Conventional In-Ground Septic System
A conventional in-ground septic system has two basic components: the septic tank and the soil absorption field.
Diagram of Conventional In-Ground Septic System
The septic tank is where primary treatment occurs, which includes the separation of solids and the breakdown of some of the organic matter by anaerobic bacteria. The separation of solids and breakdown of organic matter leaves three main layers inside of the tank: sludge on the bottom, liquid effluent in the middle, and scum on the top. The gases that are generated by the breakdown of organic matter are released into the atmosphere through a vent, the sludge that accumulates at the bottom of the tank needs to be pumped periodically, and the liquid effluent from the mid-section of the septic tank then flows, by the force of gravity, to the soil absorption field. The volume of wastewater entering the septic tank controls the volume discharge to the soil absorption field.
Diagram of Septic Tank
The soil absorption field is where secondary treatment occurs, which includes the “removal of pathogens, organic matter, and suspended solids… via physical filtration, biological reduction of contaminants by aerobic microorganisms, and ion bonding to negatively charged clay particles (State of Wisconsin Department of Commerce).” The soil absorption field is made of several perforate pipes in trenches, approximately 2 feet beneath the ground surface, which distribute the liquid evenly over a large area. In order to keep the perforated pipes from getting clogged, gravel or crushed stone is filled in over the pipes. Above the layer of gravel is a soil filter and then top soil. In order for the effluent to receive proper treatment there should be at least 3 feet of suitable soil beneath the trenches before the bedrock or groundwater is reached. In conventional in-ground septic systems the volume of wastewater entering the septic tank controls the volume of discharge to the soil absorption field.
Picture of Fabric Cover and Gravel over Pipes
From Waukesha County Health Department
At-Grade Septic Systems
The components and process of treatment found in at-grade systems is similar to convention in-ground systems, except that at-grade systems also have a pump chamber and pressure distribution system.
They are called at-grade systems because the soil absorption field is placed at the ground surface, instead of approximately 2 feet below as found in conventional in-ground systems. These systems are used when there is less then the minimum 5 feet of suitable native soil present between the ground surface and bedrock or groundwater. The liquid effluent that is discharged from the septic tank then goes to the pump chamber where, in controlled pressurized doses, it is pumped up to the soil absorption field. The perforated pipes and gravel in at-grade systems are covered with sand and soil to protect them from freezing.
In-Ground Pressure Distribution System
An in-ground pressure distribution system is a combination of a conventional in-ground system and an at-grade system. The soil absorption field is laid out the same as a conventional in-ground system, in that it is placed 2 feet below the ground’s surface and must have at least 3 feet of suitable native soil beneath it and either bedrock or groundwater. It is similar to at-grade systems in that it uses a pump chamber to deliver controlled pressurized doses of effluent from the septic tank to the soil absorption field.
The advantage to the in-ground pressure distribution system is that with the controlled doses of effluent release, it provides a certain amount of drying time in-between doses. It is thought that these drying times can result in enhanced treatment in regards to pathogens and nutrient removal.
Diagram of an In-Ground Pressure Distribution System
From State of Wisconsin Department of Commerce