Caustic soda (sodium hydroxide)
The reagent that puts back the alkalinity your coagulant destroyed, and the one most likely to injure somebody on the plant. Both facts deserve equal weight.
Caustic soda, NaOH, is supplied as a 32% or 50% solution or as 99% flake, pearl or prill. In water treatment it raises pH and replaces the alkalinity consumed by acidic coagulants. One milligram per litre of NaOH provides 1.25 mg/L of alkalinity as CaCO₃, so replacing the 100 mg/L an alum dose destroys takes about 80 mg/L of caustic.
It is faster-acting, cleaner and easier to dose than lime, and it costs substantially more per unit of alkalinity. Where the plant has an operator and a pH loop, caustic wins on simplicity; where the alkalinity demand is very large, lime wins on cost.
Specification and format choice
| Parameter | 32% solution | 50% solution | 99% flake |
|---|---|---|---|
| NaOH content | 31.0–33.0% | 49.0–51.0% | ≥ 99.0% |
| Sodium carbonate | ≤ 0.3% | ≤ 0.3% | ≤ 0.8% |
| Sodium chloride | ≤ 0.05% | ≤ 0.05% | ≤ 0.1% |
| Iron as Fe | ≤ 10 ppm | ≤ 10 ppm | ≤ 20 ppm |
| Density at 20 °C | 1.35 kg/L | 1.53 kg/L | Bulk 1.1–1.3 kg/L |
| Crystallisation point | ≈ 2 °C | ≈ 12 °C | n/a |
| HS code | 2815.12 | 2815.12 | 2815.11 |
Dose calculation
| Alkalinity to replace, as CaCO₃ | 100% NaOH required | 50% solution | 32% solution |
|---|---|---|---|
| 25 mg/L | 20 mg/L | 40 mg/L (26 mL/m³) | 63 mg/L (46 mL/m³) |
| 50 mg/L | 40 mg/L | 80 mg/L (52 mL/m³) | 125 mg/L (93 mL/m³) |
| 100 mg/L | 80 mg/L | 160 mg/L (105 mL/m³) | 250 mg/L (185 mL/m³) |
| 150 mg/L | 120 mg/L | 240 mg/L (157 mL/m³) | 375 mg/L (278 mL/m³) |
| 200 mg/L | 160 mg/L | 320 mg/L (209 mL/m³) | 500 mg/L (370 mL/m³) |
Safety: the part that is not optional
Caustic soda causes deep, painless-at-first liquefactive burns. Unlike acid, which hurts immediately and stops at the surface, alkali keeps penetrating tissue while the person is still deciding whether it is serious. Eye contact with 50% caustic can destroy sight in under a minute.
- Always add caustic to water, never water to caustic. Dissolving flake generates a great deal of heat — enough to boil a poorly made batch and eject it from the tank.
- Dilute below 20% w/v when making up from flake, in stages, with cooling or with enough time between additions for the heat to dissipate.
- Protection: full face shield over splash goggles, not one or the other. Neoprene or butyl gloves, chemical apron, closed footwear.
- Emergency provision: safety shower and eyewash within ten seconds of the transfer and dosing points, tested weekly and actually working. This is the most common gap we see on African plants dosing caustic.
- Never store with acid. Segregate caustic and sulphuric acid bunds. A mixed spill is violently exothermic.
Materials of construction
- Suitable: carbon steel and 316L stainless for solution below 50 °C, HDPE, polypropylene, PVDF, PTFE, EPDM and butyl elastomers.
- Unsuitable: aluminium and its alloys — the reaction generates hydrogen; galvanised steel; brass and bronze; glass in prolonged contact; PVC above about 40 °C; polycarbonate sight glasses, which craze.
- Caustic is one of the few water treatment chemicals for which carbon steel is acceptable, which makes storage cheaper than for the acidic coagulants.
Frequently asked questions
Caustic soda or lime for pH correction?
Caustic is a clean solution, dissolves instantly, gives fast pH response, needs no slaking plant and does not scale the dosing line. Lime is roughly a third to a half the cost per unit of alkalinity, adds calcium that assists floc weight, but requires a slurry system, blocks lines and needs an operator who will maintain it. Below about 200 kg/day of alkalinity demand, caustic is normally the right call. Above that, lime starts to justify its handling burden.
32% or 50% caustic?
50% ships less water and is the default on long freight, but it crystallises at about 12 °C and needs trace heating on any exposed line. 32% remains liquid to about 2 °C and is safer where trace heating is not available or not maintained. On most African coastal sites 50% is fine; on highland sites and in North Africa in winter, specify 32% unless the installation is heated.
How much caustic do I need to replace alum alkalinity?
An alum dose destroys 0.5 mg of alkalinity as CaCO₃ per mg dosed, and caustic provides 1.25 mg per mg. So caustic required is roughly 0.4 times the alum dose. A 200 mg/L alum dose needs about 80 mg/L of 100% NaOH, which is 160 mg/L of 50% solution — about 105 mL per cubic metre treated.
Does caustic soda affect the coagulation?
Only through pH, but that is the whole point. Dose it upstream of the coagulant where the objective is to raise raw water alkalinity, and downstream where the objective is to lift final pH back to the discharge consent. Dosing caustic and coagulant at the same point produces a locally alkaline zone that dissolves the floc you have just formed.