Onsite Sewage Treatment & Disposal Systems (OSTDS)

An onsite sewage treatment and disposal system (OSTDS) or commonly referred to as a septic system or septic tank is a system that partially treats and disposes of domestic wastewater on site.  The wastewater generated within a building is collected in the building’s plumbing system, which drains into a septic tank (typically made of concrete) where the solids are partially separated from the liquid by either floating or settling.

During this process, some bacteriological breakdown of the organic waste occurs and the liquid flows out of the septic tank into the drainfield. The drainfield is an underground network of perforated pipes or chambers for distributing partially treated wastewater from the septic tank to the soil for final treatment and recharge to the groundwater. Once the effluent leaves the drainfield, additional treatment, in the form of filtration, adsorption, aerobic decomposition and assimilation, occurs within the unsaturated soil layer. For this treatment to take place, a minimum separation between the bottom of the drainfield and the water table must exist and remain unsaturated.

This approach to treating sewage originated in the late 1800s and while the methods, materials of construction, and the sizing of systems have changed over time, the use of conventional septic systems for only primary treatment of sewage and pollutant dilution has stayed the same for over a century.