Sulphuric acid is one of the most commonly used chemicals in industrial water and wastewater treatment. It is a strong biprotonic acid with the molecular formula H₂SO₄ and is used in numerous applications due to its versatile properties. Its main functions include pH regulation, neutralization of alkaline wastewater, softening and support in chemical precipitation processes.
Table of contents
Chemical properties of sulphuric acid
Molecular structure and acid strength
Sulphuric acid is a strongly polar compound that dissociates completely in water. It releases protons (H⁺) in two steps.
The first dissociation step is complete, while the second is a weaker acid (pKa ≈ 1.99). This makes sulphuric acid one of the strongest mineral acids available.
Physical properties
- Concentration: Commercially available sulphuric acid has a concentration of 96-98 %, diluted sulphuric acid with 10-50 % is often used in water treatment plants.
- Density: 1.84 g/cm³ (at 98 % concentration).
- Corrosiveness: Sulphuric acid is highly corrosive and therefore requires special materials for storage and dosing (e.g. plastic or acid-resistant metals such as 316 stainless steel).
- Heat generation: Considerable heat is released when diluting sulphuric acid, which must be taken into account during handling.
Areas of application in water and wastewater technology
1. pH regulation
Sulphuric acid is often used to lower the pH in water treatment and wastewater treatment plants, especially in neutralization plants. A controlled pH value is crucial for many physico-chemical processes:
- Neutralization: Alkaline wastewater (e.g. with a high hydroxide content) is neutralized by adding sulphuric acid.
- Precipitation plants (CP plants): Many precipitants, such as iron or aluminum salts, are only effective in a certain pH range. Sulphuric acid is used to set the optimum pH value for precipitation.
2. softening and water treatment
Sulphuric acid is used in cooling water circuits and steam boiler systems to reduce carbonate hardness and prevent limescale deposits.The calcium sulphate formed has a higher solubility than calcium carbonate and is less prone to scaling.
3. regeneration of ion exchangers
In water softening and demineralization, sulphuric acid is used for the regeneration of cationic ion exchangers used. These ion exchangers remove cations such as calcium (Ca²⁺) and magnesium (Mg²⁺) from the water. Sulphuric acid is used to reload the exchange groups of the resins with hydrogen ions (H⁺).
Photo: Dosing station for sulphuric acid and caustic soda in our ALMA Neutra neutralization system
Technical aspects of the use of sulphuric acid
1. dosage
Sulphuric acid is dosed using acid-resistant dosing pumps (e.g. diaphragm or peristaltic pumps). It is important to control the dosing quantity precisely, as overdosing can lead to an excessive drop in pH. Online pH measuring devices and control circuits are used to precisely control the pH value.
2. materials and corrosion protection
Sulphuric acid is highly corrosive and requires special materials for storage and transportation:
- Storage tanks: Often made of HDPE (high-density polyethylene) or acid-resistant stainless steel.
- Pipes and valves: Made of PTFE, PVDF or acid-resistant stainless steel.
- Seals: Fluoroplastics such as FKM (Viton) or PTFE are preferred.
3. dilution and safety
When diluting sulphuric acid, the acid must be added slowly to the water, never the other way round, to avoid the risk of sudden heat development.
4. measurement and control technology
Online pH measurements are integrated into control loops for the safe use of sulphuric acid. These systems continuously monitor the pH value and automatically regulate the dosing to ensure stable process conditions.
Safety aspects when handling sulphuric acid
Sulphuric acid is a hazardous chemical that requires strict safety measures when handled:
- Personal protective equipment (PPE): Safety goggles, acid-resistant gloves and acid-resistant protective clothing are mandatory.
- Ventilation: Handling concentrated sulfuric acid produces vapors that are highly irritating and must be handled in well ventilated areas.
- Emergency measures: Neutralizing agents (e.g. lime water or sodium bicarbonate) and eyewash facilities should be available.
Alternatives to sulphuric acid
Depending on the application, other acids can also be used:
- Hydrochloric acid (HCl): Often used for pH corrections, but less suitable for applications with high chloride limitations.
- Phosphoric acid (H₃PO₄): Used in specific processes, e.g. food water treatment, where phosphates are desirable.
Nevertheless, sulphuric acid remains the preferred choice in industrial water and wastewater technology due to its availability, cost-effectiveness and versatility.
Conclusion
Sulphuric acid is an indispensable chemical in industrial water and waste water treatment. Its versatility, combined with a strong acidic effect, makes it ideal for pH correction, neutralization systems, softening and many other applications. A sound understanding of the chemical and technical properties as well as safety measures is essential in order to utilize the benefits of sulphuric acid safely and efficiently. In combination with modern dosing and control technology, it contributes significantly to the optimization of industrial water treatment processes.
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