750.5 Sanitary Sewers

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750.5.1 Sanitary Sewers

The item of sanitary sewers generally consists of relocating or adjusting existing sewer facilities to fit the requirements of a proposed highway improvement.

750.5.2 Type of Pipe

The permissible types of sewer pipe are reinforced concrete pipe, non-reinforced concrete pipe, and vitrified clay pipe. Usually the same type pipe as that in place is used for making adjustments and relocations unless a different type is approved by the owner.

750.5.3 Pipe Size

The State Division of Health of Missouri prohibits the use of sewer pipe less than eight inches in diameter.

750.5.4 Encasement

The requirements for encasing sewer pipe underneath roadways are given in EPG 643 Utility Procedures.

750.5.5 Pipe Slopes

Sanitary sewers are designed with slopes sufficient to give mean velocities, when flowing full, of not less than 2.0 fps based on Manning's Equation and a roughness coefficient of 0.013. Slopes which will provide this velocity are shown in the table below.

Minimum Pipe Slopes for Sanitary Sewers
Pipe Diameter (in.) Minimum Slope (ft/ft)
8 0.0033
10 0.0025
12 0.00194
15 0.00144
18 0.00114
21 0.000925
24 0.00077

750.5.6 Pipe Alignment

Sanitary sewers 30 in. or less in diameter are laid with uniform slope and straight alignment between manholes. Larger sewers are laid with uniform slope, but may have breaks in alignment without manholes.

750.5.7 Manholes

Manholes are installed at all changes in pipe grade or size; at changes in pipe alignment where the pipe diameter is 30 in. or less; at all intersections; and at intervals not greater than 400 ft. for sewers 15 in. or less in diameter, and 500 ft. for sewers 18 in. or more in diameter. Manholes are not placed in traffic lanes.

Foul water drops are provided for sanitary sewers and combination sewers when the inlet pipe elevation is more than two ft. above the manhole flow line. If practicable, connections are made at the flow line of the manhole to avoid the use of foul water drops.

750.5.8 Lamp Holes

Lamp holes are used only for special conditions, and are never used in trunk sewers or substituted manholes. They may be installed at the ends of laterals less than 150 ft. in length if approved by the owner.

750.5.9 Plans

The plans show complete details for all sanitary sewers to be constructed, including locations, grades, and profiles. The plans show all items such as type of pipe, encasement, manholes, etc., and all quantities necessary for the construction.

750.5.10 Rest Area Sewer Lagoons

The design of rest area sewer lagoons is based on guide lines developed by the Missouri Clean Water Commission. These guidelines are based on the biochemical oxygen demand (BOD) with the applied load estimated at 0.008 pounds per person per day. The allowable load for the primary cell will be 34 pounds per acre of water surface per day. When the primary cell exceeds 10,000 square feet, the cells will be built in a series of three cells, the second and third cells being 30% and 10% respectively the size of the primary cell.

The critical period can be expected to occur during the summer months. The total traffic will be highest during this time and the percentage of cars stopping could also be expected to be higher. Use design or AADT traffic for the year 2000. The predicted AADT should be expanded by 1.3 to compensate for the peak summer months. Eleven percent of the adjusted traffic volume should be used to estimate the number of cars entering the rest area.

The formula for determining the area of the primary cell for the lagoon is:

LA = AADT \times 1.3 \times 0.11 \times 2.7 \times 0.70 \times 0.008 / 34\,
LA = Required surface area of the primary cell for the lagoon, ac.
AADT = Predicted design traffic for a given year
1.3 = Predicted design traffic expansion factor to compensate for peak summer months
11% = Estimated percent of cars entering the rest area
2.7 = Average occupancy of each car entering the rest area
70% = Estimated percent of people stopping who use the facilities
0.008 = Estimated applied load, lbs./person/day
34 = Allowable load, lbs./acre/day

To prevent suspended solids from leaving the effluent of the third cell of the lagoon, a submerged sand filter should be incorporated into the third cell. The area of the sand filter is determined by using a filtration rate of five gallons per day per square foot of filter and sizing the filter to meet the demand based on three gallons per person using the restroom facilities. This procedure may result in the third cell being larger than 10% of the primary cell area and should be designed accordingly.

Details for the submerged sand filter, including specifications are available on request from the Divisions.

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