What is the function of rapid mixing in the typical water treatment flow scheme for ground water with high hardness ?
- ATo drive out the objectionable dissolved gas such as H₂S and CO₂
- BTo permit the settlement of chemical precipitates under gravity
- CTo reform the calcium and magnesium bicarbonates to prevent settlement of CaCO₃ under Mg(OH)₂ precipitates in the distribution lines
- DTo disperse the lime and soda ash to form the chemical precipitatesCorrect
Solution
Concept
Lime–soda softening
Step-by-step solution
1. In lime–soda softening the chemicals are added to the water and rapidly mixed so that they disperse uniformly. 2. They react to form precipitates of CaCO₃ and Mg(OH)₂, which are then flocculated and settled.
Explanation
Rapid mixing disperses the lime and soda ash through the water so that the precipitation reactions begin uniformly.
Why the other options are wrong
Gas removal is done by aeration, settlement by sedimentation, and recarbonation reforms bicarbonates.
Shortcut
Rapid mix = disperse chemicals.
Common mistake
Choosing the settling function.
Correct answer: D) To disperse the lime and soda ash to form the chemical precipitates