If you want to learn about septic tanks and septic systems, this one of the best places on the web; let’s get right to it. In general, most septic systems consist of one or more septic tanks (usually 1,000 to 2,000 gallons or 4,000 – 7,500 liters in size) with an inlet wastewater pipe from your plumbing on one end and an outlet pipe leading to the septic drain field on the other end. The connections on both ends are made via a T-pipe, which lets the septic effluent enter and exit without disturbing the crust on the surface of the septic tank contents. Most modern septic tanks have two compartments of the same size; each has its own access point closed with a manhole cover. The two compartments are separated with a dividing wall that has openings half way down the wall that allow fluid to pass from one side to the other while concentrating the solids on the surface and floor of the first compartment.
Aerobic Septic Tank Schematic
When wastewater (including solids) enters the first compartment the solids settle to the bottom and the scum floats to the top. Anaerobic metabolism of the solids occurs in the first compartment, which reduces the volume of the solids (these solids will eventually have to be removed by pumping your septic tank). The water is allowed to flow through the dividing wall into the second compartment where further settling takes place. Following that, the relatively clear septic effluent exits the septic tank and makes its way down to the septic leach field (also known as the drain field or seepage field).
In the septic leach field, the remaining impurities in the septic effluent are filtered by the soil. The remaining water then percolates through the soil and eventually becomes part of the groundwater. Some of the water will be purified through evaporation and plants that will draw the water up through their root system and release pure water to the atmosphere through transpiration. The leach field is a underground piping network that distributes septic effluent throughout the field. The water is able to exit through multiple drainage holes and move into the stone-filled trench (weeping tile) that surrounds the leach field pipes. When gravity pulls the water through the stone-filled trench the next stop is the natural soil for further filtering.
The size of the leach field is inversely proportional to the porosity of the drainage material and proportional the volume of septic effluent arriving at the leach field. In other words, the more porous (the more easily) the water flows through the drainage field, the smaller the field can be and the higher the volume of wastewater the large the field has to be.
If the topography of your lot allows, the entire septic system can be run by gravity. If the topography is a little more challenging then a septic lift pump will be required.
Septic leach fields are prone to clogging; that is when septic tank failure happens. There are septic system accessories including siphons that will increase the velocity of the water arriving at the leach field. This way, all parts of the drainage field are loaded and that will prevent premature clogging and failure.
There exist a multitude of septic tank designs. For example, the Imhoff septic tank uses a two-stage septic treatment system in which the digesting sludge is in a different compartment from the incoming, fresh sludge. Other designs digest the sludge anaerobically in the first compartment and then aerate the septic effluent in the second compartment before it is sent to the leach field.
There will be waste that is not digested and it will remain in the bottom of the septic tank, which has to be removed through septic tank cleaning. Cleaning in this case is another word for pumping and the septic tank pumping cost is usually not that high. For the average household, pumping your septic tank every 1 to 3 years will be sufficient. If you do not have your septic tank cleaned then the undigested sludge will keep rising and eventually make its way to your leach field. The sludge will clog your leach field and cause septic failure. Replacing a clogged leach field can cost up to $50,000; probably not something you want to do.
Although I say 1 to 3 years is a good time between septic cleanings, there really is no solid rule of thumb. If you are using an old septic system and there are a lot family members contributing to the waste water then you will have to have your septic tank pumped more frequently than if you have a new system with few family members. Some people can get away with having their system pumping every 15 or 20 years. It depends on the quality of your system and number of family members.
A well-designed septic system releases no odors and should last decades without the need for regular maintenance besides periodic inspection and pumping.