The effectiveness of alum as a coagulant is highly dependent on
- AThe temperature of the water
- BThe presence of bicarbonate alkalinity and the pH of waterCorrect
- CThe dissolved oxygen levels
- DThe chloride concentration
Solution
Concept
Alum coagulation dependence on alkalinity and pH
Formula
Al2(SO4)3 + 3 Ca(HCO3)2 -> 2 Al(OH)3 + 3 CaSO4 + 6 CO2
Step-by-step solution
1. Alum [Al2(SO4)3] reacts with calcium or magnesium bicarbonate alkalinity to produce Al(OH)3 floc. 2. Insufficient alkalinity causes pH drops, preventing floc formation and leaving soluble aluminum. 3. The coagulation reaction has an optimum pH window of 6.5 to 8.3. 4. Therefore, alum effectiveness relies directly on bicarbonate alkalinity and pH. Answer: B
Explanation
Alum [Al2(SO4)3.18H2O] requires bicarbonate alkalinity in raw water to react and precipitate insoluble gelatinous aluminum hydroxide flocs. The reaction is strongly governed by pH, requiring an optimum range between 6.5 and 8.3 for effective flocculation and minimal residual dissolved aluminum.
Why the other options are wrong
A) Temperature influences viscosity and reaction rates but does not dictate stoichiometric floc formation. C) Dissolved oxygen plays no role in chemical coagulation. D) Chloride concentration does not control alum precipitation.
Shortcut
Alum coagulation needs: Bicarbonate Alkalinity + pH 6.5 to 8.3.
Common mistake
Assuming temperature or dissolved oxygen is the primary chemical governing factor.
Correct answer: B) The presence of bicarbonate alkalinity and the pH of water