By Kayla Hanson, P.E.
Water and wastewater treatment structures play crucial roles in our daily lives. Like most aspects of underground infrastructure, however, society remains largely unaware of their presence and the breadth of their impact unless something goes wrong that threatens our health and safety.
These structures – specifically septic tanks and grease interceptors – often serve as the heart of onsite treatment systems. Each subsequent step of treatment and conveyance relies on the tank’s performance. It is imperative for these critical structures to be designed, manufactured, tested and installed properly to ensure they perform reliably throughout their service lives.
Using these tips for wastewater tanks will help ensure they operate efficiently and dependably for decades to come.
1. Require Thorough QA/QC Practices
All septic tanks and grease interceptors, regardless of material, should be manufactured in accordance with a detailed quality assurance/quality control plan. At a minimum, and where local codes do not specify otherwise, precast concrete septic tanks should be produced in accordance with ASTM C1227, “Standard Specification for Precast Concrete Septic Tanks,” and precast concrete gravity grease interceptors should be manufactured in accordance with ASTM C1613, “Standard Specification for Precast Concrete Gravity Grease Interceptors.” These standards set forth requirements for design, construction materials, manufacture and testing of these structures. Precast concrete manufacturers should also have a quality program that goes beyond just inspecting the final product. A thorough QA/QC program is active from raw material acquisition to product shipment.
2. Design for Vehicular Traffic
Precast concrete septic tanks and grease interceptors are often exposed to vehicular traffic, and as such must be designed to accommodate the corresponding loads. Traffic-rated tanks require proper design of both the tank and its lid in accordance with ASTM C890, “Standard Practice for Minimum Structural Design Loading for Monolithic or Sectional Precast Concrete Water and Wastewater Structures,” or other applicable standards in the jurisdiction.
3. Require Baffles and Filters
Each tank inlet and outlet shall be outfitted with a baffle or tee to help control the flow pattern. Outlet filters should be used to retain any larger particles that did not settle in the tank to prevent them from being discharged to the leach field or sewer. Baffles and tees must be made of corrosion-resistant material and can be cast monolithically in the tank.
4. Conduct Watertightness Testing
Septic tanks and grease interceptors may be tested for watertightness using the hydrostatic or the vacuum method, which are outlined in ASTM C1227, ASTM C1613 and ASTM C1719 for installed systems. The tests can be conducted at the manufacturing facility, on the jobsite or after the tank has been placed in its excavation prior to backfill. Testing should not be conducted after backfill due to both safety concerns as well as challenges with identifying the location of a potential leak. Testing frequency requirements vary with each jurisdiction; however, at least one tank from each form used in manufacturing should be tested annually.
5. Execute Joints Properly
Mid-seam tanks, also referred to as two-piece or clamshell tanks, often use interlocking joints like shiplap or tongue and groove. Top-seam or one-piece tanks can employ interlocking joints, lap joints or slab joints. Regardless of the type or location, all joints must be manufactured and sealed properly to ensure the structure’s watertightness. According to ASTM C1227, the maximum allowable gap between two mating joint surfaces before applying sealant is 3/8 inch.
6. Use Preformed, Flexible Joint Sealants
Preformed, flexible joint sealants conforming to ASTM C990, “Standard Specification for Joints for Concrete Pipe, Manholes, and Precast Box Sections Using Preformed Flexible Joint Sealants,” must be used to seal joints and access points. Sealants should be applied to surfaces that are clean, dry and free of foreign substances. Follow each sealant manufacturer’s instructions regarding proper application and compression. Avoid using rigid materials for sealants and connectors as they are likely to crack during backfill and service and are unlikely to create a watertight barrier.
At a minimum, when applying joint sealant, ensure:
- The sealant is positioned as close to the center of the joint as possible so the entire rope of sealant is contained within the joint.
- The rope of sealant travels around the corner of the tank and extends at least 12 inches beyond the corner before terminating.
- The rope sealant is not stretched, and its cross sectional area is preserved during installation.
- Adjoining pieces or ends of sealant overlap at least 12 inches and are thoroughly kneaded together to create one homogeneous, continuous rope of sealant.
7. Require Resilient Connectors
All pipe-to-tank penetrations should be outfitted with watertight, resilient connectors conforming to ASTM C1644, “Standard Specification for Resilient Connectors Between Reinforced Concrete On-Site Wastewater Tanks and Pipes.” These connectors are designed to prevent infiltration and exfiltration of liquids, as well as loss of vacuum. Resilient connectors may be cast into the structure at the manufacturing facility or assembled on the jobsite. Like sealant materials, resilient connectors – rather than rigid connection materials – ensure the penetration will remain watertight after backfill and in service while allowing some movement of the pipe.
8. Ensure Proper Bedding
Proper evaluation of the jobsite’s soil conditions and water table is necessary to determine the appropriate bedding and backfill materials. Tank bedding generally consists of a well-compacted, level, uniform base topped with at least 4 inches of sand or other granular material to serve as a bearing surface for the tank. The bedding and bearing surface must be free of debris or larger rocks, which could introduce point loads on the base of the tank. Providing a well-compacted, uniform bearing surface for the base of the tank will help ensure the structure remains level and prevent future settling.
9. Use Structurally Sound, Watertight Risers
Concrete tanks may use adjustment rings, riser bases, risers and riser lids made of concrete, HDPE or fiberglass. Risers shall be structurally sound and used in accordance with manufacturer’s instructions. Casting the adjustment ring or riser base into the tank lid at the manufacturing facility will help ensure a watertight connection and eliminates the need to install and seal the adjustment ring or riser base on the jobsite. However, risers installed after casting can be made watertight by following the manufacturer’s installation instructions and ensuring proper application of an appropriate ASTM C990-compliant sealant. Openings with any dimension larger than 12 inches must have lids weighing 59 pounds or more or must be outfitted with a lock, per ASTM C1227. Whenever possible, openings should also be outfitted with secondary safety devices to prevent children or pets from accidentally entering the tank in the event the lid is removed or damaged.
10. Ensure Proper Backfilling Practices
Whether the backfill material is native to the jobsite or transported from off-site, it should be free of debris and consist of material no greater than 3 inches in diameter. Backfill should be placed in uniform layers no greater than 24 inches thick to aid in compaction and prevent movement of the structure.
Precast concrete is an inherently strong, durable and low-maintenance material perfectly suited for wastewater applications. Families, schools, businesses, restaurants and entire communities put their trust in these structures, as the performance of the entire wastewater treatment system often resides with the septic tank or grease interceptor. Keep these ten tips top of mind to help ensure these structures provide safe, reliable treatment for decades to come.
Kayla Hanson, P.E., is NPCA’s director of technical services.