ANS:C - 50, 000 to 60, 000 litres/sqm/day

Overflow rate in sedimentation tanks refers to the volume of water that flows over a unit area of the tank per unit time. It is an important parameter in the design and operation of sedimentation tanks, especially when coagulants are used to aid in the settling of suspended solids. Here's a more detailed explanation:

1. Sedimentation Tanks: These tanks are used in water treatment processes to remove suspended solids from water. When water enters the sedimentation tank, the suspended particles settle down due to gravity, forming a sludge layer at the bottom.
2. Coagulants: Coagulants are chemicals added to the water to facilitate the aggregation of fine particles into larger, heavier flocs. This aggregation process helps the particles settle more quickly during sedimentation.
3. Overflow Rate: Overflow rate is a measure of the flow rate of water over a given area of the sedimentation tank. It is typically expressed in units of volume per unit area per unit time (e.g., litres per square meter per day).
4. Significance: The overflow rate indicates the hydraulic loading on the sedimentation tank. Higher overflow rates mean more water is passing through the tank per unit area and per unit time. The choice of overflow rate is crucial because it affects the efficiency of particle removal. If the overflow rate is too high, it may lead to reduced settling time and inadequate removal of suspended solids. On the other hand, if it's too low, the tank may become oversized and inefficient.
5. Normal Ranges: The given normal ranges of overflow rates for sedimentation tanks using coagulant are provided based on practical experience and engineering considerations. These ranges (25,000 to 35,000 litres/sqm/day) have been found to be effective in achieving optimal sedimentation performance while balancing factors like tank size, coagulant dosage, and settling efficiency.