How Exactly do Septic Tanks Work?

The Anatomy of a Septic System

By Mark J. Donovan

For those of us who own a home and are not on a public sewage system we have private septic systems. A septic system comprises of a septic tank and a leaching field that are interconnected together via large pipes. It is important that a septic system be used and maintained properly in order to prevent major repair costs. The installation of a new leach field, for example, could cost upwards of $25,000. Consequently it is important to understand exactly how a septic tank and leach field work to ensure that you do not inadvertently abuse it and maintain it properly.

Summarized below is an explanation of how waste water is processed in your septic system after it exits your home.

Waste water exiting the home first enters a septic tank where the solids in the waste water are allowed time to settle to the bottom of the tank. The septic tank is a large, water tight, rectangular concrete tank that is buried into the ground relatively near the home. It is meant mainly for containing waste solids and somewhat clarifying waste water 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 2 or more chambers. Waste water entering the septic tank, first flows into the large chamber. This chamber is twice the size of the other chambers in the septic tank. It is this chamber where most of the waste solids, or sludge, settle. The sludge quickly settles due to the fact that there are no currents flowing in the septic tank.

While the sludge sits in the bottom of the septic tank anaerobic bacteria digests and decomposes the sludge. In the process, the bacteria produces methane and carbon dioxide that needs to be vented from the tank.

The Anatomy of a Septic System. Here is the lid of a septic tank.

Sometimes you may notice a small PVC pipe near the septic tank. This pipe is for venting the gases from the tank. 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 waste water is clarified, or purified, further. This water is then referred to as grey-water. The grey water then exits the septic the tank to the leach field for final purification.

The leaching field is a network of drainage pipes that are sandwiched between crushed stone, and covered by topsoil. If the grey-water in the septic tank needs to be pumped up to the leach field, the grey-water may enter a third chamber where it is then pumped-up to the leaching field.

Waste water 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. It is preferable that a septic tank be gravity fed whenever possible. This eliminates the need for any type of pump and related concerns of the pump failing. There has been many a home that has had its waste pump fail and waste water 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 vertical positioned pipes. This enables semi-clarified water to flow from one chamber to the other such 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, however the time the waste water is in this second chamber is approximately half the time of the first chamber, due mainly to the fact that the second chamber is half the size of the first.

The grey-water then exits the septic tank where it goes on to the leach field for final cleansing. If a pump is required to transfer the grey-water to the leach field, the grey water enters a third chamber where the pump is located for pumping the waste water up to the leach field.

The frequency of emptying the solids from a septic tank varies with the number of occupants in the home and the size of the septic 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 4, plan to have the septic tank pumped every 3 to 4 years. You may be able to go one or two years longer, however 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 so long to pump your septic tank.

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