Ensuring Proper Septic Tank Access for Future System Maintenance

Ensuring Proper Septic Tank Access for Future System Maintenance

Cast-in-place risers must be protected during installation. If damage occurs, it is difficult if not impossible to repair.

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Tanks must be reasonably accessible to facilitate inspection and maintenance activities after installation is complete. 

Before the tank is covered, take photos related to the tank access. Risers and lids may be used to elevate access to a point closer to finished grade. If lids will be subject to vehicular traffic, they should conform to the American Association of State Highway and Transportation Officials H-20/HS-20 standard (wheel load of 16,000 pounds on a tire contact area of 8 inches by 20 inches).

Risers that are integral with the tank are optimal to achieve and maintain a watertight connection. Concrete risers may be cast into concrete tanks with a “cold joint.” The riser itself is produced separately and allowed to cure. It is then placed into the tank (or tank lid) form and the structure is poured. This cold joint may require further sealing (mastic or other appropriate sealants) to ensure watertightness. Polyethylene, polypropylene and PVC risers can also be cast directly into concrete tanks. Because of concerns regarding an effective bond between concrete and some of these materials, supplemental seals are typically used to ensure watertightness. 

A concrete riser using a cold joint.
A concrete riser using a cold joint.

Note that cast-in-place risers are the best choice in high-groundwater conditions and in cold climates where frost heave might otherwise cause separation of a riser that was added after the tank is produced. Risers with a smooth external wall are the best choice in frost-prone areas as well to minimize frost heaving. Cast-in-place risers must be protected during installation. If damage occurs, it is difficult if not impossible to repair.

Instead of being cast in place, risers may be attached to tanks on site. Make sure that the tank-riser seam, as well as any seams between riser sections, is made watertight to prevent infiltration of water. When concrete risers are attached to a concrete tank after it is made, a tongue-and-groove connection in combination with mastic or other appropriate sealant is more likely to remain undisturbed and watertight compared to a mortared seam. If additional concrete riser sections are added, these should also be made with tongue-and-groove joints and sealed with mastic. Wrapping seams provides additional protection, especially in high water table areas and freezing-thawing soil conditions.

Polyethylene and polypropylene risers are typically connected to a precast tank using an adapter ring cast into the tank. Another option is to mechanically attach a flange to the tank top using butyl rubber and stainless bolts. The riser is then sealed in place using appropriate adhesives per the manufacturer’s recommendations. 

There are varied opinions about whether riser lids should be accessible at the surface or buried under a few inches of soil. Some homeowners object to the aesthetic appearance of exposed lids. Unless the lids are of heavy concrete or have childproof or locking fasteners, they can pose a safety or vandalism hazard. There are devices that can be used to locate lids buried under a few inches of soil. Small screw-type markers can be installed flush with the ground or fastened to the lid. These can later be located with a metal detector. However, in practicality, there is no substitute for safe access at grade. The presence of risers provides the homeowner with a constant reminder of the location of the tank and may trigger more frequent maintenance. Ideally, risers should extend to the final finished grade (or 1 to 3 inches above) or comply with local regulations. Other safety precautions such as an interior grate or netting may be required and are recommended.

Tanks must be vented to prevent accumulation of gases. Venting also minimizes accumulation of hydrogen sulfide gas that may be converted to sulfuric acid in the head space of tanks, which can cause corrosion in concrete tanks. Most residential systems are designed to vent through the tanks and out the plumbing stack, but additional vents may be included at the tank and these may include a filter to deal with odor issues.

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


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