The rate of domestic or sanitary sewage produced is generally expressed in litres per capita per day (lpcd). The rate of domestic or sanitary sewage produced may be assumed to be equal to about 70 to 80% of the rate of water supply.
Thus knowing the per capita rate of domestic or sanitary sewage produced and multiplying it by the present population (i.e., at the beginning of the design period) as well as by the prospective or future population at the end of the design period, the total quantity of domestic or sanitary sewage likely to flow through the sewer per day at the beginning as well as at the end of the design period may be determined.
This will, however, give only the average flow of domestic or sanitary sewage per day, because the rate of flow of domestic or sanitary sewage is not constant but it varies as indicated below:
Variations in Rate of Flow of Domestic or Sanitary Sewage:
The rate of flow of domestic or sanitary sewage varies from season to season (or month to month), from day to day and from hour to hour. These variations in the rate of flow of domestic or sanitary sewage are termed as seasonal or monthly variations, daily variations and hourly variations.
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The seasonal or monthly variations are due to climatic effect. As more water is used in summer than in winter, a higher rate of flow of domestic or sanitary sewage would occur in summer than in winter. The daily variations are due to local conditions involving habits and customs of the people.
For example in India on Sundays and holidays activities such as washing, etc., are undertaken which involve use of large quantity of water, consequently higher rate of flow of domestic or sanitary sewage would occur on these days. The hourly variations are because of varying rates of water consumption in different hours of the day.
Fig. 3.1 shows a typical plot of hourly variation in the rate of flow of domestic or sanitary sewage. It may be observed that the flow of domestic or sanitary sewage starts increasing by about 6 a.m. and attains a maximum value between 8 and 10 a.m.
The flow then gradually decreases and a lower value of flow is reached between 1 and 4 p.m. The flow again increases and a higher value is reached between 6 and 8 p.m., after which it gradually decreases and attains a minimum value between midnight and early hours of morning.
The maximum seasonal (or monthly), daily and hourly rates of flow of domestic or sanitary sewage may be expressed in terms of the annual average daily rate of flow. Thus if annual average daily rate of flow of domestic or sanitary sewage is taken as 100 then the maximum seasonal (or monthly) rate of flow of sewage may be about 130 to 140, maximum daily rate of flow of sewage may be about 150 to 180, and maximum hourly rate of flow of sewage may be about 200 to 300.
Maximum or Peak Rate of Flow of Domestic or Sanitary Sewage and Peak Factor (or Peaking Factor):
For the design of sewers maximum or peak rates of flow of domestic or sanitary sewage are adopted. The maximum or peak rate of flow of sewage may be obtained by considering peak factor which is defined as the ratio of maximum to average rates of flow of sewage.
The value of peak factor depends on the contributing population, and the values recommended in the Manual on Sewerage and Sewage Treatment prepared by Central Public Health and Environmental Organisation may be adopted.
The value of peak factor may also be obtained from the following empirical formulae:
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(i) Babbit’s Formula:
(ii) Harmon’s Formula:
Where,
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Qmax = maximum rate of flow of domestic or sanitary sewage;
Qav = average rate of flow of domestic or sanitary sewage; and
P = population in thousand
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The Babbit’s formula is restricted to a maximum value of P = 1000 and a minimum value of P = 1. However, there is no such limitation in the case of Harmon’s formula. The values of peak factor obtained from these empirical formulae are in general less than those given in Table 3.4.
The peak factor also depends on the density of population, topography of the site and hours of water supply, which should be considered while estimating the value of peak factor for the design of sewers. Further for various sewers in a sewer-network different values of peak factor are adopted.
This is so because the variation between maximum and average rates of flow of domestic or sanitary sewage is different for different sewers. The variation between maximum and average rates of flow of domestic or sanitary sewage is large for domestic (or house) and lateral sewers because they receive the flow directly from the source.
This variation gradually diminishes as the flow reaches the branch or sub main sewers and the main sewers. As such for domestic (or house) and lateral sewers a higher value of peak factor is adopted.
Minimum Rate of Flow of Domestic or Sanitary Sewage:
For the design of sewers it is also necessary to determine the minimum rate of flow of domestic or sanitary sewage. This so because the sewers should be laid at such a longitudinal gradient that silting will not occur during the period of minimum flow of sewage. The minimum rate of flow of domestic or sanitary sewage may vary from 1/2 to 1/3 of the average rate of flow of domestic or sanitary sewage.