How to Determine the Structural Integrity of a Septic Tank

Jim Anderson
Jim Anderson

One concern service providers have when troubleshooting a system is whether the septic tank or holding tanks are structurally sound and in good condition. Investigation of the tank for structural soundness starts as soon as the lid is removed. If the lid and tank are concrete the bottom of the lid should be evaluated. If the concrete shows signs of corrosion and flaking, the service provider should be alert for other corrosion problems. For a plastic tank, if the lid or access hole is misshapen it is an indicator that the tank has been structurally deformed. 

The top of a concrete tank should be evaluated for corrosion and cracks. If the lid and top are corroded and if there is any rebar showing (a telltale sign of corrosion is rusting rebar), the tank top and lid should be replaced. The rest of the tank’s appearance will determine whether the entire structure needs to be replaced or only the lids. Be careful if the access lids are square; these lids can fall through the access hole into the tank. 

When the contents of the tank are evaluated and the liquid level is below the invert of the tank outlet, the tank is not watertight and effluent is moving out of the tank into the soil. When the contents are pumped out the service provider should look for evidence of cracks or other holes. Cracks typically show as dark areas on the tank walls. If there are not obvious cracks on the sidewalls, another possible hole is at the bottom of the tank. Often when tanks are stored outside after construction, manufacturers place a hole in the bottom to allow water to drain. Sometimes during installation the contractor forgets to seal the hole. If the tank is cracked or if there are holes, an evaluation needs to be made as to whether it is realistic to patch the tank or to abandon or remove the tank and replace it. The answer depends on the entire condition of the tank, when the tank was originally installed and where the cracks occur. 

With the number of houses in the last few years that have been vacant for longer than four weeks, the next question is whether the level in the tank indicates evaporation. A drop of more than two inches of the scum layer indicates the tank is leaking and there are cracks or holes. 

For plastic tanks, if the tank walls are deformed it indicates that the tank is not structurally sound. Plastic tanks gain their strength through their particular structure so if they are warped it indicates a structural problem and the tank needs to be replaced. When the tank is replaced the installer needs to follow the manufacturer’s recommendations for anchoring and backfilling to avoid the same problem in the replacement tank.

About the Author: Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil Water and Climate. He works with the National Association of Wastewater Technicians and is a recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Send Anderson questions about septic system maintenance and operation by email to


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