 We are talking about the oxygen delivery system but in our previous module we talk about the first part of the oxygen delivery system that is the compressor. But in this module we try to discuss the air sterilization system that is very critical. So the sterilization of the inlet air is undertaken to be prevent from the contaminants because that has to enter into the reactor or a fermenter. So by the same side the exit air on the other hand is should be sterilized not only to keep the contaminant from entering but also prevent the organism to release from the vessel just for containment level just to save the informant. The common method on inlet on the exit that is mostly the cheapest way is a filtration. So the small scale fermenters with the volume less than 5 liters, disc shape, hydrophobic Teflon membrane in a polypropylene housing filters are used. Tough nature of the Teflon and the reusable properties of such filters give them a positive point for their use. You can see in this slide that Teflon type fermenter and then you can see in this picture that how that is designed. You can see that is having a Teflon membrane inside that and then this whole Teflon membrane is packed under in a polypropylene material because that polypropylene material has the property to withstand its shape and quality under sterilization conditions because we have to autoclave it before using that. So that is mostly used up to 1000 liters. So by plating the membrane it is possible to create the compact filters with a very large surface because the Teflon membrane when you compact inside the polypropylene that create a maximum area because as much as there will be the more area then there is a more chance and there will be a less pressure is required to pass the air from the filter. So if the filter is very tight and then there is a less area and the pores are available then the air quantity that we require for the fermentation media is not sufficient. So that can reduce that pressure which we require inside the fermentation vessel because when there is a less pressure from the outside and the compressor having not the required and the filter either having a small pores either choke with some dust particles or with some others then there is a chance of the less supply of the air to the fermenter vessel or a fermentation medium. So that is why when we say that air filtration increase the filtration area then decrease the pressure required to pass a given volume from the air through the filter compressor give the suitable pressure air and then that is why the compressor is needed and then the suitable pressure of 250 kilo Pascal is required and then the type of that filter is very critical in this whole air sterilization system. So in our later module we will talk about another type of filters that mostly used on industrial scale fermenters but on laboratory scale fermenters mostly these are the small filters are used in the fermentation design.