8/17/2010 SEWER COMMISSION Minutes

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- 3 - on Manning's formula using an "n" value of 0.013. The £ollov.•ing are the minimum slopes which should. be provided, however, slopes greater than specified are desirable: Minimum Slope in Feet Per Sewer Size 100 Feet (m/100m) 4 inches �tQ5 cm. 0.47 6 in cm.� 0.28 8 inches 20 cm.) 0.19 (B) While insufficient empirical data has been generated to demonstrate the absolute, unqualified acceptability of slopes less than those specified in the preceding paragraph , a system of small diameter gravity sewers conveying settled sewage may be approved if designed and constructed to give mean velocities, when flowing full, of not less than 1.0 foot per second based on Manning's formula using an 'Intl value of 0.013, provided that the cost saving qualification of 40 CFR 35.2005 (23) is met. The purpose of this is to ensure that the saving to be gained by installing the sewers at these flatter slopes is sufficient to justify the potential risk involved. The following are the minimum slopes which should be provided for systems to be considered under these criteria: Minimum Slope in Feet Per Sewer Size 100 Feet (m/lOOm} 4 inches �10 cm.) 0,21 6 inches 15 cm<) 0.12 8 inches 20 cm.) 0.08 No pipes of less than 4 inches in diameter may be specified for gravity flow, It is imperative that adequate septic tank maintenance procedures be estab- lished to ensure the retention of solids in the septic tanks. While important for flows at 1.5 feet r second it is per , especially important below that velocity. Bitumnincus-fiber pipe, commonly known as Orangeburg pipe, is unacceptable for public (or community) collection systems. Since the small diameter gravity sewers will be conveying anaerobic septic tank effluent which will be corrosive and odorous, pipe and pumping and treat- ment hardware should be manufactured of appropriate materials to withstand these adverse conditions. Acceptable pipe materials are ABS, PVC, polyethylene, vitrified clay and fiberglass/polyester composite. Concrete, asbestos cement, steel, iron and other metals are not acceptable. Maximum allowable infiltration-exfiltration rates will be 200 gallons per j inch of pipe diameter per mile, per day (0.19 m3fcm. of pipe'dia.Jkm.lday) as detailed in Section 24.82 of the GLUMRB Recommended Standards for Sewage Works 1978 Edition.