Gravity Septic System

Professional Gravity Septic System

A conventional gravity flow septic system has three working parts:

  1. The septic tank.
  2. The drainfield with its replacement area.
  3. The surrounding soil.

Gravity Septic Tank System Installation - Step 1 — Aberdeen, WA — Stangland Septic Service

The Septic Tank

Gravity Septic Tank System Installation - Step 2 — Aberdeen, WA — Stangland Septic Service

The typical septic tank is a large buried rectangular or cylindrical container made of concrete, fiberglass or polyethylene. Wastewater from your toilet, bath, kitchen, laundry, etc. flows into the tank. Heavy solids settle to the bottom where bacterial action partially decomposes them to digested sludge and gases. Most of the lighter solids, such as fats and grease, rise to the top and form a scum layer.


Septic tanks may have one or two compartments. Two compartment tanks do a better job of settling solids and are required for new systems. Tees or baffles are provided at the tank's inlet and outlet pipes. The inlet tee slows the incoming wastes and reduces the disturbance of the settled sludge. The outlet tee keeps the solids or scum in the tank. All tanks should have accessible covers for checking the condition of the baffles and for pumping both compartments. If risers extend from the tank to or above the ground surface, they should be secure to prevent accidental entry into the tank.


Solids that are not decomposed remain in the septic tank. If not removed by periodic pumping, solids will accumulate until they eventually overflow into the drainfield. Most septic tanks need to be pumped every 3 to 5 years, depending on the tank size, and the amount and type of solids entering the tank.

"Early Warning" Levels Inside Your Septic Tank

Septic Layers - Gravity Septic Tank System Installation - Step 3 — Aberdeen, WA — Stangland Septic Service

The septic tank should be pumped whenever:

  • The bottom of the scum layer is within 3 inches of the bottom of the outlet tee or baffle.
  • The top of the sludge layer is within 12 inches of the bottom of the outlet fitting.

Some septic tank additives on the market with chemicals, yeast, bacteria, or enzymes claim to improve septic tank performance or reduce the need for routine pumping. Such products are not necessary for the proper functioning of a septic tank.


Some can cause solids to carry over to the drainfield, which results in early soil clogging and the need for a new drainfield. Products containing organic solvents contribute to groundwater pollution.


The wastewater leaving the septic tank is a liquid called effluent. It has been partially treated but still contains disease causing bacteria and other pollutants. Discharging effluent onto the ground's surface or into surface and ground water is against Washington State law.

The Drainfield

The Drain-field - Gravity Septic Tank System Installation - Step 4 — Aberdeen, WA — Stangland Septic Service

The drainfield receives septic tank effluent. It has a network of perforated pipes laid in gravel filled trenches (2-3 feet wide) or beds (up to 10 feet wide) in the soil. Wastewater trickles out of the pipes, through the gravel layer, and into the soil. The size and type of drainfield depends on the estimated daily wastewater flow and soil conditions. Every new drainfield is required to have a designated replacement area. It must be maintained should the existing system need an addition or repair.

The Soil

Soil Process - Gravity Septic Tank System Installation - Step 5 — Aberdeen, WA — Stangland Septic Service

The soil below the drainfield provides the final treatment and disposal of the septic tank effluent. After the effluent has passed into the soil, most of it percolates downward and outward, eventually entering the groundwater. A small percentage is taken up by plants through their roots, or evaporates from the soil. The soil filters effluent as it passes through the pore spaces. Chemical and biological processes treat the effluent before it reaches groundwater, or a restrictive layer, such as hardpan, bedrock, or a clay soils. These processes work best where the soil is somewhat dry, permeable, and contains plenty of air for several feet below the drainfield.
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