pH Control: The invisible factor behind efficient water treatment

In water treatment systems, pH is often overlooked, yet it is one of the most important parameters for ensuring reliable system performance. It directly affects process efficiency, equipment lifespan, and the quality of the treated water.

The pH value indicates whether water is acidic or alkaline and is measured on a scale from 0 to 14. A pH of 7 is considered neutral. Values below 7 indicate acidic water, while values above 7 indicate alkaline water.

What happens when pH is too low?

When the pH drops below 7, water becomes more aggressive toward metal surfaces and treatment equipment.

The main risks include:

• accelerated corrosion of pipes, pumps, and storage tanks;
• release of metal ions into the water;
• reduced lifespan of membranes and ion exchange resins;
• increased chemical consumption for pH correction and neutralization.

When is a lower pH used in RO systems?

In most cases, the optimal feed water pH for reverse osmosis systems ranges between 5.5 and 6.5. However, under certain conditions, the feed water is intentionally acidified.

This approach is commonly used when:

• the feed water is hard and no water softening system is installed;
• bicarbonate concentrations are high;
• there is an increased risk of calcium scale formation.

By dosing acid into the feed water, the likelihood of scale formation on the membranes is reduced, helping maintain stable system performance.

What happens when pH is too high?

When the pH rises above 7, sparingly soluble mineral deposits begin to form.

This may lead to:

• calcium carbonate and magnesium hydroxide deposits;
• clogging of pipelines and membrane surfaces;
• increased scaling risk in RO systems;
• reduced efficiency of coagulation and flocculation processes.

When is a higher pH used?

In some applications, the pH is intentionally increased.

This is most commonly done when:

• high concentrations of dissolved carbon dioxide (CO₂) affect the conductivity of the treated water;
• specialized membrane systems require specific operating conditions;
• alkaline cleaning solutions are used during membrane Clean-in-Place (CIP) procedures to remove organic and biological fouling.

It is important to note that elevated pH levels during CIP are temporary and are not part of the normal operating conditions of the system.

What is the recommended pH range?

The ideal pH depends on the treatment technology being used.

Typical operating ranges are:

• Reverse Osmosis (RO): generally between 6.0 and 7.0;
• EDI Systems: typically between 6.5 and 7.5;
• After Softening and Sand Filtration: usually between 6.5 and 8.5.
• Whenever necessary, pH is adjusted through chemical dosing.

The most common chemicals used are:

To decrease pH:

• sulfuric acid (H₂SO₄);
• hydrochloric acid (HCl).
• To increase pH:
• sodium hydroxide (NaOH);
• in some cases, calcium hydroxide (Ca(OH)₂).

Why is continuous pH monitoring important?

In modern water treatment facilities, pH should not be checked only periodically. At critical process points—especially before and after reverse osmosis systems—online pH sensors are typically installed and connected to the plant’s automation system.

Continuous monitoring allows operators to detect deviations immediately, prevent equipment damage, and maintain consistent treated water quality.

pH control is one of the key factors in ensuring the reliable operation of any water treatment system. Whether the process involves reverse osmosis, EDI, or pretreatment, maintaining the correct pH helps minimize corrosion, prevent scale formation, extend equipment life, and improve overall treatment efficiency.