Section 123-5-002 - Gravity Sewers00 2.01 Gravity sewers will be designed to achieve a minimum velocity of 2.0 feet per second when flowing full based on Manning's Equation, using an "n" value of 0.013 (Refer to Chapter 6, 002 for approval of alternative designs).002.02 The minimum diameter of public gravity sewer mains carrying raw domestic wastewater will be eight inches, except that six-inch sewers may be used if the distance between manholes is not greater than 400 feet, the total length of six-inch sewer is not greater than 800 feet, and the sewer is located where it will not need to be extended for future growth.002.03 Gravity sewers will be designed with a uniform slope between manholes. This does not apply to inverted siphons.002.04 Where velocities greater than 15 feet per second are attained, special provisions will be made to protect sewers from displacement by impact forces and erosion.002.05 Sewers with slopes of 20% or greater will be reinforced with concrete anchors. Erosion collars will be provided around steep sewers where migrating storm water could cause erosion of pipe bedding.002.06 Curvilinear sewers are restricted to simple curves. The radius of curvature will not be less than the pipe manufacturer's recommendation or be less than 100 feet. Elbows or couplings with deflections greater than three degrees will not be used at pipe joints to create a curvilinear sewer. Curvilinear alignments are not to be achieved by bending the pipe barrel.002.07 Inverted siphons will include at least two barrels, each not less than six-inches in diameter. Adequate head will be provided to assure velocities of at least 3.0 feet/second in all barrels when flowing at their maximum hydraulic capacity. Inlet and outlet control structures will be designed with adequate clearances for inspections and cleaning to occur. For two-barrel siphons, the inlet and outlet control structures will be designed so that flow can be diverted to either barrel to accommodate cleaning.002.08 Sewers will be laid at least 10 feet horizontally from any existing or proposed water main. The distance will be measured from the outside edges of the two pipes. Where it is not practical to maintain the 10 foot horizontal separation, the sewer may be laid closer if the water main is in a separate trench or on an undisturbed earth shelf located on one side of the sewer, and the bottom of the water main is at least 18 inches above the top of the sewer. Where 10 feet of separation cannot be met, both the sewer and water main will be constructed of water main quality pipe and pressure tested to 150 pounds per square inch.002.09 Sewers crossing water mains will be laid to provide 18 inches of vertical separation, measured from the outside edges of the two pipes. A full length of sewer pipe will be centered on the crossing so that its joints are as far as possible from the water main. When providing 18 inches of vertical separation is not achievable, one of the following two alternatives will be met to protect the water main: (a) either the sewer main will be constructed of water main quality pipe and pressure tested to 150 pounds per square inch, or (b) one of the mains will be encased in a watertight encasement pipe. Where the crossing is at a right angle, the protection will extend for 10 feet on each side of the crossing. Where the crossing is at an acute angle, the protection will extend to all points within 10 feet of the water main, measured perpendicular to the water main.002.10 Sewers will not be laid closer than 50 feet horizontally from any water well.002.11 Sewer pipe will be installed with proper bedding, haunching, and backfill material to adequately support the pipe and maintain its grade and alignment.002.12 The following acceptance tests will be performed on newly constructed gravity sewers. 002.12A Alignment Test. For straight sections of sewer, an alignment test using either a lamp or a laser beam will be performed. The light or laser beam will be visible through the sewer between adjacent manholes. 002.12B Water Infiltration/Exfiltration Test. The measured leakage will not exceed 100 gallons per inch of pipe diameter per mile per day, with a minimum head pressure of two feet above the highest point of the sewer pipe. The Low-Pressure Air Test may substitute for this test. 002.12C Low-Pressure Air Test. The American Society for Testing and Materials (ASTM) test will be specified for the pipe material being tested. The Infiltration/Exfiltration test may substitute for this test. 002.12D Deflection Test. A 5% deflection test will be preformed on flexible sewer pipe. A rigid ball or mandrel, sized at 95% of the inside diameter of the pipe, will pass freely through the pipe without the aid of mechanical pulling devices. The test will be performed after the backfill has been in place at least 30 days. 002.13 Manholes will be provided at sewer main intersections, at changes of slope, pipe diameter, or alignment of straight sections, and at distances not greater than 400 feet. Where modern sewer cleaning equipment is