Stage1: Settlement
heavy solids settle to the base of the tank to form a sludge which must occasionally be removed; about 80 percent of the
suspended solids can be separated from the liquid in a well designed tank.
Stage 2: Floatation
Grease and oil float to the surface to form a layer of scum; over time this scum layer becomes thick and the surface may be hard.
Stage3: Floatation
The sludge at the bottom of the tank is compressed by the weight of new material settling on top, increasing its density; and organic matter in the sludge and scum layer is broken down by bacteria which convert it to liquid and gas.
Stage4: Stabilization
The liquid in the tank undergoes some natural purification but the process is not complete; the final effluent is anaerobic and will contain pathogen
The final effluent leaving the septic tank must be disposed of in an appropriate location such as a sealed pit or sewerage system.