The correct answer is A. 25,000 to 35,000 litres/sqm/day.
A secondary sedimentation tank is a type of wastewater treatment tank that is used to remove suspended solids and biological floc from wastewater. The overflow rate is the rate at which wastewater flows over the top of the tank. The normal values of overflow rates for secondary sedimentation tanks range from 25,000 to 35,000 litres/sqm/day.
Option B is incorrect because the overflow rate is too high. A high overflow rate can cause the tank to overflow, which can lead to the release of untreated wastewater into the environment.
Option C is incorrect because the overflow rate is too low. A low overflow rate can cause the tank to become overloaded, which can lead to the release of untreated wastewater into the environment.
Option D is incorrect because the overflow rate is not within the normal range for secondary sedimentation tanks.