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How a Septic System Works

When it doesn't make sense to build an entire sewage collection and treatment system, septic systems are the go-to option. Simple but effective, these combinations of tank and soil remove solids from waste and send wastewater into the ground, where filtration, microbial activity and time eventually render clean water. It all seems a bit miraculous, given what we're working with. But for all the impressive work septic systems do, few people - even some of their users - really know how they work.

The tank is the first (and sometimes only) thing that comes to mind when we mention septic systems. Made of concrete, plastic, fiberglass or steel, they're the first and most important step in the treatment process. Water and waste from every drain in the home finds new residence inside the tank, where it remains until further notice: water separates and is eventually displaced into a drain field (also called leach field), while solid material (sludge) accumulates in the bottom of the tank until it's pumped out. An equally gross layer of scum - mostly composed of fats, oils and grease - floats atop it all.

Septic tanks need to have all that sludge pumped out regularly - should it be allowed to build up, the problems are numerous, disgusting, and potentially catastrophic for the entire system. How often this needs to happen varies, so it's always best to have annual inspections performed by a professional to determine when the next pumping should be, and to gauge the overall health of your system.

Most tanks these days utilize multiple chambers and baffles to maximize holding time in the tank. This gives solid waste more time to separate and bacteria more time to digest it, limiting the amount that ends up in the drain field (where an accumulation can lead to clogs and system failure). Incoming waste is directed downward, minimizing the disruption to the rest of the tank contents. A wall baffle between chambers helps keep most of the sludge out of the final chamber, whose contents are first into the drain field.

There are tons of additives out there, but in a properly functioning system, all the bacteria that's needed to process waste is already there. Bacteria that aid our own digestion travel with waste into the tank, where they feed and reproduce. With each flush of solid waste, new colonies are added!

A series of gravel-filled trenches and perforated pipes buried underground make up the drain field, the other half of a septic system. Once displaced by fresh waste, clarified wastewater from the tank makes its way into these perforated pipes, either by gravity or assisted by pump. Distribution boxes can help evenly distribute the water about the field. In the ground, as in the tank, bacteria are the real stars. As the semi-treated wastewater trickles out of the pipes, through the gravel, and into the soil, bacteria (and some other organisms) digest and convert the nasty stuff, leaving clean water to trickle into groundwater reservoirs.

Extremely permeable, impermeable, or shallow soil? High water table? Just because a traditional drain field won't work doesn't mean you're out of luck. In a mound system, the flat field is replaced by a mound that serves the same purpose. Pumps are used to move water from the tank to pipes inside the mound.

Septic systems go by a few different names - onsite wastewater treatment systems, onsite sewage systems - but however they're referred to, they're a lifesaver for those in remote, rural areas and other locations where sewer systems can't be constructed. But because they're onsite, there is a greater degree of responsibility on the part of the homeowner to treat them right. In Maintaining Your Septic System, we highlight the most important tips for septic system care, as well as the habits that need to be formed and broken to keep things running right.


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