Standard gravity design and pressure distribution design are two basic ways of designing septic fields. The design for standard gravity systems includes a series of trenches below the level of the septic tank, which collect discharge drained from the tank. The pressure distribution design employs a pump or siphon to pressurize the discharge before it passes through a small PVC pipe to the drainfield. Sites with low soil depth usually require an advanced treatment system.
The advanced treatment system filters and cleans the effluent prior to its disposal in the ground. Dispersal through a pressure distribution drainfield usually follows the treatment process.
Septic system design considerations include tank size, location and retention time. The right choice for septic tank size depends on the anticipated volume of water used on a daily basis. 1000 gallons is usually the minimum required capacity. Building codes specify the distance required between septic system components, as well as the minimum distance from buildings, property lines, wells, bodies of water and trees. Adequate septic tank retention time and liquid levels are necessary to ensure that solids settle to the bottom of the tank, keeping them from flowing out and clogging the drainfield.
Recommended trench dimensions are 18 to 36 inches wide, 2 feet deep, and at least 3 feet above hardpan, the water table or rock. To correctly size the drainfield, consider the estimated daily flow of waste water and test the absorption rate of the soil through a percolation test.