Septic Basics

Anatomy of on-site sewage system (OSS)

There are many different types of on-site sewage systems (OSS), but the most common is the gravity-flow system. Understanding the anatomy of your system can help you to identify and care for problems better if they arise. THis type consists of the septic tank, the drainfield with a replacement area, and the soil. THe following information will outline the various parts and how they operate. This information has been adapted from the washington State Department of Health Understanding and Caring For Your Septic Tank System.

The Septic Tank

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, ect. 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 by the State of Washington 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.

Soils 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.

The Drainfield

The drainfield receives septic tank effluent. It has a network of perforated pipes ( sometimes referred to as laterals) laid in gravel-filled trenches (2-3 feet wide) or beds ( up to 10 feet wide) in the soil. Wastemaster trickles out of the pipes, through the gravel layer, and into the soil. 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

The soil below the drainfield provides the final treatment and disposal of the septic tank effluent. After the effluent has passes 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. The natural filtering, biological and chemical processes of the soil assist in further purifying the astute before it reaches the ground water 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.