What is the perforation of fine screen used in the removal of suspended solid from sewage?

Explanation:

As per IS 6280: 1971, Clause 2.1:

Screens:

A screening device comprising of flats of rectangular or trapezoidal sections placed vertically, inclined or curved, and spaced at close and equal intervals across a channel through which sewage flows. It is used for the removal of certain materials such as pieces of wood, floating debris, leaves, frigs, rags, etc, found in sewage. Depending on the clear spacing between flats, the screens shall be classified as under:

1. Coarse screens; Above 50 mm
2. Medium screens; 20 to 50 mm
3. Fine screens; Less than 20 mm

​Fine Screens: 

Fine screens are mechanically cleaned devices using perforated plates, woven wire cloth, or very closely spaced bars with clear openings of less than 20 mm (2 cm). Fine screens are used to remove solids from primary effluent to reduce the clogging problem of trickling filters. 

The perforation value of the fine screen given in option (2) is most appropriate. Hence option (2) is correct.