SECTION 7: Septic Tanks - Onsite Sewage Treatment Program ...

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SECTION 7: Septic Tanks 7-25FIGURE 7.20 Two-Piece FRP TankWhile their one-piece design implies that poly tanks should be watertight, it is imperativethat they (and tanks of all other materials) be tested via appropriate methods. Most localcodes specify where and how these tanks may be used; the reader is thus advised to reviewthe strength requirements included in local and state codes when assessing the use of anytank in onsite systems.Fiberglass reinforced plastic septic tanksFiberglass reinforced plastic (FRP) tanks are also relatively new compared to precast concretetanks. Like the poly tanks discussed in the previous section, FRP tanks are lightweight;so the same concerns regardingflotation and installation must be addressed.Manufacturers of FRP tanksadvocate their use on the basis that theyare not subject to rust or corrosion andare resistant to the chemicals and gasespresent in sewage and soil. There areestablished industry standards that addressmaterials such as plastic laminates,rigid plastic, and cured reinforced resins.These may have a bearing on theproduction of FRP tanks. Specific standardsfor using FRP in the manufacturingof septic tanks are being consideredby industry groups. Clearly, their use inthe onsite industry dictates that theymeet the same requirements for structuralstability and watertightness as allother septic tanks. The reader is againadvised to review the strength requirementsincluded in local and state codeswhen assessing use of these tanks in onsitesystems.Some FRP tanks are produced in one piece. Others are produced in two pieces using aninjection molding process. As stated previously, sufficient, high-quality raw material andcareful attention to manufacturing standards are essential for structural soundness andwatertightness. It is possible (though not common) for FRP tanks to leak as a result ofshipping damage, a substandard batch of adhesive, uneven application of adhesive, orstress placed on the midseam during installation.Two-piece FRP tanks are often shipped unassembled and must be permanently fastenedtogether before placement. The assembly process must be carefully done so that the jointwill not leak or separate. Generally, this is achieved using appropriate adhesives and stainlesssteel bolts, as shown in Figure 7.20. The bolts are primarily used to hold the halves inplace while the adhesive cures. As with tanks made of other materials, pipe penetrationsand access riser joints must be properly sealed to make sure they do not leak. Rubber andplastic pipe seals are routinely used in the production of these tanks.
7-26 SECTION 7: Septic TanksConstructing pour-in-place tanksIf the design specifies a pour-in-place tank, check with the system designer for specificdesign and installation requirements. An engineer must design these tanks.Overall Quality of Precast Concrete, FRP,and Poly Septic TanksNotwithstanding the wastewater source, the septic tank is the first component of any septicsystem. As such, it is important that high-quality, watertight tanks be used. If you areunsure about the quality of a tank, consult with a qualified structural engineer regardingpotential problems with structural integrity. Ultimately, a hydrostatic or vacuum test ofthe tank after installation will indicate the status of watertightness.A quality concrete tank should have the following characteristics: Reasonably smooth surface No honeycombing or cracks No efflorescence (the changing of crystalline compounds to a whitish powderthrough loss of water) that may indicate a very old tank or a bad pour No exposed rebar or wire (inside or outside) Smooth, well-made tongue-and-groove or lap joint with properly applied masticA quality poly tank should have the following characteristics: Uniform wall thickness No pin holes No deformation of tank or riser openingsA quality FRP tank should have the following characteristics: Properly sealed mid-seam (if two-piece) No imperfections in lay-up Uniform wall thickness No de-lamination No cracks or dings from handlingAdditionally, tanks made from any of these materials should have: Flexible, watertight pipe seals at all pipe penetrations Cast-in-place or mechanically-attached riser with tight fitting lidKeep in mind that a tank may have cosmetic deficiencies that do not affect performancein any way. Likewise, a tank may have an attractive appearance and still have structuraldeficiencies. Ultimately, hydrostatic or vacuum testing should be employed to measuretank quality. These tests are discussed in the next section, Watertightness Testing.Watertightness testingAll tanks used for MSTS must be tested for watertightness. The test shall be conductedto include the watertightness of all connections and risers (7081.0240, Subp.5). Also all holding tanks must be tested (7080.2290 (B)).
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SECTION 7: Septic Tanks - Onsite Sewage Treatment Program ...

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