1. Zone formation

• The region which is to be served by the drainage system is divided into different zones. It is necessary to study the layout of the roads to fix the location of the sewers.
• These zones are marked on the map and also contours are also drawn to indicate the general direction of the slopes and the degree of the slopes.
• The expected flow of the sewage due to the steepness of the ground.

2.The Qauntity of the sewage:-

• The design of the sewer is based on the quantity of is to be observed i.e. either combined sewer or the separate.
• The variation in the quantity of the sewage is marked and a suitable multiplying factor is applied to calculate the average quantity of sewage for which the sewers are to be designed.