At what pH value is alum most effective for phosphorus removal?

• A. 5
• B. 6
• C. 7
• D. 8

Answer: (B)

Alum, or aluminum sulfate, is most effective for phosphorus removal at a pH of around 6. At this pH level, the conditions are optimal for the precipitation of aluminum phosphate, which occurs when alum is added to the wastewater. When the pH is at 6, there is a good balance between solubility and precipitation. The effectiveness of alum decreases significantly at pH values lower than 6, where aluminum ions remain largely soluble and less likely to form the necessary precipitates with phosphate. At higher pH levels, such as 7 and 8, the solubility of aluminum decreases and can lead to the formation of aluminum hydroxide instead of the desired aluminum phosphate precipitate. Therefore, a pH around 6 ensures maximum efficiency in phosphorus removal through the effective action of alum.