(A) Design considerations.
(1) Slope.
(a) Gravity sewer mains shall be installed with slope adequate to maintain flow velocities of at least two feet per second (fps) when depth of flow is at one-third of the sewers main inside diameter, based on Manning’s equation with an “n” value of 0.013.
(b) Recommended minimum pipe slopes listed in Section 33.41 of Circular DEQ-2 will be considered adequate.
(2) Capacity. Public sanitary sewers and appurtenances shall be designed to accommodate peak hourly flows, including an allowance for infiltration, while flowing no more than half full when no additional connections are possible and a quarter full when future growth is anticipated.
(a) The development must upsize the existing mains if the capacity of the sewer main is calculated to be three quarters full.
(b) The allowance for infiltration shall be 150 gallons per acre of coverage area per day or otherwise approved by Public Works Department.
(3) Diameter. Gravity sewer mains shall have a minimum diameter of eight inches. Increasing the diameter in order to meet the minimum pipe slope requirements will not be allowed.
(4) Manholes. Shall be a minimum for 48 inches less the 13 feet of bury for all manholes with bury depth greater than 13 feet a minimum of 60 inches manhole is required.
(5) Flow direction. On the infrastructure plans all sewer pipes shall be labeled as to the flow direction.
(6) Accessibility.
(a) Sewer mains shall be installed in the public right-of way wherever possible.
(b) Where mains cannot be installed in the right-of-way, a 20-foot wide exclusive easement with a 14-foot all-weather surface road must be constructed in the easement.
(B) Materials.
(1) Gravity piping. Gravity sewer main piping shall consist of any of the following materials:
(a) PVC meeting ASTM D3034, SDR 35 and 26 (8" to 15");
(b) PVC meeting ASTM D679, SDR 26 or ASTM F794 (18" and larger);
(c) HDPE meeting ASTM D3350 and ASTM F714;
(d) Concrete meeting ASTM C14, C76 or C655; and
(e) Other pipe materials specifically approved by the City PWD.
(2) Pressure piping. Pressure sewer piping (force mains) shall consist of PVC pressure pipe, ASTM D2241, Class 200 SDR-21, or AWWA C900 DR-18.
(3) Manholes.
(a) All manholes shall be constructed using pre-cast RCP, unless specifically allowed by the Public Works Department in writing. Structural strength shall withstand H-20 design load.
(b) All manholes installed at outfall lines must have PVC liner installed to protect against H2S gas.
(4) Manhole ring and cover. Manhole cover shall be stamped “Sewer.” The manhole ring/cover shall have an O-ring type gasket to protect the lid from inflow during storm events.
(C) Installation.
(1) Alignment and grade. Public sanitary sewers shall be installed with a straight alignment and grade between manholes as required in MPWSS.
(2) Location. Municipal wastewater system facilities shall be designed and constructed so that all such facilities are readily accessible for maintenance and repair.
(a) In addition, such facilities shall be situated so as to preclude the entrance of surface water into said facilities.
(b) All sewer mains shall be centered in the right-of-way or easement to the greatest extent possible.
(3) Depth.
(a) Sanitary sewers shall be buried to a depth sufficient to prevent freezing and shall have a minimum depth of four feet.
(b) Shallower depths may be allowed by the City PWD if suitable pipe insulating provisions are provided.
(D) Extension.
(1) Any extension of an existing city sanitary sewer main must be extended through the entire frontage length of the property to be served, with a standard manhole located at the terminus of the new sewer main.
(2) Sewer main extensions shall include all manholes, clean-outs and appurtenances deemed necessary by the city.
(E) Water line crossings.
(1) Crossings. A minimum of 18-inch vertical separation is required when a sanitary sewer main crosses above or below a water main, measured outside to outside of pipe.
(a) Please refer to MPWSS for further information on water line crossings.
(b) Less than an 18-inch vertical separation may be allowed when the gravity sewer at the crossing is made using a single 20-foot length of AWWA pressure pipe and the crossing is approximately 90° and the length of pipe is centered over the crossing.
1. Specific authorization from the Montana Department of Environmental Quality and the City PWD is required for a vertical separation of less than 18 inches.
2. No exception of the minimum 18-inch vertical separation requirement is permitted when the sewage pipe is a force main.
(2) Parallel. A minimum of ten-foot horizontal separation is required when a sanitary sewer main and water main are installed parallel.
(Prior Code, § 11.05.030.020) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)