Sodium hydroxide that corrects pH, restores alkalinity and strips heavy metals
Caustic soda for water treatment is sodium hydroxide (NaOH) dosed to raise pH and alkalinity: it protects pipes from corrosion, keeps alum and ferric coagulation working, neutralises acidic wastewater and precipitates metals as hydroxides. It takes 0.80 mg/L of NaOH to add 1 mg/L of alkalinity as CaCO₃. This guide covers the chemistry, a dosing calculator, the right form for your plant and the grade to specify.
- Forms supplied
- Flakes 98 / 99%, pearls 99%, liquid
- Packing
- 25 kg PP bags · 1 MT FIBC
- Minimum order
- 1 FCL (20ft)
- Loading ports
- Mersin · Jebel Ali
- Drinking-water standards
- EN 896 · NSF/ANSI/CAN 60 · AWWA B501
- HS / UN
- 2815.11 · UN 1823 Class 8
Current export levels: caustic soda flakes price and caustic soda price today.
- 0.80mg NaOH per mg alkalinity
- pH 8.5Cr(III) hydroxide precipitation
- 7.14mg alkalinity used per mg NH₄-N
- 24 hFirm quote turnaround
What is caustic soda used for in water treatment?
Caustic soda for water treatment does one thing in many places: it supplies hydroxide ions (OH⁻) that neutralise acidity, convert dissolved carbon dioxide into bicarbonate and carbonate, and push pH to the point where unwanted metals drop out as solids. Because sodium hydroxide dissolves completely and is supplied as a clean liquid or soluble solid, a metering pump can dose it precisely under pH control.
Drinking-water utilities use it for pH and corrosion control and to restore alkalinity after coagulation. Wastewater and industrial effluent plants use it for acid neutralisation, heavy metal precipitation and to replace alkalinity consumed by nitrification. It also regenerates anion exchange resins, cleans RO membranes and scrubs hydrogen sulphide from odour-control air.
NaOH dose (mg/L) = 0.80 × alkalinity to add (mg/L as CaCO₃)| Application | What NaOH does | Typical control point | Where |
|---|---|---|---|
| pH and corrosion control | Raises pH and converts CO₂ to carbonate alkalinity, reducing lead and copper release | Final pH set by the corrosion-control plan | Drinking water, after disinfection |
| Coagulation support | Replaces alkalinity consumed by alum or ferric salts so floc forms | Coagulation pH from jar tests | Rapid mix, before flocculation |
| Acid neutralisation | Neutralises acidic effluent before discharge or biology | Discharge permit pH range | Equalisation and neutralisation tanks |
| Heavy metal removal | Precipitates metals as insoluble hydroxides | pH of minimum solubility, e.g. 8.5 for Cr(III) | Plating, mining and metal-finishing effluent |
| Nitrification support | Replaces the 7.14 mg alkalinity lost per mg of NH₄-N oxidised | Residual alkalinity, often 70–80 mg/L as CaCO₃ | Activated sludge aeration tanks |
| Softening | Converts bicarbonate to carbonate so calcium precipitates as CaCO₃ | High pH set by the softening design | Softening clarifiers |
| Ion exchange and RO | Regenerates anion resins, cleans membranes, raises pH for boron rejection | Resin and membrane maker limits | Demineralisation, desalination |
| Odour and gas scrubbing | Absorbs hydrogen sulphide and acid gases as sodium salts | Scrubber liquor pH | Headworks, sludge and biogas scrubbers |
Scroll the table sideways to see every column.
How does caustic soda raise pH in water?
Sodium hydroxide is a strong base: it dissociates completely into sodium and hydroxide ions the moment it dissolves.
The hydroxide ions react first with free acid (H⁺), then with dissolved carbon dioxide, turning it into bicarbonate (HCO₃⁻) and, at higher pH, carbonate (CO₃²⁻). That is why caustic soda raises pH and total alkalinity at the same time, and why stripping excess CO₂ and dosing NaOH are both standard ways to lift the pH of soft, aggressive water.
The same strength is the main operating risk. In water with little buffering, a small overdose moves pH a long way, so caustic dosing is normally flow-paced and trimmed by an online pH probe rather than set by hand. Measure alkalinity and pH before and after dosing, and use the dose factors below as the starting point for a jar test, not as a substitute for one.
