What is hydrochloric acid used for in drinking water treatment?

Hydrochloric acid (HCl), also known in some contexts as “muriatic acid," is a strong, widely used mineral acid with important applications in water and wastewater treatment. In an aqueous solution, HCl dissociates completely into hydrogen (H⁺) and chloride (Cl⁻) ions, making it highly effective for pH adjustment, chemical regeneration, scale removal and other water-treatment tasks. 

Below is an overview of why HCl remains a valuable chemical in drinking water treatment, how it works, what to avoid and how it compares with other chemicals.

Why water treatment plants use hydrochloric acid

pH adjustment and corrosion control

One of the most common uses of hydrochloric acid in water treatment is pH correction. In many water sources — especially those with high alkalinity — pH can be too high, leading to scaling, undesirable taste or odor, and corrosion of pipes or equipment.

By adding HCl in controlled amounts, treatment operators can lower the pH to target levels suitable for safe distribution, disinfection, and corrosion control. This makes HCl especially valuable in systems where other acids (e.g., sulfuric) might introduce problematic ions — for example, sulfate — into the water.

Regeneration of ion-exchange resins and demineralization

Drinking water systems often use ion-exchange resins to soften water, remove hardness (calcium, magnesium) or remove unwanted ions. Over time, resins become saturated and require regeneration. Hydrochloric acid is commonly used to regenerate cation-exchange resins, by replacing bound ions with H⁺, thereby restoring resin capacity.

This use of HCl supports ongoing water softening and demineralization, which are critical for delivering high-quality drinking water — especially in areas with hard water or high carbonate concentrations.

Descaling and maintenance of treatment equipment

In water treatment infrastructure — pipes, filters, membranes, and chemical reactors — mineral deposits (scale) and rust can build up over time. Hydrochloric acid is effective at dissolving scale (such as calcium or magnesium carbonates), clearing deposits, and cleaning equipment.

By using HCl for descaling and maintenance, plants can preserve flow rates, maintain efficiency, and prolong lifespan of treatment assets.

As precursor chemical and in combined processes

Beyond direct use, hydrochloric acid is sometimes used as a raw material to produce other water-treatment chemicals — for instance, certain metal chlorides used as coagulants or flocculants.

In some treatment schemes, HCl also plays a role in processes like decarbonization, which removes carbonates from hard water and reduces scaling propensity.

What does hydrochloric acid do when added to water?

Upon addition to water, HCl dissociates completely:

This proton (H⁺) release lowers the water’s pH, neutralizing excess alkalinity. Lower pH — within safe, controlled limits — can:

• Prevent scale formation and buildup

• Improve corrosion control

• Optimize chemical coagulation, disinfection, and other treatment processes

• Restore ion-exchange resins by replacing bound ions

Because HCl is strong and acts quickly, it can achieve these outcomes with relatively small volumes and precise dosing — important for operational efficiency in drinking water systems.

What chemical is “best” for water treatment?

There is no universal “best chemical” for all water-treatment needs. Instead, the optimal choice depends on water chemistry, treatment goals, regulatory constraints and system design. That said, hydrochloric acid remains one of the most widely used and versatile acids in drinking water treatment — especially when:

• pH needs to be lowered without introducing sulfate or other problematic ions

• Ion-exchange resins must be regenerated

• Equipment cleaning and descaling are required

• Scale reduction or decarbonization is needed

Other chemicals (like sulfuric acid, CO₂, or various weak organic acids) may be used in different contexts — but HCl’s strong acidity, reactivity and chlorine-free byproducts often make it a preferred choice where high reactivity and minimal residual contaminants are critical.

What you should never mix with hydrochloric acid

Because HCl is highly reactive and corrosive, certain substances should never be mixed with it — especially in treatment or maintenance settings:

• Chlorine-based disinfectants or oxidizers (e.g., sodium hypochlorite): mixing HCl with oxidizers or strong chlorine solutions can generate toxic chlorine gas (Cl₂), posing severe health risks.

• Strong oxidizing agents, peroxides, or metals prone to hydrogen evolution: HCl can react violently, release hydrogen gas, or corrode metals — risking fire, explosion, or system damage.

• Uncontrolled high concentrations in closed or poorly ventilated areas: always dilute acid properly (add acid into water, not water into acid), use acid-resistant materials (HDPE, PVC, PP tanks/piping), and provide appropriate personal protective equipment (PPE) — gloves, goggles, ventilated areas.

Following these precautions is essential for safe, effective, and compliant use of HCl in water treatment.

Why hydrochloric acid remains essential in drinking water treatment

Hydrochloric acid remains a cornerstone chemical in drinking water treatment — because of its reliability, versatility, and effectiveness. It supports pH adjustment and corrosion control, enables ion-exchange resin regeneration, dissolves scale and deposits, and helps maintain the efficiency and safety of water treatment systems. When used properly, with careful dosing, material selection, and safety practices, HCl can deliver high-quality water treatment outcomes with minimal residual impact.

Because water chemistry, regulatory requirements, and treatment objectives vary widely, no single chemical is “best” in every scenario. But for many drinking water systems — especially those dealing with high alkalinity, scale, or hard water — hydrochloric acid remains among the most practical and effective tools available today.

This piece was created with the help of generative AI tools and edited by our content team for clarity and accuracy.