Which of the following is not a factor for the satisfactory operation of a septic tank/sand filter system?

The correct choice identifies the ratio of septic tank size to sand filter size as not being a critical factor for the satisfactory operation of a septic tank/sand filter system. This is because while the individual sizes of the septic tank and sand filter are important for overall system performance, their specific ratio does not fundamentally influence the operational efficiency. What matters more is how well each component functions in its own right according to its design parameters and local regulations.

In contrast, the other options are crucial for the effective functioning of such systems. The rate of sewage application affects the hydraulic load on the system. A rate that is too high can lead to system overload, whereas too low a rate can cause solids build-up in the tank. Additionally, the effective size of sand is significant because it determines the permeability and retention characteristics of the sand filter, allowing for proper treatment of effluent. Lastly, the uniformity coefficient of sand is also important since it indicates the variation in sand particle sizes, affecting the filter’s ability to perform well without clogging or bypassing.