The pre-treatment tank is usually the first tank in a multiple tank septic system. The separation of solids which could be either heavier or lighter than water is performed in a pre-treatment tank which is also known by it’s functional name of settling tank. The settling tank is the first tank or chamber (in the case of a multiple chamber tank) which may also be referred to as the pre-treatment tank or trash tank. This separation is accomplished in the first tank by allowing gravity to work on the wastewater as it passes through. In order for gravity to be effective the flow of the wastewater through the first tank must be slow enough that the heavy material has time to sink to the bottom and the lighter than water material (fats and grease) has time to float to the surface. The out flow opening on the tank should have a PVC tee on it which only allows the wastewater from the center of the tank to exit while the lighter and heavier solids stay behind. Here is an example of a new (2016) pre-treatment tank equipped with two openings over the input and output “t’s”.