A septic tank must be filled with water before it is used. The water helps start the treatment of the sewage by the bacteria.
The sewage treatment by the bacteria turns the waste matter into effluent (wastewater) and a solid substance called sludge. The effluent gets carried to the leach drain, French drain or lagoon.
The material in the septic tank gets covered by a hard crust known as a scum blanket. This blanket acts as an air seal keeping air away from the sewage. The lack of air helps in the breakdown of the sewage by the bacteria.
The sludge gathers at the bottom of the tanks. Eventually there will be too much sludge in the tank and it must be pumped out and the sludge disposed of correctly.
By having two tanks or a rectangular tank divided into two sections, most of the sludge stays in the first tank or section. In the second tank or section, the sewage undergoes further treatment to remove solid matter.
The effluent is then piped to the effluent disposal system, such as the lagoon.