Tertiary filtration

Created for the final treatment of so-called primary potable and industrial water, rapid sand filtration has also proved to be a very valid tool for the final treatment of wastewater, where a particularly limpid effluent with a reduced presence of organic substances is of interest, with a view to its reuse.
Filters consist of several layers of granular material, supported by a draining bottom, crossed from top to bottom by the current of water to be filtered; they can be small tanks or large basins. Filtration occurs in a discontinuous cyclic process: the filter is maintained until either the water leaving the filter is affected by excessive turbidity, or the pressure drops induced by the impurities collected have reached excessive values.
At this point the flow of water sent over the filter is interrupted, and the filter material is washed, in countercurrent, by means of a vigorous flow of water and air.

The particles in the filter medium are subjected to the action of the upward current and the intense agitation and hydrodynamic action of the water and air cause the particles accumulated in the filter to be detached and removed by hydraulic transport.

Activated carbon filters are used for the thorough abatement of organic substances and micro-pollutants such as insecticides, pesticides, heavy metals and various toxic substances that manage to break through the barrier of chemical and biological processes. The purifying effect of activated carbon is due to surface attraction phenomena caused by the huge active surface area of the carbon mass, which is able to capture pollutant particles by adsorption.

We use powdered activated carbon in small and medium-sized plants as an auxiliary treatment to improve the efficiency of chemical and biological treatments. For example, the ability of biological processes to remove dissolved organic substances can be combined with the ability of activated carbon to adsorb non-biodegradable colloidal organic substances.

Powdered activated carbon is suitable for short, discontinuous treatments or for strong purification. For other continuous or high capacity plants, granular activated carbon is used, which is less effective but more economical, recoverable and easily regenerated.