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How Does a Septic System Work?

A septic system is an alternative to a municipal sewer system. Homes that are not connected to a municipal sewer must instead use an underground septic system to handle the sewage and waste water from a home.

Septic systems are made from two main parts, the septic tank and the leaching or drain field. The waste water and solids from toilets, sinks, showers, laundry and kitchen are all carried away from the home and deposited into a septic tank.

A septic tank can be an actual metal, plastic, fiberglass or precast concrete tank, or it can be constructed on site with concrete or other materials. The tank has an inlet near the top, to which the home's sewer line connects. The tank also has an outlet that connects to the drain field. Additionally, a septic tank has one or more manholes and view ports for service and inspection.

As waste water flows into the tank, the solids sink to the bottom. Three distinct layers develop in a properly maintained septic system. At the bottom is the sludge, clear water and at the top, a scum layer. Anaerobic microorganisms digest the waste and break it down into tiny organic particles which can then be carried away with the clear water. These organisms do not require oxygen and thus can function effectively in the oxygen starved water of a septic tank. Some disease causing germs or bacteria which are associated with sewage, are actually inhibited by the lack of oxygen in the water.

The water level in the tank rises as water from showers, laundry and regular household activities flows out from the house. As the solids are broken down, they mix with the clear water. When the water level reaches the outlet, water flows into the drain field.

The drain field is typically an array of buried pipes with holes along their length. The pipes are surrounded by gravel or similar permeable material to allow the free flow of water. A second stage of water treatment occurs as, this time, aerobic organisms digest the remaining organic material and neutralize the remaining disease causing germs. The water seeps out of the drain lines and into the ground where natural filtration also occurs in the soil.

For a septic system to work, several things must happen. First, the septic tank must be large enough to handle the effluent delivered daily. If more gallons of water are delivered over a period of time than the system can dissipate, the sewage may overflow the tank and flow into the drain field. This can lead to solids clogging the drain field, which then leads to even less waste water being dissipated, exacerbating the problem. A septic tank should have a minimum of 250 gallons of capacity per person for each of the members of the household.

Secondly, the system relies on active colonies of both aerobic and anaerobic organisms. Use of drain clearing chemicals and bleach both have harmful effects on these helpful bacteria. If too much chemical waste enters the septic system it can damage the colonies which then slows down the waste water treatment process. If the colonies are completely destroyed, the septic system will fail until new colonies can be established.

The area of the drain field must have permeable soil and a supply of oxygen. Paving over the field will reduce the oxygen levels in the soil and thus it will not support a large colony of useful bacteria and reduce the effectiveness of the system. Allowing vehicles to drive or park over the field will result in soil compaction which reduces the ability of the soil to take up oxygen or to dissipate water.

Finally, inorganic solids such as plastic and rubber must not be flushed into the septic system because they can lead to blockages. Just like with excessive water flow into the system, inorganic waster can clog the drain field and then reduce the effectiveness of the entire system. Never flush trash or anything that will not dissolve or break down in water.

Some of the solid material in the tank cannot be digested by the bacteria. Over time this layer of undigestable material will build up and must be pumped out. Failure to periodically pump out the septic tank will result in reduced handling capacity, which then leads to increasingly frequent overflows and back ups in the home.

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