Okla. Admin. Code § 252:656-5-2

Current through Vol. 41, No. 19, June 17, 2024

Section 252:656-5-2 - Design standards

(a)Standard. Design and construct sewers with hydraulic slopes sufficient for obtaining a velocity of 2 fps (feet per second) or greater. Base the design on Manning's formula using an "n" value of 0.013. Gravity sewers shall not be smaller than 8-inch diameter, except those sewer lines meeting the requirements in Subchapter (c) below.

(b)Slope. The depth of flow and the slope of the conduit affects the velocity of a liquid flowing under gravity conditions. The following table gives minimum slopes for different sizes of pipe to meet the required flow velocity.

(1) 4" sewer: 1.00 feet/100 feet

(2) 6" sewer: 0.50 feet/100 feet

(3) 8" sewer: 0.40 feet/100 feet

(4) 10" sewer: 0.29 feet/100 feet

(5) 12" sewer: 0.22 feet/100 feet

(6) 14" sewer: 0.17 feet/100 feet

(7) 15" sewer: 0.15 feet/100 feet

(8) 16" sewer: 0.14 feet/100 feet

(9) 18" sewer: 0.12 feet/100 feet

(10) 21" sewer: 0.10 feet/100 feet

(11) 24" sewer: 0.08 feet/100 feet

(c)Exceptions. The following may be approved where the proper slope cannot be achieved.

(1)Pipe Diameter. Under the following conditions, DEQ may approve a smaller pipe diameter than stated in (b) above (but not less than 8 inches) if:

(A) The available ground slope or an obstruction dictates a different pipe diameter to meet the slope/velocity criteria.

(B) A smaller diameter pipe (but not smaller than 8-inch) will provide better hydraulic flow characteristics than the larger pipe (i.e., greater depth of flow, higher velocity, etc.). The applicant must furnish computations and compare the hydraulic conditions in the pipe at average, high and low flow conditions. Computations shall show capacity in the pipe at projected peak flow conditions.

(2)No future expansion. Up to 400 feet of 6-inch pipe may be installed at the end of a line that is isolated from future expansion.

(3)Privately-owned collection lines. Under the following conditions four- and six-inch diameter lines may be installed in privately-owned developments such as mobile home parks, recreational vehicle parks and similar establishments:

(A) Individual lots or units within the development are not intended for sale or transfer of ownership or where the collection system will not be dedicated to a public entity.

(B) No more than 10 mobile homes or 180 fixture units shall be connected to a four-inch line, and no more than 40 mobile homes or 700 fixture units shall be connected to a six-inch line.

(C) The minimum slope for a four-inch line shall be 1/8 inch per foot and for a six-inch line 1/16 inch per foot.

(4)Small diameter gravity sewers. A small diameter gravity sewer system is acceptable where only settled sewage will be transported and consist of septic tanks and small diameter collection mains. They may only be considered for small municipalities or rural sewer districts with less than 100 connections or a population equivalent less than 250 with no or low potential for population growth. Locate septic tanks so all sewage is settled before the connection to small diameter sewers. Grinder pumps shall not be used in lieu of a septic tank.

(A)Hydraulic design. The design flow shall be at least 0.3 gpm per connection. The velocity in lines carrying only settled effluent may be reduced to not less than 1.0 fps based on Manning's open channel flow formula using a "n" value of not less than 0.013 and depth of flow at one half the pipe diameter.

(B)Collector mains. The horizontal alignment may bend so long as the radius of a bend does not exceed the manufacturer's recommendations. Use a positive gradient. The pipe diameter shall be at least 4 inches, and determined through hydraulic analysis. Determine burial depth by the elevation of the interceptor tank outlet invert elevation, frost depth and anticipated trench loadings.

(C)Service laterals. Lines between septic tanks and collector lines shall be 3-inch PVC or larger.

(D)Tanks, pumps and service lines. Septic tanks shall meet the design requirements of OAC 252:641 with routine maintenance. Septic tanks, pumps and service lines from them must be regarded as integral components of the wastewater system and not part of the individual home plumbing.

(E)Manholes and cleanouts. Cleanouts may be used instead of manholes, except at major junctions of mains. Cleanouts are required at the upstream ends of mains, at minor main junctions, at changes in main diameter and at least every 400 feet. Cleanouts shall be flush with the ground and designed to prevent damage from vehicular traffic.

(F)Corrosion. Use corrosion resistant materials in lift stations.

(G)Vents. Vents are necessary to maintain free-flowing conditions in the main and are commonly used in combination with cleanouts.

(H)Testing. In addition to leak testing the small diameter system, conduct vacuum or hydrostatic tests on interceptor tanks. Typical acceptance criteria are less than 1.0 inch loss of Hg vacuum after five minutes with initial vacuum of 4.0 inches of Hg or a drop in water level of 1.0 inch after 24 hours in an overfilled tank.

(5)Pressure sewers. Pumping units, septic tanks and holding tanks shall be regarded as integral components of the wastewater system and not part of individual home plumbing. There must be at least one pump per housing unit and a minimum of an audio/visual alarm for malfunctions.

(A)Sewer design. Flow velocities must be in the range of 2 to 5 feet per second for grinder pump installations. Lower velocities may be approved only for pipes carrying settled effluent from septic tanks. Systems must have air relief valves, a means to flush all lines, cleanouts and rerouting procedure in the event of maintenance. Each line without a grinder pump must have a septic tank meeting OAC 252:641, Individual and Small Public On-Site Sewage Treatment Systems, to separate solids.

(B)Pumps. Pump size must meet the volume and head conditions. Grinder pumps must be at least two-horsepower unless the applicant can show that the manufacturer of the complete pump system has a minimum of 5 successful installations permitted by the DEQ serving ten (10) or more homes. Systems will be designed to provide back-flow prevention. Pumps shall be housed in a tank separate from the septic tank with at least 12 hours holding capacity to allow for power outages and equipment failures.

(C)Equipment Inventory. A minimum number of pumps shall be purchased by the system to provide back up for maintenance of the system. The system is required to provide one pump for the first 1-10 homes, one (1) additional pump for the next fifteen (15) homes and one (1) additional pump for each additional twenty-five (25) homes thereafter.

Okla. Admin. Code § 252:656-5-2

Added at 17 Ok Reg 1140, eff 6-1-00; Amended at 23 Ok Reg 937, eff 6-15-06; Added at 28 Ok Reg 1282, eff 7-1-11