How does a septic system work?

Text by L. Broadfoot

A septic system is an efficient, simple method of disposing of household wastewater. Consisting of two main parts – a holding tank and a drain field – the system uses natural processes to treat and dispose of all wastewater from a home’s kitchen, bathrooms and laundry room.

Water first flows from a home’s sewage line into a septic tank, a large underground watertight container, where the solids and liquids remain long enough to separate into three layers. Grease, soap suds and light solids will float, creating a scum layer on the top. Heavier solids sink to the bottom, forming a layer of sludge, leaving partially clarified liquid in the middle.

Bacteria found naturally in wastewater will gradually decompose most of the solid waste while it remains in the septic tank. This initial treatment of wastewater is an anaerobic process, the decomposition of organic materials without oxygen. Additives are not usually needed to initiate this biological process, and in some cases may disturb the sludge layer and cause clogs further down the system. While most of the solids will be broken down by bacteria, some non-decomposed solids will remain and eventually must be pumped out of the tank.

The size of the tank, the number of people in a household and the volume of solid waste will determine how often the tank should be pumped out. Most will need cleaning every three to five years, but the timing should be confirmed with a certified septic contractor.

Some septic tanks have two compartments, allowing the sludge layer to form with little disturbance from new wastewater entering the tank.

As new wastewater enters the tank at one side it displaces the fluid already there. This partially-clarified water then flows through pipes to the drain field.

The tank outlet may have a filter or baffle system to prevent solids from leaving the tank and clogging the drain field system.

Most septic systems simply use gravity to move the liquid from the tank to the drain field, but some will have an added pump tank for pressure distribution, which powers the liquid to the drain field in cycles.

The clarified liquid flows to the drain field, also called a leach field or an absorption bed, and through a series of perforated pipes or tiles, one to three feet below the soil surface. The fluid then filters through layers of gravel or soil and returns to the natural water system. The type of soil and how well it absorbs water will determine the size of the drain field. If the ground absorbs water slowly the pipes may rest on a bed or in trenches of gravel or sand.

As the fluid is dissipated and leaches through the ground, oxygen helps complete the process of biological decomposition, and the drain field provides the final step in the treatment and disposal of wastewater.

The entire drain field area should be free of deep-rooted plants, and left undisturbed other than a mowed lawn at the surface. Measurements and diagrams of pipe placement will ensure that the area is protected from future development. Some municipalities require a designated replacement area, should the initial drain field need repair or installation of additional pipes.

Unpleasant odors around plumbing fixtures or around a septic tank or slow-running drains may indicate a problem with a septic system. The most common reason for system failure is from a build-up of sludge which clogs either the drain field pipes or the soil itself. Lush green growth or standing water over the leach field, even in dry weather, may indicate the water is not filtering downward fast enough, and may flow, untreated, into surrounding groundwater or wells. Drinking water wells should be tested regularly for nitrates and bacteria.

Normal amounts of household cleansers and detergents won’t affect the biological septic processes, but excessive amounts of chemicals or latex paint shouldn’t be flushed into the septic system. Non-decomposable items should not be added to the system – cigarette butts, paper towels, hygiene products, coffee grounds or grease. The tank will have to be cleaned more often if solid waste is added from a garbage disposal.

If a tank is cleaned regularly, a properly operating septic system won’t require any other maintenance, and should last for twenty to thirty years.

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