Septic Systems 101: The Septic Tank

A septic tank is a large watertight container that begins the treatment process for wastewater. While most tanks are made from concrete, there are lightweight versions made of plastic or fiberglass. Most plastic or fiberglass tanks are used when a truck cannot deliver a concrete tank. Most residential homes with a septic system have a tank ranging in capacity size from 500 to 1,500 gallons. Capacity sizes are increased with the number of bedrooms in a home. For example, a 3-bedroom home will most likely have a 1,000-gallon septic tank.

All liquids, solids, and organic materials that go down a household’s drains (from sinks, laundry machines, showers, toilets, etc.) flows first into the septic tank through plumbing connected to the tank. After the liquids, solids, and organic material settle into the tank, the wastewater, or effluent, is separated and filtered out into the drainfield to be treated. This process is done naturally as the liquids, solids, and organic materials settle into three layers.

The floating top layer is called the scum layer. The substances that make up this layer are lighter than water, such as oil, grease, and fats. The bottom layer is called the sludge layer and is made up of dense, solid materials that will sink such as tissue, soil, and unconsumed food particles. The sludge layer is eaten away by underwater aerobic bacteria, which eventually die off, create gases and then become part of this bottom layer. It’s crucial the tank be pumped out every 3-5 years as it builds up over time leaving less room for the watery middle layer and causing serious issues in the drainfield or the home.

The middle layer is called the effluent layer. Effluent is the clarified wastewater left over after the separation of scum and sludge. The effluent is carried from the septic tank to the first distribution box and out to the drainfield by an outlet baffle. The outlet pipe has a T shape turned on its side with one spout leading out of the tank. The side of the outlet tee facing north is capped so the scum layer cannot enter while the south facing side leads effluent out of the tank. Again it’s crucial the sludge layer not build up enough to be able to exit the tank through the outlet tee, which is why routine pumping is necessary.

Some traditional septic tanks have more than one compartment. Two compartment tanks work the same way as a one-compartment tank, described above, would work with the exception they have an additional outlet baffle in between the compartments. This second compartment allows for an additional catch basin to separate the solids and scum from the effluent. Most two-compartment tanks are installed when the state mandates it or the homeowners see value in having an extra filtration step. However, some wastewater engineers believe a one-compartment tank performs better than a two-compartment tank.