The correct answer is: D. All of these
Surface loading, overflow rate, and overflow velocity are all terms used to describe the same thing: the ratio of design discharge to the surface area of a sedimentation tank.
Surface loading is the most common term used, and it is simply the amount of water that flows through the tank per unit area of surface. Overflow rate is the same thing, but it is expressed in terms of the depth of water that flows over the tank per unit time. Overflow velocity is the speed at which the water flows over the tank.
All of these terms are important because they affect the performance of the sedimentation tank. The higher the surface loading, overflow rate, or overflow velocity, the more likely it is that particles will be removed from the water. However, if the surface loading, overflow rate, or overflow velocity is too high, it can cause problems such as floc break-up and carryover of solids.
The optimal surface loading, overflow rate, or overflow velocity for a particular sedimentation tank will depend on a number of factors, including the type of tank, the nature of the wastewater, and the desired level of treatment.