The design parameter for flocculation is given by a dimensionless number Gt, where G is the velocity gradient and t is the detention time. Values of Gt ranging from 104 to 105 are commonly used with t ranging from 10 to 30 min. The most preferred combination of G and t to produce smaller and denser flocs is
- Alarge G values with short tCorrect
- Blarge G values with long t
- Csmall G values with short t
- Dsmall G values with long t
Solution
Concept
Flocculation Parameters
Formula
G = √(P / (μV))
Step-by-step solution
1. Identify the effect of velocity gradient (G), where a higher G value provides high shear energy to break down larger aggregates into smaller, denser flocs. 2. Consider detention time (t), where a shorter t prevents these dense flocs from continuing to clump into larger, less dense masses. 3. Combine these factors to determine that large G values with short t produce smaller and denser flocs, and select Option (a).
Explanation
A higher velocity gradient (large G) provides rapid, high-energy mixing that breaks down larger flocs and packs particles closely together, forming smaller and denser flocs. A shorter detention time (short t) prevents these flocs from combining into larger, lighter masses.
Shortcut
To remember the rule for smaller and denser flocs, use the rule "High Shear, Quick Clear": - High Shear (Large G): A high velocity gradient packs particles tightly into small, dense flocs. - Quick Clear (Short t): A shorter detention time prevents dense flocs from over-aggregating into large, loose masses.
Common mistake
A common mistake is selecting option D or B by incorrectly assuming that smaller G values or longer detention times (t) are always preferred for flocculation to allow larger flocs to grow. Students often confuse the requirements for generating large, light flocs with those needed for smaller, denser flocs, failing to realize that high velocity gradients (large G) provide the high shear energy required to compact particles into dense flocs, while a shorter detention time (short t) prevents them from over-aggregating into large, loose masses.
Correct answer: A) large G values with short t