Which factor is NOT considered in the calculation of drainfield area?

In the context of calculating drainfield area for on-site sewage facilities, the primary factors considered typically include top surface area, water usage, and local evaporation. Each of these factors contributes to understanding how effectively the drainfield will perform in dispersing wastewater.

The top surface area is crucial because it determines the amount of soil available for absorption and treatment of wastewater. Water usage is important as it informs how much wastewater will be produced and, consequently, how extensive the drainfield necessary to treat it must be. Local evaporation impacts the moisture retention of the soil and the overall effectiveness of the drainfield, which can bolster its capacity to handle effluent.

The sidewall area, however, is not a factor in the drainfield area calculation. This is because the design and function of a drainfield primarily depend on the top surface area through which wastewater is dispersed and absorbed rather than the sidewalls, which do not contribute to the area utilized for absorption of effluent into the soil.

In summary, while top surface area, water usage, and local evaporation are all vital in determining the appropriate size and efficiency of a drainfield, the sidewall area does not play a role in these calculations.