The sewers are always designed for the partial flow conditions. The purpose of maintaining partial flow in sewers is to maintain an open channel flow condition and to provide space above surface of sewage for ventilation. Sewers may be designed to run for a depth of flow of d, which should be at least half the diameter, D, i.e., 0.5D but not more than 0.8D at ultimate peak flow conditions.
Sewage consists of considerable amount of organic and inorganic solids, which remain floating or suspended. If velocity of flow in the sewer is less, these solids get deposited at the invert of the pipe and cause obstruction to the flow of sewage. Hence, it is necessary to maintain a minimum velocity of flow at which no solids get deposited in the sewer.. The velocity of flow that prevents settlement of solids is known as self-cleansing velocity. This velocity should be maintained at least once in a day during the peak flow at all sections of the sewerage system.
Similarly, the velocity of flow in sewer should not be too high, as the suspended solids will cause wear to contact surface of the pipe and erode the pipe material of sewer. This will reduce the life of the sewer. The permissible maximum velocity to prevent eroding is termed as non-scouring velocity and it should be limited to 3.0 m/s.
Some important facts about velocity of flow in partial-flow sewers are as follow:
The slope or grade of the sewer is selected in such a way that the velocity of flow is maintained between the self-cleansing and non-scouring velocity. The sewer sizes and slopes should be such that the velocity of flow increases progressively or steadily from the head section through the entire length of sewers. It has been shown that for sewers running partially full, for a given flow and slope, velocity of flow is little influenced by pipe diameter. The slopes recommended by the manual (CPHEEO) present peak flows up to 30 Lps are given in Table 4.6, which would ensure a minimum velocity of 0.6m/s.
The minimum size of public sewers should not be less than 150 mm. However for major towns and cities and where the topography is flat, the minimum size of sewer should be of 200 mm in diameter.
The starting sewer should be laid at such a depth that the sewer connection from the house is possible. Further the minimum depth should be on the criteria of the stress on the pipe due to external load is negligible. The minimum depth of cushion over the top of the sewer should be at least 0.9 m. Pipes laid under roads should have a cushion of 1.20 m over the top of the pipe. The maximum depth of cutting for trenches for construction of sewers is decided based on the depth of groundwater table and sub-soil conditions. Normally a depth of 5.0 to 6.0 m may be the upper limit considering the difficulty in implementation and economy. However, greater depths for short lengths for negotiating a hump or avoiding a lift station are permitted if the site condition permits