 As we have seen in our previous module about the filter sterilization of the medium, but in this module we will talk about the filter sterilization of the fermentation air. So as concern, when our fermentation process is aerobic, then there is the need of the supply of the air or the oxygen to the fermentation vessel just to meet the metabolic requirement of the fermentation. So when we are dealing with the aerobic fermentation, then there is the sterilization of the air is very critical. So we have the two choices. One is the direct heat method or a steam and the most commonly used method for the air sterilization is the filtration. So when we are dealing with the sterilization of the air with the filtration, then the fixed pore filters, which have an absolute rating are a very widely used in the fermentation industry. And the several manufacturers produce the filtration system for air sterilization. So these systems like those of the sterilization of the liquid consist of pleated membrane cartage designed to be accommodated in stainless steel modules. So the most common construction material used for the pleated membrane for air sterilization is the polytetrafluoroethylene, commonly known as PTFE, polytetrafluoroethylene, which is the hydropobic and is therefore resistant to the wetting. So because this having the hydrophobic in nature, then there is the resistant to the wetting. So the filter will not be wetted. So as also the PTFE filters may be steam sterilized because if we have to use those filter, we have to first steam sterilize just to ensure that the filter should be free from that. So our resistant to ammonia, which may be injected into the air steam prior to the filter for pH control. Sometime we have to control the pH. So as far as the filter sterilization of the liquid, it is essential that the pre-filter is incorporated upstream of the absolute filter. So the pre-filter tap large particles, such as dust, oil, carbon, that mostly come either from the air or from the compressor and the pipe scale and the rust. Because when there is some chemical work, then there should be some scaling and some the rest particles that can become from the air. So the use of the coal sink pre-filters. So we can design these course filters by just in different mechanisms. So the pre-filters also ensure the removal of water from the air because sometimes of having large humidity. So entrained water is coal assessed in the filter. So air flow being from the inside of the filter to the outside and is discharged via an automatic drain. So if we have the filtration and then the removal of the water molecules, then that should be, so if you see that in our previous, so if it's the filter, so then the whole water is contained and that be removed by the discharge. So you can see in this slide that is the poor filtration. This is the filter mostly used for air sterilization. So you can see here on the top that 0.2 micron meter having PTFE filter. So this filter is mostly used for small scale, lab scale filters, small scale, lab scale, I can say that lab scale fermenters. So we are dealing with the small scale air sterilization. But as concerned, the large scale filtration, we are mostly using the HEPA filters.