How a Septic System Works
Private on-site wastewater treatment systems (POWTS) are the common way that household wastes are treated and disposed of in all areas where city sewer is not available. These systems treat most waste through a natural process, the balance of the waste must be periodically pumped out.
There are three main stages in the treatment of household waste. The first is the Septic Tank. Wastewater exits the house via a sewer pipe and enters the septic tank. The septic tank acts as a holding tank allowing the solids to settle-out. The larger solids sink to the bottom where naturally occurring anaerobic bacteria begin breaking them down and destroying pathogens. The lighter solids, like grease, float to the top and form a scum layer. The relatively clear layer in the middle is the effluent.
Next, liquid effluent flows from the tank to the absorption area. The inlet and outlet to the septic tank are covered by baffles which prevent the scum layer from entering the absorption area where it would clog the field. There is also a filter in outlet of the tank that does not allow solids to pass through to the absorption area. A hydraulic pump is sometimes needed if the absorption site is higher than the septic tank, or if an elevated sand mound is used.
Finally the effluent arrives at the absorption field or drainfield where it is distributed to the soil for treatment. There are numerous different types of Materials that can be used to create the drainfield. As the effluent enters the drainfield it percolates into the soil where a large portion of the pathogens are destroyed. As the effluent exits the drainfield microorganisms in the natural soil consume the organic pollutants and complete the treatment process. This percolation process is a natural biological one that can safely treat the liquid before it reaches groundwater, if the system has been properly designed and maintained.
If a soil has shallow bedrock or a high water table, the effluent can reach the groundwater before it is fully treated. To avoid this a Mound System design is used to maintain 3 feet of separation from your septic system to these limiting factors. The system will require a hydraulic pump to lift the effluent up into the mound. Another option where high groundwater, poor soil conditions or lot size limitations exist is a Holding Tank.