A Few Good Websites That Explain How Septic Systems Work
How Stuff Works
"In rural areas where houses are spaced so far apart that a sewer system would be too expensive to install, people install their own, private sewage treatment plants. These are called septic tanks."
"A septic tank is simply a big concrete or steel tank that is buried in the yard. The tank might hold 1,000 gallons (4,000 liters) of water. Wastewater flows into the tank at one end and leaves the tank at the other."
"Anything that floats rises to the top and forms a layer known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the middle is a fairly clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids."
"Wastewater comes into the septic tank from the sewer pipes in the house."
"As new water enters the tank, it displaces the water that's already there. This water flows out of the septic tank and into a drain field. A drain field is made of perforated pipes buried in trenches filled with gravel."
"The water is slowly absorbed and filtered by the ground in the drain field."
Don Vandervort's Home Tips
"A septic tank separates and processes wastes. From the waste water that flows into the tank, heavy solids settle to the bottom, forming a layer of sludge. Greases, oils, and lighter solids rise to the top, creating a layer of scum. The area between these two layers is filled with liquid effluent that can flow through the outlet pipe to the drainfield system."
"Inside the tank, anaerobic and facultative micro-organisms feed on the solids in the sludge and scum, breaking down their volume."
"Because only a portion of the sludge and scum are broken down (about 40 percent), a septic tank must be pumped periodically. Otherwise, accumulated solids fill the tank and are forced out into the drainfield, where they hinder the soil's ability to percolate."
Virginia Department of Health
This page has an interesting visual showing the growth of scum (greases) and sludge (solids) in a septic tank if left untreated over a period of seven years.
"The septic tank appears simple at first glance but in actuality provides a simple and elegant means of achieving several complex actions."
"Solid matter entering the tank slowly settles to the bottom of the tank if it's heavier than water, or floats to the top if it's lighter than water. Fats and greases will tend to float. Some solids create problems. Vegetable material contains cellulose and is not readily digested by the bacteria in the septic tank. This causes solids to accumulate more rapidly. For this reason, garbage grinders are not recommended for homes having onsite sewage systems. Excessive hair is a potentially problematic solid. Hair is generally neutral in buoyancy and neither floats or settles and presents clogging problems when present in large amounts, while most of the other solid matter will settle. Anaerobic (without oxygen) digestion takes place and most of the solids are converted to carbon dioxide, water, and other byproducts. The process is not completely efficient and solids will accumulate over time."
Maryland Cooperative Extension – University of Maryland
"A septic system has two major components: a septic tank and a drainfield."
"Septic Tank: Waste water flows from the house to the septic tank. The tank is designed to retain waste water and allow heavy solids to settle to the bottom. These solids are partially decomposed by bacteria to form sludge. Grease and light particles float, forming a layer of scum on top of the waste water."
"Drainfield (Trench): A solid pipe leads from the septic tank to a distribution box where the waste water is channeled into one or more perforated pipes set in trenches of gravel. Here the water slowly infiltrates (seeps) into the underlying soil."
"We are changing the world, one septic system at a time."