Alkaline effluent is common in textile processing, dairies, bottle washing and many chemical plants. Before it can go to biological treatment or discharge, its pH has to come down, usually to between 6.5 and 8.5. Concentrated sulphuric acid is the cheapest way to do that per unit of acidity. It is also one of the most dangerous chemicals on an ETP, so the dilution and dosing set-up deserves care.
Always add acid to water
Diluting 98% sulphuric acid releases a large amount of heat. If water is poured into concentrated acid, the first water to touch it can boil instantly and spray acid out of the tank. Always add the acid slowly into a larger volume of water with mixing. The old rule is worth repeating to every operator: "Acid into water, never water into acid."
Dose it neat or dilute it first?
Many larger plants dose 98% acid directly through a metering pump into a well-mixed tank. This avoids handling the dilution, but it needs pumps, lines and fittings rated for concentrated acid, and very good mixing at the dosing point. Smaller plants usually dilute to 10–20% in a day tank. Dilute acid is easier to meter accurately at low flows and less likely to cause local hot spots.
Dilution maths: to make 100 kg of 10% w/w acid from 98% acid, weigh 10 ÷ 0.98 = 10.2 kg of acid and add it slowly into 89.8 kg of water. Concentrated sulphuric acid is heavy (about 1.84 kg per litre), so convert weights to litres before measuring by volume. 10.2 kg is about 5.5 litres.
Tank and pipe materials: concentration changes the answer
| Material | 98% acid | Dilute acid (below about 70%) |
|---|---|---|
| Carbon steel | Commonly used for storage tanks at ambient temperature | Not suitable; corrodes fast |
| HDPE / PP | Limited; check the supplier's rating | Good choice for day tanks |
| PVDF, PTFE-lined | Good for pumps, valves and lines | Good |
| FRP (correct resin) | Only with a resin rated for it | Good with the correct resin |
The key point: a steel tank that has stored 98% acid safely for years will corrode quickly if water gets into it. Keep concentrated-acid tanks dry and vented through a desiccant breather. Our guide on safe handling of mineral acids covers storage in more detail.
Getting the dosing right
- Dose into a mixed tank, not a pipe end. The acid should meet the effluent where there is strong mixing, or you will see pH swings.
- Use pH control, not a fixed rate. Effluent alkalinity changes through the day. A pH controller linked to the dosing pump saves acid and avoids overshoot.
- Expect CO₂. Effluent with carbonate or bicarbonate alkalinity will fizz as acid goes in. Allow headroom and ventilation in the neutralisation tank.
- Watch sulphate. Sulphuric acid adds sulphate. If your discharge consent limits sulphate or TDS, consider hydrochloric acid or CO₂ instead.
- Find out what is driving the high pH. Sometimes a process change upstream, such as caustic recovery, cuts acid use more than any dosing tweak.
Safety basics
Face shield over goggles, acid-resistant gloves and an apron are the minimum for anyone handling 98% acid. Keep an emergency shower and eyewash close to the dilution area, and keep sulphuric acid away from caustic, hypochlorite and organic chemicals. For broader effluent chemistry, see chemicals for effluent treatment plants. We supply sulphuric acid 98% in carboys, drums and bulk.
Frequently asked questions
How much sulphuric acid is needed to lower effluent pH?
It depends on the effluent’s alkalinity, not just its pH. Titrate a measured sample with dilute acid in the lab to the target pH, then scale up to your flow. Re-check regularly, because alkalinity changes with production.
Can 98% sulphuric acid be stored in a mild steel tank?
Yes, concentrated sulphuric acid is commonly stored in carbon steel at ambient temperature. Dilute acid is not: it corrodes steel quickly. Keep water out of steel acid tanks and use HDPE, PP, FRP or lined tanks for diluted acid.
Why should acid be added to water and not water to acid?
Diluting sulphuric acid releases a lot of heat. Adding water to concentrated acid can boil the first water instantly and spray acid. Adding acid slowly into water spreads the heat safely.
This article is general guidance. Always follow the Safety Data Sheet and your own site procedures for the specific product and grade you use.