The first phase of the secondary treatments consists in a filtration through a sand bed (usually sand and gravel of quartz origin), with the aim to remove the suspended substances present in the water.
During the operation, the substances, retained in the filter bed, causes an increase of the load losses; when these reach a predetermined limit value, it is necessary to proceed to the washing of the bed by using filtered water, to remove the substances retained by the filter. Since the filtering material is quite heavy, the backwash is generally not sufficient to lift it and to allow a good cleaning; therefore, a backwashing with air and water is normally performed.
The filter material with fine particle size ensures good filtration, but involves frequent washing with consequent interruptions of operations (short cycle operation). The use of coarse-grained filter material ensures exercise cycles much longer, but provides less clean filtered water. To combine the advantages of the coarse and fine materials, it is possible to use multilayer filters with particle size ranging from a minimum of 0.3 mm to a maximum of 8 mm.
The efficiency of a filter depends primarily on a good distribution of the inlet water, on the distribution of the backwash water and air and on a homogeneous drainage of the filtering surface. In pressure filters, drainage and distribution of backwashing are obtained with the use of a double bottom. The upper plate is provided with at least 40-50 filtering nozzles per m², able to push the water and to retain the filtered material. In fact, the cracks of the nozzles, made of polypropylene, are normally 0.25 mm, that is smaller than the size of the smallest grain of quartz sand.
The upper distribution, instead, is generally carried out by a special device with truncated cone or through nozzle.
Usually the quartz filter involves the use of pneumatic valves and appropriate instrumentation, in order to manage all the cycles of filtration and backwashing in completely automatic mode.
L’The efficiency of this kind of filtration can be up to 100 microns.