Neutralising acid
H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O
0.82 kg of NaOH neutralises 1 kg of sulphuric acid; 1.10 kg neutralises 1 kg of hydrochloric acid.
Building alkalinity
CO₂ + NaOH → NaHCO₃
Each 0.80 mg/L of NaOH adds 1 mg/L of alkalinity expressed as CaCO₃, and 0.575 mg/L of sodium.
Caustic soda dosing calculator for water treatment
Enter the plant flow, what the caustic has to do and the product you buy. The calculator converts the dose into kilograms of product per day, monthly tonnage, bags and containers, using the stoichiometric factors on this page.
Factors: alkalinity 0.80, H₂SO₄ 0.82, HCl 1.10, alum (594 g/mol) 0.40, FeCl₃ 0.74 mg NaOH per mg. A month is 30 days; one 20ft container carries about 25 MT of bagged flakes or pearls.
A planning estimate from stoichiometry, not a process design. Confirm the dose by jar test or titration, and size pumps for peak flow.
Worked example: a 20,000 m³/day drinking-water plant
The plant wants to raise alkalinity by 10 mg/L as CaCO₃ after alum coagulation and buys 98% flakes.
| NaOH dose | 10 × 0.80 = 8 mg/L |
|---|---|
| Pure NaOH per day | 20,000 × 8 ÷ 1,000 = 160 kg |
| 98% flakes per day | 160 ÷ 0.98 = 163 kg |
| Per month (30 days) | about 4.9 MT · 196 bags of 25 kg |
| Per year | about 59.6 MT · 2.4 containers |
Caustic dosing in water treatment: the factors operators use
Stoichiometric factors calculated from equivalent weights. They give the theoretical dose; real plants add a margin found by testing.
| What you are treating | NaOH needed | Basis |
|---|---|---|
| Add 1 mg/L alkalinity as CaCO₃ | 0.80 mg/L | Equivalent weight 40 vs 50 |
| Neutralise 1 mg/L sulphuric acid | 0.82 mg/L | 2 NaOH per H₂SO₄ |
| Neutralise 1 mg/L hydrochloric acid | 1.10 mg/L | 1 NaOH per HCl |
| Offset 1 mg/L alum, Al₂(SO₄)₃·14H₂O | 0.40 mg/L | Alum consumes 0.50 mg/L alkalinity |
| Offset 1 mg/L ferric chloride, FeCl₃ | 0.74 mg/L | FeCl₃ consumes 0.92 mg/L alkalinity |
| Nitrify 1 mg/L ammonia-nitrogen | up to 5.7 mg/L | 7.14 mg/L alkalinity consumed; dose only the deficit |
| Sodium added by 1 mg/L NaOH | 0.575 mg/L Na | 23 / 40 |
Applications of caustic soda in water and wastewater treatment
Where sodium hydroxide is dosed, what it achieves and the mistakes that cost plants money.
Drinking water: pH adjustment and corrosion control
Raising pH and converting excess carbon dioxide into carbonate alkalinity is one of the corrosion-control treatments the US EPA lists for limiting lead and copper release, and it names caustic soda among the chemicals used for it. The target pH comes from the plant’s corrosion-control study, not from a general rule. Because chlorine disinfects mainly as hypochlorous acid, which weakens as pH climbs above about 7.5, plants usually complete chlorine contact before the final pH lift.
Coagulation and flocculation
Alum and ferric coagulants consume alkalinity as they hydrolyse: about 0.50 mg/L as CaCO₃ for every mg/L of alum and 0.92 mg/L for ferric chloride. In soft water this drives pH below the coagulation window and floc fails to form. Dosing caustic soda at the rapid mix restores the alkalinity, and jar tests set the coagulation pH that gives the lowest settled turbidity.
Wastewater neutralisation
Acidic streams from metal finishing, batteries, chemicals, textiles and mining are neutralised with sodium hydroxide before discharge or before biological treatment. Two-stage neutralisation with flow-paced dosing and pH trim avoids the overshoot that a single tank fed with strong caustic tends to produce. Compare caustic soda in mining and caustic soda for textile processing for the process side of those effluents.
