The self-cleansing velocity of a sewer with a diameter of 30 cm to 60 cm is:

Concept-

Self-cleansing velocity-

• The silting or deposition of particles of solid matter is undesirable in sewers and hence, the sewers should be laid at such a gradient that a minimum velocity that will prevent the silting of particles in sewers is developed over a wide variation in the discharge of sewage.
• Such a minimum velocity is known as self-cleansing velocity and for keeping the sewers free from any trouble, this velocity should be developed at least once a day, preferably twice a day.
• The self-cleansing velocity depends on the nature of the suspended matter in sewage and the size of the sewer.
• The following table shows the self-cleaning velocities for sewers of different sizes as recommended by Badwin Latham. Usually, a self-cleaning velocity of about 80 cm to 90 cm per second is adopted for normal sewage.

• Diameter of sewer in cm	Self-cleansing velocity in cm/sec
  15 to 25	100
  30 to 60	75
  Above 60	60

• The maximum velocity of flow is also to be taken into consideration. If the velocity of flow exceeds a certain limit, the particles of solid matter start to damage the inside smooth surface of sewers or in other words, a scouring action takes place.

• The maximum permissible velocity at which no such scouring action will occur is known as non-scouring velocity and it will mainly depend on the material used in the construction of sewers.