A local plant has been administering Alum to aid in settling. Which of the following BEST describes why settling rates have actually decreased?

The correct answer highlights the importance of alkalinity in maintaining optimal pH levels during the use of Alum as a coagulant in the wastewater treatment process. Alum, or aluminum sulfate, functions effectively in settling processes by aggregating fine particulates into larger flocs that can settle out of the water column. However, the effectiveness of this process can be significantly hampered if the pH of the wastewater is not properly balanced.

When Alum is added, it can lower the pH of the solution. If the alkalinity—the water’s ability to resist pH changes—is inadequate, it may not sufficiently buffer the pH against the acidifying effects of the Alum addition. A low pH can inhibit the coagulation and flocculation processes, resulting in decreased settling rates. This is because a lower pH can negatively influence the charge interactions between the floc-forming particles and the Alum, reducing the overall efficiency of solids removal.

In such instances, the settling rates can decline, leading to poor operational performance. Therefore, maintaining appropriate alkalinity is crucial to ensure that the pH remains within an ideal range that allows for optimal coagulation and flocculation processes to take place, which in turn supports effective settling and clarification in wastewater treatment.