available, manhole spacing up to 600 feet may be used. Manhole spacing beyond 600 feet will be considered for sewers 30 inches in diameter or greater. Cleanouts may be installed only at the end of six-inch or eight-inch sewer laterals that are less than 400 feet long if the minimum slope requirement is met.002.14 Drop manholes will be provided wherever the invert of the entering sewer is 24 inches or more above the invert of the manhole. For new construction, the drop pipe will be outside of the manhole and encased in concrete to protect the manhole from unequal backfill pressures. Drop manholes will not necessarily be required for manholes installed on large diameter trunk or interceptor sewers where the invert of sewer laterals are set above the manhole invert to prevent standing water in the sewer laterals.002.15 Manhole ring and cover assemblies exposed to street traffic will weigh at least 300 pounds. Manhole steps, if provided, will be made of corrosion resistant material.002.16 The minimum diameter of manholes will be 48 inches. The minimum diameter of the access opening will be 22 inches.002.17 The flow channels through manholes will be shaped to conform to the shape of the connecting sewer pipes. Concrete benches will be installed on each side of the flow channels and slope towards the channels with a minimum 1.0 % slope.002.18 Sewers will be sized for the peak hourly flow, which is the average daily flow times a peak factor (F). Where historical flow records are not adequate to determine the peak factor in a service area, the peak factor will be calculated by the following formula, where P is the population equivalent of the service area in thousands. Click here to view
002.19 Privately owned sanitary sewers for mobile home parks, recreational vehicle parks (RV parks) or campgrounds may be designed according to the following design standards as an alternative to the standards listed in 002.01- 002.18 above. 002.19ASanitary sewers within the park or campground will be at least six inches in diameter. The minimum slope for six-inch sewers will be 1.0%. Wyes and tee pipe connections are allowed at intersections in place of manholes.002.19B Cleanouts will be installed at the upstream end of each sanitary sewer, at each change of direction, at each sewer intersection, at each change in pipe diameter, and installed at least every 150 feet of sewer length if one-way cleanouts are used or every 300 feet if two-way cleanouts are used. 002.19C Service lines will be at least four-inches in diameter. No more than two living units or campsites will connect to a four-inch service line. The minimum slope for four-inch service lines will be 2.0%. 002.19D Service lines for RV parks or campgrounds will include a trap below the frost line or be provided with removable gas-tight caps. 002.20 Small diameter gravity sewers will meet the following design standards: 002.20A Septic tanks or interceptor tanks, used to provide primary treatment, will have a tank capacity of at least 1000 gallons and meet all other design criteria of NDEQ Title 124, Rules and Regulations For the Design, Operation and Maintenance of On-Site Wastewater Treatment Systems. 002.20B Manholes will be installed at major impact points where flow measurement or sampling is necessary for regulatory or planning purposes. 002.20C Where manholes are not installed, cleanouts will be installed at the upstream end of each small diameter gravity sewer, at each change in pipe diameter, at significant changes of grade, at pipe intersections, and installed at least every 500 feet of sewer length if one-way cleanouts are used or every 1000 feet if two-way cleanouts are used. 002.20D Small diameter gravity sewers will have a minimum diameter of four inches. 002.20E Small diameter gravity sewers will be installed below the frost line.002.20F Small diameter gravity sewers will pass the American Society for Testing and Materials (ASTM) low-pressure air test before being placed into service (See 002.12C). 002.21 Small diameter pressure sewers, using grinder pumps or septic tank effluent pumps, will meet the following design standards.002.21A Cleanouts will be installed at the upstream end of each small diameter pressure sewer, at each change in pipe diameter, at pipe intersections, and installed at least every 500 feet of sewer length. 002.21B Isolation valves will be installed for each branch sewer line and not less than every 1000 feet of force main length. 002.21C Air release valves will be installed at local high points in the pressure sewer profile. 002.21D Small diameter pressure sewers will be installed below the frost line,002.21E Each grinder pump or effluent pump will have an audio or visual high-water alarm provided. 002.21F Each service line will include an isolation valve and two check valves. A check valve included in the pump unit counts towards this requirement. 002.21G Standard dimension ratio (SDR) 26 polyvinyl chloride (PVC) pipe will not be used in diameters smaller than two inches.123 Neb. Admin. Code, ch. 5, § 002
Amended effective 9/4/2019