Water and wastewater often leave a treatment step with a pH that is too low. Reverse osmosis permeate, acid-cleaned systems, some industrial effluents and desalinated water all tend to be acidic or aggressive. Caustic soda (sodium hydroxide) is the most common chemical used to bring the pH back up. This guide explains when to use it, how to dose it, and how plants in Oman source and handle it.

Why pH correction matters

• Corrosion control: low-pH water corrodes pipes, pumps and tanks.
• Discharge limits: treated effluent must usually be within a pH range set by the regulator before discharge.
• Downstream treatment: coagulation, biological treatment and disinfection all work best within certain pH ranges.
• Product water quality: desalinated and RO water often needs pH adjustment and remineralisation before distribution or use.

Caustic soda or another alkali?

Caustic soda is strong, fast and easy to dose accurately as a solution. Lime and soda ash are cheaper per unit of alkalinity for some large applications, but lime creates slurries and sludge, and soda ash releases carbon dioxide. For most small and medium plants, caustic soda solution dosed through a metering pump is the simplest choice.

Preparing a dosing solution

Most plants dose caustic as a 10 to 20% solution, prepared from flakes or diluted from liquid caustic:

1. Fill the day tank with the required volume of water first.
2. Add caustic soda flakes slowly while mixing. Never add water to flakes; the heat released can make the solution boil.
3. Allow the solution to cool before starting the dosing pump.
4. Label the tank with the strength and the date.

Roughly 100 g of 99% flakes per litre of water gives a solution close to 10%. Use polyethylene or stainless steel tanks and pipework, and avoid aluminium and galvanised parts.

Working out the dose

The amount of caustic needed depends on the water's starting pH, its alkalinity and its carbon dioxide content, so it cannot be calculated from pH alone. The practical approach is:

1. Jar test: take a known volume of water and add the dosing solution in small measured steps, recording the pH after each step.
2. Scale up: convert the dose per litre to the plant flow rate to set the pump.
3. Control: use an online pH probe after a mixing point, linked to the dosing pump, and calibrate it regularly.

Dose slowly and measure. Overdosing raises the pH too far, wastes chemical and can cause scaling.

Safety and handling

Caustic soda is corrosive (UN 1823, Class 8). Operators should wear splash goggles, gloves and an apron when making up solutions, and an eyewash should be close to the dosing area. Store bags dry and sealed, away from acids. Keep the safety data sheet at the dosing station.

Sourcing caustic soda for plants in Oman

For water treatment plants operating in Oman, Muscat Chemical is a caustic soda supplier in Oman holding 99% flakes as ready stock in Barka, with the daily price, the SDS and a sample certificate of analysis published online and next-day delivery across Oman.