There are three main types of residential sewage systems, which are standard gravity, pressure distribution and advanced treatment, according to Ever-Green Septic Design. These systems each use different methods of dispersing waste water.
The standard gravity system allows sewage to drain using gradients and gravity. It requires about 3 feet of good draining soil underneath the system.
Pressure distribution uses a pump to force waste water over a wider area for better treatment. These systems require a minimum of 2 feet of drainage beneath the system.
Advanced treatment systems use multiple filtering options to pre-treat the waste water before it enters the soil. These options can include sand filters, aerobic, anoxic and anaerobic treatment and textile filters. Advanced treatment systems require only 12 to 30 inches of drainage.