HOW A SEPTIC SYSTEM WORKS
So, you need to learn about a septic system! If you read some of the information at this site, you will find that a septic system is an interesting device. Why? Because it is a simple technology that effectively protects human health and the environment from the effects of human domestic wastewater. However, though a simple technology, the processes that treat the wastewater are quite complex and most people do not really know how they work. This article is intended to take some of the mystery out of the thing we call a septic system.
A typical septic system contains three major components:
- A septic tank
- An effluent distribution system
- An absorption field
When a person flushes the toilet or empties a bathtub or washing machine, the wastewater follows the plumbing, usually by gravity, to the septic tank. At the tank, the solids settle out and the liquid stays long enough to become fermented and to become enriched with beneficial bacteria.
The septic tank is usually made of concrete, fiberglass, or plastic, is typically buried and should be watertight. All septic tanks have baffles (or tees) at the inlet and outlet to insure proper flow patterns. Most septic tanks are single compartment; however, some people install two-compartment tanks or two single compartment tanks in series. While typically designed to hold a minimum of 1000 or more gallons of sewage, the size of the tank may vary depending upon the number of bedrooms in the home and state and local regulatory requirements. The primary purpose of the septic tank is to separate the solids from the liquids and to promote partial breakdown of contaminants by microorganisms naturally present in the wastewater.
The solids, known as sludge, collect on the bottom of the tank, while the scum floats on the top of the liquid. The sludge and scum remain in the tank and should be pumped out periodically. Solids that are allowed to pass from the septic tank may clog the absorption field. Keeping solids out of the absorption field not only prevents clogging, but also reduces potentially expensive repair or replacement costs and helps ensure the ability of the soil to effectively treat the septic tank effluent. Therefore, an additional safeguard in keeping solids out of the absorption field is the use of effluent filters on the outlet of the septic tank. The wastewater (effluent) coming out of the septic tank may contain many potentially disease-causing microorganisms and other pollutants such as nitrates, phosphates, and chlorides.
After the effluent leaves the septic tank, it is transported either by gravity or by pumps to the distribution box and laterals. The distribution box is included as part of the system to separate the septic tank effluent evenly into a network of distribution lines that make up the absorption field.
The main pipe from the septic tank leads to the distribution box or “D-Box” from which an equal amount of effluent is channeled to each of the laterals. The laterals are located underground and become part of the zone of treatment and zone of disposal. The zone of disposal is illustrated in the figure below and works as follows. The effluent is distributed through the perforated pipes, exits through the holes in the pipes, and trickles through the rock or gravel where it is stored until absorbed by the soil. The zone of treatment, which is located in the unsaturated zone of the soil, treats the wastewater through physical, chemical, and biological processes.
The soil also acts as a natural buffer to filter out many of the harmful bacteria, viruses, and excessive nutrients, effectively treating the wastewater as it passes through the unsaturated zone before it reaches the groundwater. This treatment primarily occurs at the top of the zone of treatment, where a Biomat develops, consisting of living beneficial bacteria, organic matter, and mineral precipitates. The Biomat provides a substrate for decomposition of the “bad” bacteria. The “clean” wastewater enters the ground water again in the “Zone of Disposal”, which is typically permeable soil or rock material that is above the water table.
If the zone of treatment has adequate oxygen, which occurs when it is separated from the water table by at least 2 to 4 feet, it effectively converts ammonia nitrogen to nitrate nitrogen, and it reduces the number of harmful bacteria and viruses to levels that are safe for humans.
Even after treatment, wastewater still contains nutrients, such as nitrates and phosphates that in excessive amounts may pollute nearby waterways and groundwater supplies. Excessive nutrients in drinking water supplies can be harmful to human health and can degrade lakes and streams by enhancing weed growth and algal blooms. Some of the nutrients are retained or become assimilated by plants and microbes, but much of the nitrate nitrogen and some of the phosphates still discharge to the ground water, and may enter streams and can cause or contribute to the eutrophication. Therefore, though generally safe for humans, the conventional septic system is responsible for a certain amount of water pollution even when the system is working perfectly. Requiring distance setbacks from streams and potable wells provides the final level of protection. With the setbacks in place, and as long as the septic systems are not malfunctioning, homeowners can be assured that both drinking water and surface water are adequately protected.