Depending upon size of opening, screens may be classified as follows.

1. Coarse Screen:-

   • It is also known as rack and spacing between bar is about 50 mm or more. These screen do help in remaining flashing objects from sewage.
   • They will collect about 3 litre of solids per million litre of sewage.

2. Medium screens:-

   • The spacing between the bar is about 6-40mm.

   • This screen will collect about 30 to 90 litre of material per million litre of sewage.

   • The screening will usually contain some quantity of organic matter which may purify and become offensive and must therefore be disposed of by incineration or buried.

   • These screens are generally kept inclined about 30° to 60° to the direction of flow so as to increase the opening area.

3. Fine screens:-

   • Fine screens have performations of 15 mm to 3 mm in size.

   • The installation of these screens proves very effective.

   • They remove up to 29% of the suspended solids from sewage.

   • These screens however get clogged very often and needs frequent cleaning. They are therefore used only for treating the industrial wastewater.

   • Brass or bronze plates or wire mesh is generally used for construction of fine screens.