Heavy metal precipitation
Raising pH turns dissolved copper, nickel, zinc, cadmium, lead and trivalent chromium into insoluble hydroxides that settle with a coagulant. Two rules prevent most failures. First, hexavalent chromium does not precipitate as a hydroxide: it must be reduced to Cr(III) with a sulphite or metabisulphite at low pH, and only then precipitated, typically at about pH 8.5. Second, many metal hydroxides are amphoteric and redissolve if pH runs too high, so mixed-metal streams are run at the compromise pH found by testing.
Biological treatment and nitrification
Nitrifying bacteria consume about 7.14 mg of alkalinity as CaCO₃ for every mg of ammonia-nitrogen they oxidise. Plants with low-alkalinity influent lose pH and nitrification efficiency unless alkalinity is added. Caustic soda is one of the common choices because it is easy to pace to ammonia load; keep a residual alkalinity in the aeration tank rather than chasing a pH number.
Softening
In caustic softening, hydroxide converts bicarbonate to carbonate, so calcium precipitates as calcium carbonate (Ca²⁺ + HCO₃⁻ + OH⁻ → CaCO₃↓ + H₂O), and the sodium carbonate formed removes further non-carbonate hardness. Magnesium precipitates as Mg(OH)₂ only at higher pH. Caustic softening produces less sludge than lime softening but adds sodium to the water.
Ion exchange, RO and membrane cleaning
Strong-base anion exchange resins in demineralisation plants are regenerated with dilute sodium hydroxide. In seawater reverse osmosis, raising the pH of the second-pass feed converts boric acid to borate, which the membrane rejects far better. Alkaline cleaning of RO and UF membranes also uses NaOH, within the pH and temperature limits set by the membrane manufacturer.
Odour control and gas scrubbing
Caustic scrubbers absorb hydrogen sulphide and other acid gases from headworks, sludge handling and biogas into a sodium hydroxide solution. The scrubber liquor pH is held high enough to keep absorption efficient, and the spent liquor is bled off and replaced.
Two claims to ignore
Caustic soda does not make chlorine a stronger disinfectant: higher pH weakens free chlorine. And it does not precipitate hexavalent chromium or cyanide directly; both need a separate reduction or oxidation step, with caustic used to hold the pH.
Caustic soda vs lime, soda ash and magnesium hydroxide for water treatment
The right alkali depends on what you need: fast, precise pH control, the lowest cost per unit of alkalinity, or the least risk of overshoot.
| Alkali | kg per kg alkalinity as CaCO₃ | How it is fed | Overshoot risk | Best for |
|---|---|---|---|---|
| Caustic soda, NaOH | 0.80 | Clear solution, metering pump | high | Precise pH control, automated dosing, small to large plants |
| Hydrated lime, Ca(OH)₂ | 0.74 | Slurry or saturator | medium | Large softening plants; adds calcium |
| Soda ash, Na₂CO₃ | 1.06 | Dissolved solid | low | Carbonate alkalinity, non-carbonate hardness removal |
| Magnesium hydroxide, Mg(OH)₂ | 0.58 | Slurry | low | Slow, self-limiting neutralisation of acidic wastewater |
Per kilogram, lime and magnesium hydroxide deliver more alkalinity than caustic soda and usually cost less per tonne, but both are handled as slurries that settle, scale and need more maintenance. Caustic soda costs more per unit of alkalinity and is corrosive, yet it gives the fastest, most controllable response, which is why most plants use it for pH trim even when another alkali does the bulk work. Soda ash is the gentler choice when you want carbonate alkalinity without a high pH; see soda ash in water treatment.
