Between these two scum layers is a zone of clarified liquid effluent which is internally piped to the second compartment of the septic tank for additional settling. As incoming sewage from the house fills the first compartment, clarified liquids are forced to leave the second chamber of the septic tank and flow out to the leach field or leach pit. The typical leach field is a series of chambers or rock filled trenches where effluent is further treated as it slowly percolates through the soil.
A leach pit is a deeper, larger hole filled with rock for disposing of wastewater in a smaller footprint. Not as effective treatment as a larger leach field, where sewage percolates slowly over a larger area, leach pits are an alternative for smaller properties only where high seasonal ground water is not present.
Greywater from washing machine, sinks and showers contains soaps designed to kill bacteria (clean and disinfect things) and thus discourage optimal septic tank function. You want to encourage helpful good bacteria (digesting anaerobic cultures) to grow in your septic tank and organically treat the waste, not kill helpful bacteria with detergent laden graywater. If allowed by local building department, it is best practice to divert your household greywater to a separate leaching area".
Original article source: https://www.thenaturalhome.com/septic.html