Overview
pH is one of the most fundamental parameters in water treatment. It governs the solubility of metals, the effectiveness of coagulation, the stability of disinfection residuals, the corrosivity of water toward metal infrastructure, the activity of biological treatment organisms, and the speciation of many pollutants. Virtually every water treatment application requires deliberate pH control at one or more points in the process.
pH adjustment chemicals fall into two categories: alkalis (caustic chemicals that raise pH) and acids (that lower pH). The correct selection within each category depends on cost, handling constraints, downstream chemistry compatibility, and whether the reagent introduces undesirable ions into the treated water.
Why pH control matters for coagulation: Most inorganic coagulants such as PAC, alum, and ferric salts have a narrow optimum pH window for effective floc formation. Alum requires pH 6.0–7.5; ferric coagulants work best at 4.5–7.0; PAC is more tolerant but still has a working range. Source water that is naturally acidic or alkaline will require pH correction before or during coagulant dosing to achieve target turbidity removal.
Alkali Chemicals (pH Raising)
Acid Chemicals (pH Lowering)
Applications
| Application | Objective | Typical Reagent |
|---|---|---|
| Coagulation pH correction | Maintain source water pH within coagulant's effective operating range | Soda ash or lime (to raise); sulfuric acid (to lower if alkaline source) |
| Cooling water pH control | Maintain pH 6.8–7.5 to balance corrosion inhibitor effectiveness against scale formation tendency | Sulfuric acid (control of high-pH blowdown); caustic soda (if acid dosing overshoots) |
| Boiler feedwater | Maintain slightly alkaline feedwater to suppress oxygen corrosion; control condensate pH | Caustic soda or ammonia; CO₂ for post-RO adjustment before deaeration |
| Effluent neutralisation | Bring acidic or alkaline industrial effluent to discharge-compliant pH range (6.0–9.0 for PROPER) | Lime or caustic soda for acid effluent; sulfuric or HCl for alkaline effluent |
| RO membrane cleaning & post-treatment | Acid CIP for scale removal; pH adjustment of permeate for re-mineralisation | HCl or H₂SO₄ for CIP; CO₂ or soda ash for permeate conditioning |
| Heavy metal precipitation | Raise pH to 9.0–11.0 to precipitate metal hydroxides from wastewater | Caustic soda or hydrated lime |
Petrochemical & LNG Applications
pH adjustment chemicals are among the highest-volume commodity chemicals consumed at petrochemical complexes and LNG terminals due to demanded across multiple water systems simultaneously.
In refinery and petrochemical cooling water systems, sulfuric acid is continuously dosed to counteract the natural rise in pH caused by CO₂ stripping in the cooling tower. Maintaining pH within the 6.8–7.5 band is critical for the effectiveness of scale and corrosion inhibitor programmes: too high, and calcium carbonate scale deposits on heat exchanger tubes; too low, and the inhibitor film is disrupted and corrosion risk rises sharply.
In amine gas treating units, the amine solution pH must be carefully controlled. Acidic gases absorbed from the process gas stream lower the amine pH; a controlled pH range is essential for maintaining amine loading efficiency and preventing corrosion of the absorber and regenerator vessels. Caustic soda is used for amine pH correction and for wash water neutralisation.
In condensate treatment systems at LNG plants, condensate stripping produces water with low pH (dissolved CO₂ and organic acids from the gas stream). Caustic dosing or CO₂ stripping before discharge or reuse neutralises this acidity and brings the water into the acceptable pH range for effluent discharge or re-use as process water.