How It Works

What is a Septic System?

A septic system is an on-site wastewater treatment system that processes and purifies household waste (effluent).  The effluent consists of blackwater (toilet wastes) and greywater (kitchen sink, bathtub, and laundry wastes).

A septic system is comprised of two components; a septic tank and a drainfield.  Primary treatment occurs in the septic tank.  The septic tank is a watertight underground container where bacteria digest organic materials in the wastewater and the remaining inorganic materials are simply collected.  As wastewater flows into the septic tank, heavy solids settle to the bottom into a sludge layer, while grease and fats float to the top forming a layer of scum.  Between these two layers is a clear zone of liquid (effluent) called the clarified zone.  The effluent then flows into the drainfield for secondary treatment.  Here, bacteria, enzymes and other microorganisms complete the digestion and purification process as the wastewater slowly leaches or infiltrates back into the native soil and then eventually reaching an aquifer as clean water fit for human consumption.  The soil acts as a biological filter removing harmful substances before the effluent reaches the groundwater.

The amount or depth of soil necessary to complete this purification process varies by state, and in some cases, county regulations.  The State of Wisconsin requires a minimum suitable soil depth of 36 inches beneath any type of soil absorption system.  This is the most significant factor in determining what type of septic system is going to be necessary for a particular site.  Other site factors do come into play, but do they generally do not carry the significance of the suitable soil depth available.

Quick Description of the Common Various Types of Septic Systems

Conventional System. A conventional septic system is an in-ground soil absorption system that requires a minimum suitable soil depth of 48 inches.  The wastewater (effluent) leaving the septic tank can be gravity fed or pumped to this type of absorption system.  The conventional type septic systems are non-pressurized and typically consist of washed aggregate (stone) with 4-inch perforated pipe or engineered aggregate with 4-inch perforated pipe (EZ-Flow) or absorption chambers.

In-Ground Pressure System. An in-ground pressure septic system is an in-ground soil absorption system that requires a minimum suitable soil depth of 42 inches.  The wastewater (effluent) leaving the septic tank is pumped from a pump chamber to this type of absorption system.  The in-ground pressure septic systems are pressurized and typically consist of smaller diameter perforated pipe (1-inch to 2-inch) with either washed aggregate (stone) or engineered stone (EZ-Flow) or absorption chambers.  This type of absorption system does require about 12 inches of additional soil cover to be placed over the top of it for insulating purposes.

At-Grade System. An at-grade septic system is an above-ground soil absorption system that requires a minimum suitable soil depth of 36 inches.  The wastewater (effluent) leaving the septic tank is pumped from a pump chamber to this type of soil absorption system.  The at-grade septic systems are pressurized and typically consist of a long and narrow washed aggregate (stone) layer placed over an area of chisel-plowed native topsoil.  Typically, a single smaller diameter perforated pipe (1-inch to 2-inch) is placed on top of the layer of aggregate along the upslope side of the aggregate.  The pipe is then covered with another thin layer of aggregate and then a filter fabric.  Additional soil materials are placed over the absorption system (typically 12 inches) for insulating purposes.  When completed, the at-grade system will stand about 2 feet above the existing grade.

Mound System. A mound septic system is an above-ground soil absorption system that basically requires a minimum suitable soil depth of 6 inches.  The wastewater (effluent) leaving the septic tank is pumped from a pump chamber to this type of soil absorption system.  The mound systems are pressurized and typically consist of a layer of ASTM C-33 sand placed over an area of chisel-plowed native topsoil with a bed or multiple beds of washed aggregate (stone) or engineered aggregate or absorption chambers placed above the sand.  Smaller diameter perforated pipe (1-inch to 2-inch) are placed in these beds of aggregate or the absorption chambers.  Filter fabric is placed over the beds of aggregate and then additional soil materials are placed over the entire absorption system (typically 12 inches).  The depth of the layer of mound sand is determined by the amount of suitable soil depth.  To create the required 36 inches of suitable soil depth, ASTM C-33 sand, to a depth of 36 inches minus the depth of suitable soil on-site, must be placed over the chisel-plowed suitable soil on-site.  When completed, the mound system will stand anywhere from 30 inches to 48 inches above the existing grade.

Holding Tank. A holding tank system is basically a larger buried tank that is used to collect all the wastewater that is produced by the residence.  When this tank is nearly full, an alarm will notify the residents that they will need to contact a qualified holding tank service provider (pumper) to empty the holding tank and dispose of the wastewater at an appropriate treatment site.  The minimum required size of the holding tank is determined by the estimated wastewater flow that the residence will produce.  The minimum size holding tank per Wisconsin state code is 2000 gallons.  Residential holding tanks generally range in size from 2000 gallons to 5000 gallons.