DESIGN CRITERION ACCORDING TO IS 1742 , RECOMMEND BY THE ENGINEERS TO SEWER CLEANING ASSUMPTIONS

According to IS 1742 ,for large groups of houses , schools, public institutions, hospitals,factories etc etc maximum rate of flow of foul water for the design of sewers shall be taken three times the average dry weather flow , flowing half full with a minimum self cleansing velocity of 0.75 mt /sec .A good average rule is to allow for a flow of liquid wastes from buildings at the rate of 0.03 cu.m/ min .per 100 persons based on a water consumption of 135 litres per head per day. Some Engineers recommended as follows  , All laterals,branches and outfall sewers should be designed for a peak flow at the rate of 3 times the dry weather flow for population below one lakh, and the outfall may be designed for 2.5 times the dry weather flow if the population contributing to the flow in the out fall sewer is more than ten lakhs .Sewers of 350 mm dia and below should be designed to run half full at the peak flow , and sewers of 450 mm dia and above for two third full .Sewers of larger size than 900 mm dia may be designed to run three fourth full .Sewage production is taken from 75 percent to 90 percent of the water supplies to the community .It is necessary that all sullage drains have self-cleaning velocities as for as possible so that there are no accumulations in the sewers and the sewage does not become septic .In India sewage has been found get septic after six hours whereas it takes about 10 to 12 hours in the cold countries. Circular section has the least perimeter for a given area and gives the highest velocity when flowing full or half full , and is most suitable when the discharge is more or less constant which is in a separate system of sewers (Where only foul water flows ) but it is not very suitable for a combined system .