Which Septic Tank Material Should You Use?

Each common septic tank material has advantages and disadvantages you should consider carefully before each installation

Which Septic Tank Material Should You Use?

Interested in Septic Tanks?

Get Septic Tanks articles, news and videos right in your inbox! Sign up now.

Septic Tanks + Get Alerts

Historically, many different materials have been used to construct septic tanks. Most commonly, septic tanks are constructed using:

1. Polyethylene/polypropylene resin

2. Fiberglass-reinforced plastic

3. Precast concrete

Precast concrete tanks have traditionally been used for onsite systems. As such, there is an abundance of information available regarding concrete tanks in comparison to tanks made of other materials. Use of tanks fabricated from FRP and polyethylene is becoming more common. Advantages and disadvantages of each tank material are discussed below.

Polyethylene/Polypropylene

Polyethylene/polypropylene “poly" tanks can be rotationally molded in one piece or injection molded in two pieces. Some early poly tanks were prone to deflection and splitting during installation and in service. Newer model tanks have a ribbed/corrugated design to increase structural performance. Use of high-quality raw materials and careful attention to manufacturing practices are essential for the structural soundness and watertightness of any septic tank.

As with tanks made of other materials, access riser joints and pipe penetrations must be properly sealed to make sure they do not leak. Rubber and plastic pipe seals are routinely used in the production of poly tanks, and access risers are typically made of the same poly materials as the tank itself. While the one-piece design implies that it should be watertight, it is imperative that they (and tanks of all other materials) be tested via appropriate methods.

Most local codes have approved poly tanks, and manufacturers specify where and how poly tanks may be used; so be sure to review the strength and other requirements included in the manufacturer’s installation instructions when assessing the use of any tank in onsite systems.

Advantages

  • Ease of installation: Lighter than concrete, poly tanks can be beneficial on limited-access sites
  • Not subject to rust or corrosion and are resistant to the chemicals and gases present in sewage and soil, resulting in longer design life
  • Contractors can deliver themselves, thus not requiring a boom truck or waiting on delivery
  • The design limits seams/joints, which could leak
  • Economical

Disadvantages

  • More prone than concrete tanks to float out of the ground in areas of high water tables due to their light weight
  • Not typically available in larger capacities
  • Typically only available in a small range of sizes
  • Typically not rated for traffic
  • Have limited depth of bury (commonly 4 feet, but check with manufacturer)
  • Some brands must be filled with water/wastewater at all times
  • Have stringent installation requirements to ensure structural integrity

Fiberglass Reinforced Plastic (FRP)

Some FRP tanks are produced in one piece. Others are produced in two pieces using an injection molding process. As stated previously, using high-quality raw materials and assuring careful attention to manufacturing standards are essential for structural soundness and watertightness. It is possible (though not common) for FRP tanks to leak as a result of shipping damage, a substandard batch of adhesive, uneven application of adhesive or stress placed on the midseam during installation. Two-piece FRP tanks are often shipped unassembled and must be permanently fastened together before placement. The assembly process must be carefully executed so the joint will not leak or separate. Generally this is achieved using appropriate adhesives and stainless steel bolts. The bolts are primarily used to hold the halves in place while the adhesive cures. Rubber and plastic pipe seals are routinely used in the production of these tanks.

As with tanks made of other materials, pipe penetrations and access riser joints must be properly sealed to make sure they do not leak. Verification of watertight connections is essential; as with all tanks, if joints are not watertight, then the functionality of the tank is greatly reduced due to increased risk of water infiltrating the tank. Watertightness can be easily verified in the field by filling the tank with water (above the joints) and checking for leaks. 

Advantages

  • Lighter than concrete tanks, which can be beneficial in limited-access sites
  • Not subject to rust or corrosion and are resistant to the chemicals and gases present in sewage and soil
  • Available in larger capacities
  • Can be designed for deeper bury and can be traffic rated

Disadvantages

  • More prone than concrete tanks to float out of the ground in areas of high water tables due to their light weight
  • Have stringent installation requirements to ensure structural integrity
  • Can be less economical compared to concrete and poly tanks
  • Typically only available in a small range of sizes

Concrete

Precast septic tanks are typically manufactured in two pieces, either with a seam at the lid or midseam. Materials used to seal multiple-piece precast tanks typically consist of blended compounds such as butyl rubber-based or asphalt-based (bituminous) sealants. Inlet and outlet pipe penetrations are a potential point of leakage, particularly if the tank or piping settles or shifts as a result of improper bedding or installation. These connections should be mechanically sealed to the tank so that they are watertight and flexible.

Although bituminous seal, mastic or cementitious grout have been used for many years, newer flexible gaskets and boot fittings are available that can be cast in place at the time of tank manufacturing and provide a much more reliable seal. Rubber boot seals are particularly desirable because they are flexible and retain a seal during backfilling and settling.

Precast concrete septic tanks are manufactured in local facilities in a controlled environment. Steel reinforcement is used according to the tank design to provide additional structural capacity during handling, installation, testing and use. Usually precast septic tanks are manufactured in two pieces. The compartment walls are typically cast monolithically with the tank. 

The first section of precast concrete or plastic risers can be cast into the lid or secured and sealed. Preformed flexible joint sealants made of butyl rubber or asphalt-based materials are used to seal horizontal joints. Pipes are connected to the tank and should employ connections that are designed for this application. These connections should consist of cast-in, watertight, flexible resilient connectors to enable the tank and pipe to move without risk of leakage. If not sealed properly using the appropriate material and methods, joints or connections can be prone to leakage. As with other tank materials, it is imperative that the tank be watertight, and in-field verification at the time of installation can be easily performed for verification via appropriate methods. Precast tanks can usually be customized to house additional components such as ATUs or pumps. 

Advantages

  • Greater resistance to buoyancy due to concrete’s density
  • Less stringent installation requirements
  • Available in a wide range of sizes, including very large capacities
  • Can be designed for deeper bury and can be traffic rated
  • Easier to customize
  • Economical

Disadvantages

  • Weight of material and equipment needed for placing can be difficult on sites with limited access
  • Can be subject to corrosion

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to kim.peterson@colepublishing.com.



Discussion

Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.