Guide · 30 September 2026 · 6 min read
What is a pH adjuster?
By the Accot technical team
Effluent that is too acidic or too alkaline fails discharge limits, stops coagulants working, slows biological treatment and can cause odour and corrosion. A pH adjuster fixes that. Here is what pH is, where pH adjusters are used, the problems they solve, and how to choose the right one.
1. What is pH, and what is a pH adjuster?
pH is a measure of how acidic or basic water is. The scale runs from 0 to 14: 7 is neutral, below 7 is acidic and above 7 is basic (alkaline).[1]
The scale is logarithmic — each whole number is a ten-fold change. Water at pH 5 is ten times more acidic than water at pH 6.[1] A small change in the reading is a big change in the water, which is why pH correction has to be dosed carefully.
A pH adjuster is an acid or an alkali dosed into water to bring its pH into the target range. Alkalis such as caustic soda (sodium hydroxide) and soda ash (sodium carbonate) raise the pH; a strong mineral acid such as sulphuric acid lowers it.[2]
2. Where are pH adjusters used?
Any plant whose process uses acids or alkalis — for example in cleaning, metal pickling, dyeing or chemical processing — can produce effluent that needs pH correction. Typical uses are:
- Final neutralisation before discharge — bringing effluent within the pH limit of its discharge standard.[3]
- Before coagulation — coagulants work within relatively narrow pH bands. Aluminium and iron coagulants react with the water’s alkalinity and lower its pH, so soft, low-alkalinity water needs alkali added, while high-alkalinity water may need acid.[2]
- Heavy metal removal — adding alkali converts dissolved metals into insoluble hydroxides that settle. The best pH depends on the metal, so it is set case by case.[4]
- Biological treatment — nitrification consumes alkalinity, and as alkalinity is used up the pH drops.[5]
- Sewers, sumps and tanks — keeping the pH at or above neutral reduces the hydrogen sulphide gas released into the air above the wastewater.[6]
3. What problems do pH adjusters solve?
| Problem | How a pH adjuster helps |
|---|---|
| Effluent outside the pH limit | Brings it into range: pH 6.0–9.0 under Standard A, 5.5–9.0 under Standard B.[3] |
| Poor coagulation, cloudy water | Sets the pH where the coagulant works best. Lowering pH with acid, rather than adding more coagulant, avoids raising the dissolved metal content of the water.[2] |
| Dissolved heavy metals | Raising the pH precipitates them so they can settle.[4] Metals also tend to be more toxic at low pH because they are more soluble.[1] |
| Nitrification slowing (ammoniacal nitrogen failing) | Keeps pH and alkalinity up — nitrification and phosphorus removal rates decrease when the pH drops below 6.9.[5] |
| Rotten-egg odour, corroded concrete | A pH below 7 favours dissolved hydrogen sulphide, which escapes as gas; keeping the pH at or above neutral reduces the amount released.[6] |
The full list of Standard A and B discharge limits is in our guide What is a coagulant?
4. Choosing the right pH adjuster — and what ours do
In chemical treatment, the amount of chemical needed depends on the pH and alkalinity of the wastewater, and competing reactions make it hard to calculate — accurate doses should be determined by jar tests and confirmed in the field.[4] In coagulation testing, the amount of acid or alkali needed is first found by titration.[2]
Caustic soda or soda ash?
Caustic soda is a strong alkali: it releases hydroxide immediately, so it raises pH quickly and can push it very high. Soda ash is a milder, buffering alkali — its peak pH is lower (about 11.5) and its response is more gradual, so overshooting the target is less likely. Caustic soda is highly corrosive and needs full PPE and trained operators; soda ash is an irritant, and as a powder it must be dissolved before dosing.[7] Concentrated acids and alkalis are corrosive — always follow the safety data sheet and wear PPE.
Accot pH adjusters
Raises pH · alkali
ACCOT Soda Ash
Neutralises acidic water and wastewater and raises pH. Also used to increase the alkalinity of spray booth water. Available as powder or as a 25% solution.
Raises pH · alkali
ACCOT Caustic Soda 50%
50% alkali solution for raising pH and neutralising acidic water and wastewater. High solubility allows easy dosing and rapid reaction.
Lowers pH · acid
ACCOT Hydrochloric Acid 33%
Strong inorganic acid for balancing and neutralising the pH of alkaline water and wastewater, or where acidification is required.
Lowers pH · acid
ACCOT Sulphuric Acid 50%
Strong inorganic acid for lowering pH and reducing alkalinity in water and wastewater, for pH adjustment and neutralisation.
Used together with
- Coagulants — ACCOT K101, ACCOT K40FC and ACCOT PAC, which each work best within their own pH range. See What is a coagulant?
References
- [1]U.S. Geological Survey. pH and Water, Water Science School. www.usgs.gov/special-topics/water-science-school/science/ph-and-water
- [2]Environmental Protection Agency, Ireland (2002). Water Treatment Manuals: Coagulation, Flocculation & Clarification. www.epa.ie/publications/compliance--enforcement/drinking-water/advice--guidance/EPA_water_treatment_mgt_coag_flocc_clar2.pdf
- [3]Environmental Quality (Industrial Effluent) Regulations 2009, P.U.(A) 434/2009, made 12 October 2009 under the Environmental Quality Act 1974 — regulation 11 and Fifth Schedule (p. 4031). Department of Environment Malaysia. www.doe.gov.my/en/environmental-quality-industrial-effluent-regulations-2009-p-u-a-434-2009-2/
- [4]United States Environmental Protection Agency (2000). Wastewater Technology Fact Sheet: Chemical Precipitation (EPA 832-F-00-018). nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1001QTR.TXT
- [5]United States Environmental Protection Agency (2007). Biological Nutrient Removal Processes and Costs (EPA-823-R-07-002). www.epa.gov/sites/default/files/documents/criteria_nutrient_bioremoval.pdf
- [6]United States Environmental Protection Agency (1991). Hydrogen Sulfide Corrosion in Wastewater Collection and Treatment Systems: Report to Congress (430/09-91-009). nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=00000L69.txt
- [7]Aldayyeni, M. Q. & Alameedy, U. (2026). Performance optimization of CO₂ mineralization: a comparative study of caustic soda and soda ash as pH-risers under high-pressure reactor. Iraqi Journal of Chemical and Petroleum Engineering, 27(2), 69–75. doi.org/10.31699/IJCPE.2026.2.6