The screen to be installed at the inlet of the wastewater plant is expected to perform the basic function, i.e. to separate all solid particles of at least one dimension smaller than the width of screen bar spacing and to ensure the reliability of this function.
No screens for channel widths up to 600 mm are specifically designed to entrap gravel or sand, hair, fibres and textiles. All these undesirable components shorten the screen service life and result in higher demands for servicing.
With respect to the level of the aforesaid influences reflecting the content of harmful matter, the individual screens differ a great deal. There are treatment plants of municipal water that contain a great amount of such substances causing the above problems.
In general, the influent impurities have approximately the same dimensions in both large and small treatment plants, which is disadvantageous for the small ones.
For process reasons, small bar spacing (6 or 3 mm) is required. However, screens with small bar spacing easily become clogged, have a smaller actual flow area, and withstand poorly long-term overloading due to wastewater and amounts of impurities
In practice, two systems are used:
- with direct inflow of wastewater from the sewer piping to the screen
- with a pumping sump