First, it can filter the sewage preliminarily, because there are many regular slot on the surface of the well screen. However, the well screens can not filter the small impurities well.
Second, usually the well screen is installed under the ground where has lots of rocks and soils, it can prevent impurities from other devices.
Third, the well screen also has the function of supporting and protect the well wall. Usually the well screen is used together with pumps. When the pumps works under the borehole, it will vibrate strongly, so we have to use the well screen to support the well..
The well screens is usually used in the borehole or other equipment, therefore we should be careful to select the well screen, once the well screen has problems, the loss will be huge.