In a conventional system, effluent flows by gravity (or is pumped) to the high point of a leach field. Typically 4" diameter perforated PVC pipe is laid level over 6" of uniform round stones (variously called "gravel", "aggregate", or "drainrock") which also surround the sides of the pipe. Filter fabric is placed over the pipe and gravel to prevent silting, and the array is covered with soil. The stone has four functions:
• Storage: stores effluent that builds up during a surge in load, and stores effluent when the soil is saturated from rain, until it can be released;
• Structural: holds the sides of the trench from collapsing, supports the soil above, and supports the drain piping off the floor of the trench or bed;
• Distributive: distributes effluent along the trench, mitigating its tendency to exit from the first holes in the distribution piping;
• Aerating: the spaces between the stones provide needed air to the aerobic organisms on the floor of the field.
While stone aggregate is not ideal, it is relatively inexpensive, readily available, and familiar. The field removes most pathogens, organic matter and suspended solids by a combination of physical filtration, biological reduction of organic material by aerobic bacteria, and ion bonding to clay particles, all of which takes place in the soil under the stones.
A conventional leach field consists of a series of properly separated parallel trenches, each typically 2 to 3 feet wide; or a bed not more than 15' wide of more closely spaced pipes. A "contour system" is a single long trench that runs with the line of natural contour. Trenches over 150 feet long need to be pressurized with pumped effluent. The size of the field is determined by the slope of the land, the type of soil, the number of bedrooms, the highest level of the water table, and the distance to an impervious soil layer. Some jurisdictions allow a reduction in field size if the home is equipped with water-saving fixtures. This reduction typically may not be combined with any further reduction through the use of an innovative leach field. Typically, 3 feet of undisturbed, dry soil is necessary under the leach field.