What is the purpose of adding aluminum salts and a base to water during treatment?

Adding aluminum salts and a base to water during treatment serves several important purposes related to water purification and clarification.

1. Coagulation: Aluminum salts, such as aluminum sulfate (alum) or polyaluminum chloride (PAC), are used as coagulants. When added to water, these salts react with the natural alkalinity (bicarbonate ions) present in the water to form positively charged aluminum hydroxide species. These species attract and bind to negatively charged particles and impurities, such as clay particles, organic matter, and bacteria, forming larger flocs.

2. Flocculation: After the coagulants are added, the water undergoes a gentle mixing process known as flocculation. This allows the formed flocs to collide with each other and grow in size, making them easier to remove in subsequent steps.

3. Sedimentation: The flocs formed during coagulation and flocculation become heavy and settle down in sedimentation basins or clarifiers. This allows the particles and impurities to be removed from the water through sedimentation. The settled solids are removed as sludge, leaving the clarified water above.

4. Adsorption and Removal of Impurities: Aluminum hydroxide flocs have a high surface area and act as adsorbents. They can effectively adsorb and remove various impurities present in the water, such as organic compounds, heavy metals, and microorganisms. This process further improves the quality of the treated water.

5. pH Adjustment: Adding aluminum salts and a base also helps adjust the pH of the water. The base, such as sodium hydroxide (NaOH) or lime (calcium hydroxide), raises the pH of the water, which is necessary for effective coagulation and flocculation processes. Maintaining the appropriate pH range optimizes the performance of the treatment process and ensures the formation of stable flocs.

Overall, the addition of aluminum salts and a base to water during treatment is essential for removing impurities, clarifying the water, and improving its overall quality by coagulation, flocculation, sedimentation, adsorption, and pH adjustment processes.