More than 80% of the particles that contaminate compressed air are smaller than 2 µm in size and can therefore easily pass through the compressor's inlet Filter. From that point, the particles spread throughout the pipe system and mix with the water and oil residue and pipe deposits. This can result in the growth of micro-organisms. A filter positioned directly after the compressor can eliminate these risks.
Nevertheless, to have pure compressed air, bacteria growth after the filter must be kept fully under control. The situation is complicated further as gases and aerosol can be concentrated into droplets (through concentration or electrical charging) even after passing several filters. Micro-organisms can germinate through the filter walls and therefore exist in the same concentrations on the inlet as well as the outlet sides of the filter.
Micro-organisms are extremely small and include bacteria, viruses and bacteriophages. Typically, bacteria can be as small as 0.2 µm to 4 µm and viruses from 0.3 µm to as small as 0.04 µm. Contamination smaller than 1 µm diameter and, consequently, micro-organisms can pass easily through the compressor inlet filter. Despite their size, these micro-organisms are a serious problem in many industries, because as 'living' organisms they are able to multiply freely under the right conditions. Investigations have established that micro-organisms thrive in compressed air systems with non-dried air at high humidity (100%).
Oil and other contamination act as nutrients and allow micro-organisms to flourish. The most effective treatment involves drying air to a relative humidity of <40% (this can be achieved by using any type of dryer) and fitting a sterile filter in the system. The sterile filter must be fitted in a filter housing that allows in situ steam sterilization or that can be easily opened. Sterilization must be performed frequently to maintain good air quality.