How Does It Work?
There are several types of private sewage systems commonly found serving homes today. One type, the sewage holding tank, temporarily stores the sewage in a watertight tank for later removal by a sewage hauler to a municipal sewage treatment plant.
There are several types of on-site soil absorption systems approved for use in the State of Wisconsin. The most common system uses a septic tank for primary treatment of household wastewater before it is discharged to the soil absorption field.
When wastewater is discharged to the septic tank, the waste is directed downward by the inlet baffle allowing the larger solids to settle to the bottom of the tank forming a sludge blanket. Grease, oils and floating particles rise to the top to form a scum layer located between the inlet and outlet baffle. The baffle on the outlet side of the septic tank prevents the scum layer from being discharged to the absorption field. While in the septic tank, the wastewater is retained for a period of time to allow anaerobic bacteria to digest and breakdown the solids reducing its volume. The partially treated waste discharged from the septic tank is referred to as sewage effluent.
Effluent flows by gravity or is pumped to the absorption field for filtration by the soil. Within the absorption field, a series of perforated pipes distributes the sewage effluent. The types of absorption fields approved for use include see page beds, seepage trenches, dry wells, in-ground pressure and mound systems. The size of the absorption field varies depending on the soil permeability and building usage. For residences, the building usage is based on the number of bedrooms in the house.
Soil acts as a filter allowing sewage effluent to be treated as it moves downward through the soil. As the absorption field ages, the soil’s ability to filter and absorb sewage effluent diminishes causing the absorption field to become saturated. Although a saturated system will continue to filter sewage effluent, eventually the sewage effluent accumulates in the absorption field faster than the absorption field can filter and absorb it. As the soil absorption rate decreases due to the clogging of the soil pores, sewage back up or the surface discharge of sewage may result. When this occurs, the absorption field will need to be replaced. Since there are many factors that can effect system longevity, there is no way to determine how long a private sewage system will last.