DM plant process and regeneration, step by step
How a DM plant works
- Pre-treatment. Sand and activated carbon filters remove turbidity and chlorine, which would damage the resin.
- Strong acid cation (SAC) unit. Calcium, magnesium and sodium are exchanged for hydrogen ions. The water leaving is acidic.
- Degasser (optional). Strips the carbon dioxide formed from bicarbonate, which reduces the load on the anion resin.
- Strong base anion (SBA) unit. Chloride, sulphate, nitrate, silica and carbonate are exchanged for hydroxide, which combines with hydrogen to form water.
- Mixed bed (optional). A polishing column with both resins brings conductivity below about 1 µS/cm for boilers and pharma.
A conductivity meter on the outlet tells the operator when the resin is exhausted.
Regeneration steps
| Step | What happens | Typical time |
|---|---|---|
| 1. Backwash | Upward flow loosens the resin bed and flushes out trapped dirt and fines | 10–15 min |
| 2. Settling | The bed settles back into place | 5 min |
| 3. Chemical injection | Dilute HCl (about 4–5%) through the cation unit; dilute NaOH (about 4%) through the anion unit, using an ejector | 30–45 min |
| 4. Slow rinse | Water at the injection flow rate pushes the remaining chemical through the bed | 30–45 min |
| 5. Fast rinse | Service-flow rinse until outlet conductivity is back in spec | 15–30 min |
Exact chemical quantities depend on the resin volume and the resin maker's data. Use the resin supplier's regeneration level, usually given in grams of 100% chemical per litre of resin.
Signs your DM plant needs attention
- Shorter runs between regenerations than before: the resin may be fouled with iron or organics, or chlorine has attacked it.
- Silica slipping through the anion unit: the caustic is too cold or too weak, or the anion resin is ageing.
- Higher pressure drop: clogged distributors or crushed resin.
We supply DM plant spares including laterals, distributors, valves and resin, and build complete DM plants. For large flows, an RO plant ahead of the DM plant cuts chemical use sharply.
FAQs
What is the full form of DM plant?
DM stands for demineralisation. A DM plant removes dissolved minerals (salts) from water using ion exchange resin.
Which chemicals are used to regenerate a DM plant?
Hydrochloric acid (or sulphuric acid) for the cation resin and caustic soda (sodium hydroxide) for the anion resin.
What is the difference between a softener and a DM plant?
A softener only swaps hardness (calcium and magnesium) for sodium, so TDS stays the same. A DM plant removes almost all dissolved salts.