SAND FILTERS DESCRIPTION
Born for the final treatment of primary potable and process's water, rapid filtration on sand has proved a valuable tool for waste water's final treatment, where there is a particular interest in a clear water and with a reduced content of organic substances, for a further reuse.
Our filters (atmospheric or pressurized) are usually consisting in multiple layers of granular material, supported by a bottom draining, traversed from top to bottom by the stream of water to be filtered, can be small tanks or large basin.
We realize the filtration with a discontinuous cyclic process: the filter is maintained in operation until the water leaving the filter is affected by an excessive turbidity, or pressure drops caused by the impurities collected have reached excessive values.
In this situation the water flow sent to the filter is interrupted, and then the filter is washed in countercurrent, by means of an energetic flow of water and air. The filtered material is subjected to the action of the updraft flow and to the intense agitation. Hydrodynamic action of water and air determine the detachment and material removal, for sluicing, of the particles accumulated in the filter.
The washing water induces a limited expansion of the filtering mass, and has essentially the function of transport to the discharge of solid particles detached from the air.
Once stopped washing flow, the filter bed remains stratified, in the sense that it induces a classification of the filter material, sedimented on the bottom the coarse-grained material and / or of higher density's and therefore the finer material and / or lower density.
Once so cleaned (with time of washing in order of 15-20 minutes), the filter is ready to operate again. During the first minutes of the new stage of filtration, the filter operates under a rinse, using an appropriate valve that discharges the water.
Inside sand filters is realized a complete and rational exploitation of the adsorption capacity of all the mass. In other words, overcoming the concept of simple filtration surface, based on the interception of turbidity by the first filter layers, for the filtration of wastewater recourse was made to the application of the concept of the filtration volume or in depth, which allows to fully exploit the storage capacity of turbidity by the filtering mass, allowing filtration with sufficiently prolonged durations.
The filters surface is dimensioned so that the filtration rate is such as to ensure the proper removal of pollutants. It means velocity quite high, which are justified even in the opportunity that the solid particles penetrate deeply into the filter mass, by deploying it as much as possible, prolonging the filtration cycle.
The nozzles on the bottom must be evenly distributed and especially designed to avoid clogging risk.
The washing stage operates with hydraulic loads higher surface and with a high specific flow rate of air, which is why the number of top nozzles that are to be installed inside the filter is usually significant.