The correct answer is A. 25,000 to 35,000 litres/sqm/day.
A plain primary sedimentation tank is a type of wastewater treatment tank that uses gravity to separate solids from liquids. The overflow rate is the rate at which the wastewater flows over the top of the tank. The normal value of overflow rates for plain primary sedimentation tanks ranges between 25,000 to 35,000 litres/sqm/day. This means that for every square meter of tank surface area, 25,000 to 35,000 liters of wastewater should flow over the top of the tank per day.
The overflow rate is an important design parameter for plain primary sedimentation tanks. It affects the efficiency of the tank in removing solids from the wastewater. A higher overflow rate will result in a lower removal efficiency, while a lower overflow rate will result in a higher removal efficiency.
The overflow rate is also affected by the type of wastewater being treated. Wastewater with a high concentration of solids will require a lower overflow rate than wastewater with a low concentration of solids.
The overflow rate is also affected by the temperature of the wastewater. Wastewater that is warmer will have a higher overflow rate than wastewater that is cooler.
The overflow rate is an important parameter to consider when designing a plain primary sedimentation tank. The correct overflow rate will ensure that the tank is able to remove solids from the wastewater efficiently.
Option B is incorrect because the normal value of overflow rates for plain primary sedimentation tanks is not 40,000 to 50,000 litres/sqm/day.
Option C is incorrect because the normal value of overflow rates for plain primary sedimentation tanks is not 50,000 to 60,000 litres/sqm/day.
Option D is incorrect because the normal value of overflow rates for plain primary sedimentation tanks is not 80,000 to 100,000 litres/sqm/day.