What is a Plastic Septic Tank?

Robyn Clark

When designing a septic system, one of the most common questions asked is whether it should be concrete or plastic. Septic tanks, which store solid waste, are available in concrete, steel, fiberglass, and plastic. The bestselling models are those made of concrete or plastic. Of all of these options, a plastic septic tank is typically the least expensive and the easiest to install. Despite the advantages offered by plastic designs, the use of plastic septic tanks is prohibited in some regions due to the potential for leakage and other complications if the tank is not installed and maintained properly.

Plastic septic tanks generally do not have the problems with corrosion that are common with steel tanks.
Plastic septic tanks generally do not have the problems with corrosion that are common with steel tanks.

Plastic is not chemically reactive, and a plastic septic tank will not have the problems with corrosion that are common with steel tanks. Unlike concrete, plastic is not a porous material. Plastic tanks are not prone to damage caused by infiltrating tree roots, a major problem associated with concrete tanks. When properly installed, a plastic septic tank is watertight and better able to withstand extremes in temperatures. A septic system with a plastic tank is less likely to exhibit odor problems in hot weather, and is less likely to crack when soil temperatures drop below freezing.

The difference in weight as compared to concrete can be both an advantage and a disadvantage for a plastic septic tank. These tanks are so lightweight that they can be installed without the use of a crane or other heavy-duty equipment, but they are also more easily cracked or damaged during the installation process. Damage to the tank or to connecting pipes can also be caused by the plastic tank shifting in position or floating upward, which can occur if waste levels in the tank are very low or if the surrounding soil is inundated with storm water. There are models of plastic tanks available that feature a reinforcing ribbing structure to make the tank more durable. Most manufacturers will offer some type of warranty against damage under normal use conditions.

Comparisons of the performance of concrete and plastic tanks indicate that plastic tanks tend to have lower effluent levels under similar usage conditions. These lower levels are typically the result of waste leaking from cracks in the tank. Improperly performing routine maintenance procedures can also result in leakage due to a damaged or missing drain plug. In addition to having lower average effluent levels, plastic tanks are typically manufactured to have a smaller holding capacity than concrete tanks. For these reasons, many designers prefer using concrete tanks for high-volume commercial installations.

Properly designing and installing an effective and efficient septic system can be a challenge. Site-specific conditions, such as soil types and drainage patterns, will need to be considered when selecting the type of tank best suited for a particular application. Local ordinances may restrict or prohibit the installation of a plastic septic tank. Homeowners should consult with a licensed professional to determine which type of tank matches the design criteria for the proposed wastewater treatment system.

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