Where possible, gauging of flow in existing sewers should be made in order to determine actual variations. Recording gauges are available or can be devised that will give depths of sewage in the outfall sewer or in the main leading from a district. In order to design a system for a previously unswered town or section of a city, an estimate must be made of the fluctuations to be expected in the flow. This is of importance, as the sewers must be large enough to accommodate the maximum rate, or there may be a backing up of sewage into the lower plumbing fixtures of buildings.
As in water consumption, the rate of sewage production will vary according to the season of the year, weather conditions, day of the week, and time of day. The variations do not depart so far from the average as for water because of the storage space in the sewers and because of the time required for the sewage to run to the point of gauging. That is, the peaks are flattened because it requires considerable sewage to fill the sewers to the high flow point, and the high from various sections will reach the gauging point after various time of flow. When the peak occurs will depend upon the flow time in the sewers and the type of district served. In a residential district the greatest use of water is in the early morning. Hence if the sewage is gauged near its origin, the peak flow will be quite pronounced and occur about 9 A.M., whereas if the sewage must travel a long distance, the peak will be deferred. In commercial and industrial districts the water is used throughout the working day, and accordingly the peak is less pronounced. Observation of fluctuations in various cities indicates that the peak for small residential area is likely to be 225 percent of the average for that day. For commercial areas the peak may reach 150 percent of the average and for industrial areas somewhat less. The flow in the outfall line of a sewer system serving a city having a normal population and commercial and industrial activities will have a peak flow of about 150 percent of the daily average. Figure 20-8 illustrates the greater peak which may be expected in sewage flow from a small residential district.
Some designer uses the following formula7 to estimate the maximum rate of domestic sewage flow from small areas:
In which M is the ratio of the maximum sewage flow to the average, and P is the population served in thousands. Some engineers use 22 as the numerator of the fraction.
The maximum sewage flow will be the hourly maximum, or the peak rate of the maximum day plus the maximum infiltration. In relation to water supply this will mean the peak rate of the maximum day, multiplied by the proportion of the water supply reaching the sewers, plus the infiltration. Note that the fire demand does not enter into sewage calculations. Minimum rates of sewage flow are useful in the design of sewage pumping plants and occasionally to investigate the velocities in sewers during low flow periods. In the absence of gauging, minimum flow may be taken as 50 percent of the average.
For design purposes a number of state health department specify the following as minimum requirements: normal infiltration to be cared for but not from rain leaders or unpolluted cooling water (which should not be discharged to sanitary sewers); lateral and sub mains to be designed on the basic of 1500 1 (400 gal) per capital/day, including normal infiltration; main, trunk, and outfall sewers on the basic of 950 1 (250 gal) per capacity/day, to include normal infiltration but with addition for industrial wastes if known if know to be in large amounts.