On Site Sewage Facilities On Site Sewage Facilities (OSSF's) are wastewater systems designed to treat and dispose of effluent "on site", i.e. on the same property that produces the wastewater. The septic tank/drainfield combination is the oldest and most common type of OSSF, although newer aerobic units now exist which represent scaled down versions of municipal wastewater treatment plants. OSSF's account for approximately 25% of all domestic wastewater treatment in the US.