The Anatomy Of A Septic Tank

By Mark J. Donovan

June 20, 2012 5 min read

If you own a home and are not on a public sewage system, chances are you have a private septic system. A septic system is composed of a septic tank and a leaching field that are connected together via large pipes.

Septic systems have to be used and maintained properly in order to prevent major repair costs. The installation of a new leach field, for example, could cost upward of $25,000. Consequently, it is important to understand exactly how a properly maintained septic tank and leach field work to ensure that you do not inadvertently abuse them.

In such a setup, wastewater exits the home and first enters a septic tank, where the solids are allowed time to settle to the bottom of the tank. The septic tank is a large, watertight, rectangular concrete structure that is buried into the ground relatively near the home. It is meant mainly for containing waste solids and somewhat clarifying wastewater prior to sending it on to the leach field for further cleansing.

In order to separate the waste solids and liquids, a septic tank consists of two or more chambers. Wastewater that enters the septic tank first flows into the large chamber, which is twice the size of the other chambers in the septic tank. This is where most of the waste solids, or sludge, settle -- because there are no currents flowing in the septic tank.

While the sludge sits in the bottom of the septic tank, anaerobic bacteria digest and decomposes the sludge. In the process, the bacteria produce methane and carbon dioxide, which need to be vented from the tank. Sometimes you may notice a small PVC pipe near the septic tank. This pipe is for venting the gases. Similarly, you may also notice a couple of larger "candy cane" PVC pipes over the leaching field. Again, they are to vent off methane gas.

As the bacteria breaks down the sludge, it eventually becomes stabilized and ceases to decay any further. At that point, some of the sludge may begin to float and form a scum at the top of the septic tank.

In the second chamber, the wastewater is clarified, or purified, further. This water is referred to as graywater. The graywater then exits the septic the tank to the leaching field for final purification. The leaching field is a network of drainage pipes that are sandwiched between crushed stone and covered by topsoil. If needed, graywater can be pumped to the leaching field from a third chamber. Wastewater entering the septic tank can either be gravity-fed or pumped up from the home. It all depends upon the height of the septic tank inlet waste-pipe hole, relative to the height of the waste pipe exiting the home. A gravity-fed septic tank is preferred whenever possible. It eliminates the need for any type of pump, which could be susceptible to failure. Many a home has had its waste pump fail, causing wastewater to back up into the home. It's not pretty.

A septic tank is designed to maximize the time that bacteria and microorganisms are in contact with the sludge to enable faster digestion of it. Thus, inlet and outlet pipes are placed diagonal to one another. Also, the water is made to flow from one chamber to the other in vertically positioned pipes. This ensures that solids and larger particles are not transferred from one chamber to the other.

The digestion and settling of solid organic matter continues to occur in the second chamber -- but the wastewater is in this chamber for approximately half the time of the first chamber, mainly because the second chamber is half the size of the first.

The frequency of emptying solids from a septic tank varies with the number of occupants in the home and the size of the tank. The rule of thumb is that 0.05 cubic yards of sludge are produce per person, every year. Consequently, for the average family of four, plan to have the septic tank pumped every three to four years. You may be able to go one or two years longer, but you begin to risk the chance of sludge draining into the leach field. Again, replacing a leach field is a very expensive proposition, so it is wise not to wait too long to pump your septic tank.

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