The correct answer is: A. 25,000 to 35,000 litres/sqm/day.
The overflow rate is the rate at which water flows over the weir of a sedimentation tank. It is an important parameter in the design of sedimentation tanks, as it affects the efficiency of the tank in removing suspended solids from the water.
The overflow rate is typically expressed in litres per square metre per day (L/m2/day). The recommended overflow rate for sedimentation tanks using coagulant is 25,000 to 35,000 L/m2/day. This range of overflow rates will ensure that the tank is able to remove suspended solids effectively, while also minimizing the risk of floc breakthrough.
Floc breakthrough is the phenomenon of floc particles passing through the weir of the sedimentation tank and into the effluent. This can occur if the overflow rate is too high, or if the floc particles are not formed properly. Floc breakthrough can lead to a decrease in the quality of the effluent, and can also increase the risk of clogging in downstream processes.
Therefore, it is important to select an overflow rate that is within the recommended range. This will ensure that the sedimentation tank is able to remove suspended solids effectively, while also minimizing the risk of floc breakthrough.
The other options are incorrect because they are outside of the recommended range for overflow rates for sedimentation tanks using coagulant.