Sizing Sanitary Collecting Sewers

The design and sizing of collecting sewers, the subtrunks, and the main trunk lines are provided by engineers. However, the UT should understand the factors that contribute to the design and the requirements that must be met.

While the unit system is used to size the building sanitary piping and the building drain, the sewage quantities used in sewer design normally are computed on a contributing population basis. The population to be used in design depends upon the type of area that the sewer is to serve. If the area is strictly residential, the design population is based on full occupancy of all quarters served. If the area is industrial, the design population is the greatest number employed in the area at any time. There are exceptions to the general rule that sewers must be designed on a population basis. Among these exceptions are laundry sewers and industrial-waste sewers. The per capita contribution for sewer design varies. Typical values are 100 gallons per person per day for permanent residents and 30 gallons per person in the industrial area per 8-hour period.

The sizing of the sewer includes the average rate and the extreme (peak) rate of flow (which occurs occasionally). The ratio of the peak rate of flow to the average rate of flow may vary with the area served, because the larger the area or the greater the number of persons served, the greater the tendency for flow to average out. Typical peak flows might range from 6 for small areas down to 1.5 for larger areas.

An allowance for infiltration of subsurface water is added to the peak flow to obtain the design flow. A typical infiltration allowance is 500 gallons per inch of pipe diameter, per mile of sewer per day.

Additional capacity to provide for population increase is usually included for areas that are likely to continue to develop. Provision of approximately 25 percent additional capacity over the initial requirements is advisable.

Each length of pipe from one manhole to the next is sized to carry the design flow. However, to help prevent clogging and to facilitate maintenance, a minimum size is usually specified which may be larger than is necessary to carry the design flow at the upper ends of the system. Typical minimum sizes are 6-inch pipe for house and industrial-waste sewers and 8-inch pipe for all other sewers.

It is sometimes the practice to select a pipe size that will carry the design flow when the pipe is half full, thus allowing for expansion. More often, however, sufficient safety factors in the future population estimate and the peak flow factor are included so the pipe may be designed to carry the design flow when flowing full.

The formulas or tables used in sizing the pipe are based on experiments and experience. One of the factors taken into account is the roughness of the pipe. Asbestos-cement pipe, for example, is smoother than concrete pipe. Because there is less friction on the inside of the asbestos-cement pipe, it will carry a greater flow than concrete pipe of the same size.

Another factor is the slope at which the pipe will be laid. The slope will generally be determined by the fall available on the natural ground area through which the sewer runs. The plans for collecting sewer systems generally show slope (or grade) in terms of fall per hundred feet. Slope is sometimes expressed as a percent rather than in inches per foot. A 1 percent slope means 1 foot of frill in a 100-foot length of pipe, or about 1/8 inch per foot. A 0.5 percent slope (6 inches in 100 feet is about 1/16 inch per foot.

Table 7-6 gives the minimum slope for some of the most commonly used pipe sizes. The slope should remain constant in the section between each manhole. Each section between successive manholes should be analyzed and the slope for that particular section determined. If the fall is relatively steep, the velocity of the flow is faster and a smaller pipe size may be used. If the slope is relatively flat, the velocity is slower and a larger pipe size may be used. In the larger pipe, the depth of flow may decrease to such extent that the velocity might be no greater than a smaller pipe

Table 7-6.-Minimum Slope for Sewer Pipe

on the same grade. Therefore, an increase in pipe size to obtain the desired flow velocity is limited by the rate of flow. Typical minimum flow velocities are 2 feet per second when the design flow fills the pipe and 1.6 feet per second at the average rate of flow. Maximum velocities must also be considered; too high a velocity will erode the pipe. A typical maximum velocity is 15 feet per second for concrete pipe. Because of the differences in available slopes, smaller pipe may be used in some sections than is required in an upper section of the same sewer. The pipe size should be reduced whenever better flow conditions would result.

Manholes provide access to sewers for inspection and cleaning. They are placed where there is a change in grade, a change in pipe size, a junction of two or more sewerlines, or a change in direction. Otherwise, they are placed at intervals of 300 or 500 feet of sewerline. The manholes should be built so there is no decrease in velocity and a minimum of water disturbance. The channel should be deep enough to prevent sewage from spreading over the manhole bottom. The covers should be of a weight strong enough to support the expected traffic. Perforated covers should not be used for sanitary sewer manholes, because openings in the sewer manhole would permit the entrance of sand, grit, and surface water. The sewers are ventilated by the stacks of the building plumbing systems.

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