Water is by far the preferred cooling medium because of its chemical-physical properties. In particular when evaporative cooling towers are used, water is regularly added (Make-Up) to compensate the losses due to the evaporation in the cooling tower and the losses from the circuit to keep the chemical-physical characteristics of the circulating water within an acceptable range of conductivity, dissolved salts, hardness. In the purification of make-up water the target is to reduce to the maximum possible extent the presence of suspended solids due to the incoming well or surface water. Moreover in evaporative cooling towers the circulating water is contaminated by impurities due to the atmospheric air (air-borne dust). Therefore, filtration of a partial stream of circulating water (Side Stream) is necessary to reduce the content of suspended solids and as a consequence the purging stream.