Fabric filter wells
The term fabric filter well describes the type of well .
For fabric filter wells, filter tubes with elongated slot openings are preferred. The entry area is 40 to 45 percent.
In order to prevent the ingress of sand , the filter tube is covered with a fine-meshed fabric made of copper , bronze or brass , which is placed over a coarse underlying fabric ( wires crossing at right angles ). The mesh size of the filter fabric (body or, in the case of very fine sands, braided fabric) depends on the course of the grading curve of the aquifer. The coarse and middle grain of this layer should be retained, the fine fraction should be let through. Before the well is put into operation , these finer grains of sand are carried away by the flow from the surrounding soil into the well and removed from there by an extraction that is greater than normal. This so-called desanding process creates a natural soil filter around the filter tube with an outwardly increasing fine grain content .
Drilled wells with fabric filters are rarely used today, despite the advantage of small drilling diameters (e.g. for small wells, lowering of the groundwater level, etc.), since filter fabrics are less resistant to mechanical and chemical influences than gravel filters and also have a greater entry resistance than these.