Septic Systems 101:
A Primer to Understanding Your
Individual Subsurface Sewage Disposal System (ISSDS)
In general, a contemporary septic system consists of two basic units (Figures 1 & 2): an
underground septic tank (Primary Treatment Unit) and a soil absorption area or drainage field
(Secondary Treatment Unit) with a distribution box that connects the two and disperses the septic
tank effluent evenly over a properly sized absorption area. The septic tank itself is a durable,
watertight container that is resistant to corrosion or decay, e.g., concrete, plastic or fiberglass.
The soil absorption field is typically a network of perforated piping laid-out in trenches filled
with a filtering medium such as gravel, crushed stone or sand.
Figure 1. Typical, double-chamber septic tank depicting various features (NJDEP).
The septic tank serves as a settling basin: heavy solid wastes (sludge) sink to the bottom of the
tank where microbial action will ultimately reduce it by as much as 50% over time; and lighter
solids, e.g., grease, toilet paper, and hair, rise to the surface to form a floating layer of scum
(Figure 1). A typical septic tank will “hold” the wastewater for at least 24 hours to provide
adequate time for separation and settling. The “clarified” water between the sludge and scum
layers then enters the distribution box and is discharged into the drainage field where it is further
purified as it moves through the filtering beds and soil profile. The septic tank is specifically
designed to aid the separation process: inlet baffles or tees direct the household discharges
downward to the bottom of the tank, which prevents the wastewater from flowing straight across
the tank to the outlet end and concurrently minimizes splashing and resuspension of settled
solids. An outlet baffle or tee is also incorporated to contain the scum layer and prevent it from
entering the disposal field. Removing solids prior to the release of effluent protects the soil
absorption system from clogging and failu