EX → Mg⁻ + EX²⁺ + 2Na⁺

In these formulas, EX represents the ion exchange resin. This resin binds Ca²⁺ and Mg²⁺, releasing the Na⁺ ions it originally contained within.

Currently, commercially available ion exchange resins are spherical synthetic organic polymer electrolytes. The resin matrix contains sodium chloride. During the water softening process, sodium ions are gradually depleted, reducing the softening effect of the ion exchange resin. Regeneration is then necessary, which involves adding a specific concentration of saline solution (typically 10%) at regular intervals. The reactions are as follows:

Ca-EX2+2Na+ (concentrated saline) → 2Na-EX+Ca2+

Mg-EX2+2Na+ (concentrated saline) → 2Na-EX+Mg2+

If cation exchange softening is absent during water treatment, not only will calcium and magnesium deposits accumulate on the reverse osmosis membrane, reducing its efficiency and potentially damaging it, but patients will also be more susceptible to hard water syndrome. Water softeners can also promote bacterial growth, so the equipment needs a backflushing function to prevent excessive impurities from adsorbing onto it. Another issue worth noting is hypernatremia, because the softening and re-reduction process of dialysis water is controlled by a timer. Normally, the re-reduction process mostly occurs in the middle of the night, which is controlled by a valve. If a malfunction occurs, a large amount of saline will rush into the water source, leading to hypernatremia in the patient.