Water Treatment Coagulation – Effect of Alum (Al2(SO4)3) on pH When aluminum sulfate (alum) is used as a coagulant in conventional water treatment, what is its typical effect on the pH of the treated water (assuming normal alkalinity and no external pH control)?

Introduction / Context:
Coagulation with alum is a cornerstone of surface-water treatment for turbidity and natural organic matter removal. The hydrolysis of alum consumes alkalinity and produces acidity, which typically lowers the pH unless compensated by sufficient natural alkalinity or added chemicals such as lime or soda ash.

Given Data / Assumptions:

• Coagulant: aluminum sulfate (alum).
• Typical raw water with finite bicarbonate alkalinity.
• No deliberate pH correction during coagulation step.

Concept / Approach:

Alum hydrolyzes to form aluminum hydroxide floc, consuming bicarbonate alkalinity and releasing hydrogen ions. A simplified reaction shows that alum + alkalinity → Al(OH)3(s) + sulfate + CO2, with a net production of H+ that depresses pH. Operators therefore monitor and, if necessary, adjust pH to stay within the optimal coagulation window (commonly pH 5.5–7 for alum, depending on water quality).

Step-by-Step Solution:

Verification / Alternative check:

Jar tests routinely show pH depression after alum addition; plant practice often includes lime/soda ash addition downstream or concurrent with coagulation to maintain target pH.

Why Other Options Are Wrong:

• Increase or no change: contrary to the chemistry of alum hydrolysis for typical waters.
• “May increase or decrease”: while theoretically possible in unusual waters or with buffers, the standard expected effect is a pH decrease.

Common Pitfalls:

Overdosing alum leading to excessive pH drop and poor floc formation; not accounting for seasonal alkalinity changes affecting pH control.

Final Answer:

Results in a reduction of pH of the treated water.