H. Open Ditch Drainfield it has two to four perforated field lines installed in trenches 2 feet wide and 2 to 3 feet deep filled with crushed rock without a topsoil cover on steep sloping land. The drain lines are installed at-grade perpendicular to the land slope with a grass covered spacing of 10 to 12 feet between lines. Septic tank effluent flows by gravity through a distribution box with valve controls and will rise in the trench and flow over the grass strip for final treatment and absorption. The control valves are needed to control access to each line separately for longer site life. As wastewater enters the tank, solids settle to the bottom forming a sludge layer, and the greases, fats, and oils float to the top forming a scum layer.
Baffles prevent solids from leaving until some decomposition and waste reduction has occurred.
Microbes in this oxygen-devoid system decompose organic components of the liquid and sludge and reduce pathogens. As effluent leaves the tank, the solids are reduced both in concentration and particle size. The major function is to provide the most significant portion of treatment by delivering wastewater for final treatment. The liquid trickles through the rock, sand, or soil where pathogens are filtered out and destroyed by the microbial mat surrounding the pipes. A major concern in siting is to evaluate the soil s capacity to adequately perform this final treatment.