Sewage Flow Variation
The quantity of sewage produced depends upon the quantity of water use.
Average daily flow = (70 – 80) % average water consumption i.e.
Average Daily Flow (ADF) of Sewage = 75%
Average Daily Demand (ADD) of water consumption = 0.75 ADD
The flows in sanitary sewers vary seasonably monthly, daily, hourly. For areas of moderate sizes be expressed as;
Maximum Daily Flow = 1.5 x ADF
1.5 varies from place to place
Maximum hourly flow = (2 – 4) ADF
This is actually the peak flow
Sewers are designed on peak flow basis, however the minimum flow passing through the sewer is also important in the design of a particular sewer because at low flow the velocity will be reduced considerably which may cause silting.
So the sewers must be checked for minimum velocities at their minimum hourly flows. Generally for a moderate area the following minimum flows may be assumed.
Minimum Daily Flow = 2/3 ADF
Minimum Hourly Flow = 1/3 ADF
Tip: The sewers should be laid of such a gradient as to have minimum self cleansing velocity.