It is important to remember that septic tank inlet and outlet pipes are near the top of the tank. Thus, the septic tank always appears full. The purpose of the distribution box is to evenly distribute the wastewater flow throughout the drainage field. Once in the drainage field, the wastewater passes through perforated lateral pipes, a layer of crushed stone and finally through several feet of unsaturated soil. As the treated wastewater travels through the soil, bacteria die off and the water is purified.
Some systems have a seepage pit or cesspool instead of a lateral field.
Septic tanks, distribution boxes and seepage pits are commonly made of pre-cast concrete. The perforated pipes in the drainage field are often made of PVC pipe which lay buried in several feet of crushed stone. Town regulations, health codes and soil conditions often govern the system design in your area. You may want to contact municipal offices to obtain specific codes or a copy of the property survey. When the combined depth of the sludge and scum layers equal one-third of the septic tank capacity, the tank should be pumped.