The Role of Alum in Water Treatment: A Crucial Step Between Aeration and Flocculation

Water treatment is a multi-stage process designed to ensure the safe and efficient purification of water for public consumption. One key step in this process involves the addition of aluminium sulphate, commonly known as "alum", which plays a vital role in water clarification, particularly between the aeration and flocculation stages.

The Importance of Alum in Water Treatment

After water undergoes aeration, where dissolved gases like carbon dioxide are removed and oxygen is introduced to oxidise organic matter and dissolved metals, the next step is to add alum. This chemical is crucial because it facilitates the coagulation process, where fine particles suspended in water are aggregated into larger particles known as "flocs". These flocs are easier to remove in the subsequent sedimentation and filtration stages.

• Alum's Mechanism of Action: When alum is added to water, it dissolves and reacts to form aluminium hydroxide, a sticky substance that binds with suspended particles such as dirt, silt, and organic matter. This aggregation of particles creates larger clumps that are heavy enough to settle out of the water during flocculation and sedimentation. This process significantly reduces turbidity, improving water clarity and ensuring that harmful pathogens, which often attach to these suspended particles, are also removed.
• Determining the Right Alum Dosage: The effectiveness of alum in coagulation largely depends on the turbidity of the incoming water. For deciding the appropriate percentage of alum to add, it is essential to measure the water’s turbidity beforehand. This ensures that the right amount of alum is used to achieve optimal coagulation without excess chemical use. I will discuss the turbidity testing process in more detail in my next article.

Transition from Chlorine to Alum

Historically, chlorine was sometimes added at this stage for its disinfecting properties. However, research has shown that when chlorine reacts with organic matter present in the water, it can form disinfection by-products (DBPs), some of which are carcinogenic, such as trihalomethanes (THMs). As a result, the practice has shifted to adding chlorine at the end of the treatment process, after most organic matter has been removed, thus minimising the formation of harmful DBPs.

The introduction of alum at this stage of the treatment process ensures that the water is prepped for effective flocculation and reduces the potential for harmful chemical reactions that could compromise water safety. By ensuring the removal of suspended particles and reducing turbidity, alum not only enhances water clarity but also improves the overall efficiency of the treatment process, ensuring that subsequent disinfection stages, including the use of chlorine, are more effective and safer for consumers.

Conclusion

The use of alum between aeration and flocculation is a critical step in modern water treatment processes. It ensures that suspended particles are effectively removed, enhancing water quality and safety, while also allowing for the safer and more efficient use of chlorine in the final stages of water purification.

Technical words:

1. Alum
2. Aeration
3. Flocculation
4. Turbidity
5. Flocs