Question

Which of the following are associated with alum coagulation?

1. A decrease in alkalinity of treated water
2. Formation of hydroxide flocs of aluminium
3. A slight decrease in pH of treated water
4. An increase in permanent hardness

Select the correct answer using the codes given below:

• A. 1, 3 and 4
• B. 1,2,3 and 4
• C. 2 and 4
• D. 1, 2 and 3

Answer 1

The correct option is B 1,2,3 and 4

(c) The purpose of coagulation is to alter the colloids so that they can adhere to each other. During coagulation a positive ion is added to water to reduce the surface charge to the point where the colloids are not repelled from each other.

Aluminium can be purchased as either dry or liquid alum $\left[A{l}_2\right(S{O}_4{)}_3.14H_{2}O]$. when alum is added to a water containing alkalinity, the following reaction occurs:



Such that each mole of alum added uses six moles of alkalinity and produces six moles of carbon dioxide. The above reaction shifts the carbonate equilibrium and decreases the pH. However, as long as sufficient alkalinity is present and $C{O}_2$ is allowed to evolve, the pH is not drastically reduced and is generally not an operational problem. When sufficient alkalinity is not present to neutralize the sulfuric acid production, the ph may be greatly reduced:



If the second reaction occurs, lime or sodium carbonate may be added to neutralize the sulfuric acid. Two important factors in coagulant addition are pH and dose. The optimum dose and pH must be determined from laboratory tests. The optimal pH range for alum is approximately 5.5 to 6.5 with adequate coagulation possible between pH 5 to pH 8 under some conditions.