Liquid, flakes or pearls: the best form of caustic soda for water treatment
All three are the same chemical. The choice is about logistics, storage and how you make up the dosing solution.
| Form | NaOH content | Delivery | Watch out for | Best for |
|---|---|---|---|---|
| Liquid 50% | 50% | Road tanker or ISO tank into a bulk tank | Starts to crystallise at about 12 °C; heated, insulated storage in cold climates | Large plants near a producer |
| Liquid 25–32% | 25–32% | Tanker, IBC or made on site | Pays freight on more water per kg of NaOH | Cold sites, small plants, day tanks |
| Flakes 98 / 99% | 98–99% | 25 kg bags or 1 MT FIBC, by container | Dissolving releases heat; needs a mixed make-up tank | Importing plants, remote sites, lowest freight per kg NaOH |
| Pearls 99% | 99% | 25 kg bags or 1 MT FIBC, by container | Same make-up step as flakes | Hopper feeders, less dust than flakes |
Making a dosing solution from flakes
Flakes needed (kg) = solution mass × target strength ÷ flake purity. For 1,000 kg of 25% solution from 98% flakes: 1,000 × 0.25 ÷ 0.98 = 255 kg of flakes plus 745 kg of water. A 20–25% solution stays liquid well below the temperature at which 50% crystallises.
Add flakes to water, never water to flakes
Dissolving sodium hydroxide releases a lot of heat. Charge the water first, start the mixer, then add flakes slowly so the tank temperature stays under control. Wear a face shield, goggles, alkali-resistant gloves and apron. Full guidance: how to store caustic soda safely.
Freight, not chemistry, usually decides between liquid and solid for importers: a container of flakes carries about twice the NaOH of the same weight of 50% liquid. The trade-offs are set out in caustic soda flakes vs liquid.
Drinking-water grade caustic soda: standards to specify
For potable water the grade matters as much as the price, because everything in the product ends up in the water.
| Standard | Region | What it covers |
|---|---|---|
| EN 896 | Europe and many tenders that follow EN | Sodium hydroxide as a chemical for water intended for human consumption: purity and impurity limits |
| NSF/ANSI/CAN 60 | North America | Health-effects certification of drinking-water treatment chemicals, including maximum use dose |
| AWWA B501-2025 | North America | Physical and chemical requirements, sampling, testing, packaging and shipping of sodium hydroxide |
| E 524 | EU food additive | Food-grade purity criteria for sodium hydroxide |
Industrial wastewater plants can normally use standard industrial 98% flakes. Drinking-water plants should ask for membrane-cell sodium hydroxide with a certificate of analysis covering NaOH assay, sodium carbonate, sodium chloride, iron and mercury, and for the conformity certificate their regulator recognises. Grades are compared in types and grades of caustic soda, and the checks to run on arrival are in how to check caustic soda quality.
Handling sodium hydroxide safely at a water plant
Sodium hydroxide causes severe skin and eye burns, and the eye is the organ most at risk.
- OSHA PEL
- 2 mg/m³ TWA
- NIOSH REL
- 2 mg/m³ ceiling
- IDLH
- 10 mg/m³
- Transport
- UN 1823 solid · UN 1824 solution · Class 8
- 50% liquid
- Crystallises at about 12 °C
- First aid
- Flush eyes and skin with water at once, for a long time
Buying caustic soda for water treatment from SUHA
We export solid sodium hydroxide to water utilities, EPC contractors and chemical distributors from two loading ports.
Two origins
Mersin, Türkiye, for Africa, the Black Sea and the Mediterranean; Jebel Ali, UAE, for the Gulf, East Africa and South Asia.
Grades
Flakes 98% and 99%, pearls 99%; certificate of analysis with every lot. Tell us if your tender names EN 896 or NSF/ANSI/CAN 60.
Packing and MOQ
25 kg PP bags with PE liner or 1 MT FIBC; one full container minimum, about 25 MT.
Documents
COA, SDS, Dangerous Goods Declaration, certificate of origin; SGS, BV or Cotecna inspection at loading on request.
Caustic soda for water treatment — FAQ
References
- US EPA — Optimal Corrosion Control Treatment Evaluation Technical Recommendations (EPA 816-B-16-003)
- CWEA — How alkalinity affects nitrification
- Yokogawa — Reduction monitoring in the chromium wastewater treatment process
- AWWA — B501-2025 Sodium Hydroxide (Caustic Soda); CEN — EN 896, chemicals for water intended for human consumption
- Westlake Vinnolit — Sodium hydroxide 50% product data sheet
- NIOSH — Pocket Guide: sodium hydroxide; OSHA — chemical data, sodium hydroxide
Written by the SUHA International Trading technical desk, Dubai and Ankara. Dose factors are calculated from equivalent weights.
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