Polyethylene (Plastic) Septic Tank

About Polyethylene (Plastic) Septic Tanks

Polyethylene septic tanks are manufactured using a rotational molding process where polyethylene resin is loaded into a hollow mold that is heated and rotated on multiple axes, causing the melted plastic to coat the interior of the mold uniformly. This produces a seamless, one-piece tank with consistent wall thickness and no joints or seams that could leak. The polyethylene used is a high-density grade specifically formulated for underground structural applications, with UV stabilizers for any exposed portions and chemical resistance for wastewater contact. Rotational molding allows complex shapes including integral baffles, access risers, and structural ribbing to be formed as part of the tank in a single manufacturing step. Polyethylene tanks are the lightest option available, making them popular for sites with difficult access.

Available Sizes and Costs

The table below shows the available sizes for polyethylene (plastic) septic tanks with estimated purchase costs and annualized costs based on the expected 25-year lifespan. The annualized cost divides the purchase price by the expected service life to show the true yearly cost of ownership, which is the most meaningful way to compare tank materials of different lifespans. Delivery, excavation, and installation labor are not included in these prices and typically add $1,000 to $3,000 to the total installed cost.

Advantages

• Most affordable lightweight option with good balance of cost and durability
• Completely waterproof one-piece rotational molded construction with no seams or joints
• Resistant to all forms of chemical corrosion from wastewater and soil contact
• Lightweight enough for two-person manual installation without heavy machinery
• Flexible material absorbs ground movement without cracking in shifting soils
• Available at most home improvement and plumbing supply stores for easy procurement

Disadvantages

• Shorter lifespan of 20 to 30 years compared to concrete at 40 plus years
• Very susceptible to flotation in high water table conditions requiring concrete ballast
• Can be crushed by heavy soil loads or vehicle traffic if not properly backfilled
• Not available in sizes above 1500 gallons limiting use for larger homes
• Flexible walls can deform under soil pressure if backfill is not compacted properly
• May not be approved in all jurisdictions as some states require concrete or fiberglass only

Installation Considerations

Polyethylene tanks are the lightest and easiest to install, often requiring only two workers. However, their flexibility demands careful attention to backfill quality and compaction. Use only clean sand or pea gravel as backfill, never native soil with rocks that could puncture the wall. Fill the tank with water as backfill progresses to equalize internal and external pressure. In high water table areas, concrete ballast is mandatory to prevent flotation. Never drive heavy equipment over an installed polyethylene tank as the walls cannot support concentrated point loads.

Regardless of tank material, all installations must comply with state and local code requirements for setback distances from wells, property lines, buildings, and water bodies. A licensed installer familiar with local requirements should perform or supervise all tank placement and connection work. Post-installation inspection by the permitting authority is required before the tank can be backfilled and put into service. Keep accurate records of the tank location, size, material, and installation date for future maintenance reference and potential property sale disclosure requirements.

Frequently Asked Questions

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