Thank you for the links.
For application purposes, think of a water line that enters into a tank, then at the other side of the tank is the inlet (really the outlet for the tank) of another water line. At the top of the tank is an opening (vent) that has a filter such that when air and water pass into the tank, the air rises to where the filter is. The air that rises will be under some pressure as the water will be under pressure (transmitting of pressure undiminished and what not). The pressure will push the air through the filter but as soon as the air gets through and the water reaches the filter, the water can't get through. The flow rate through the filter need only be able to keep up with the incoming air. Such things can be fixed by using the right shape of filter, increasing the surface area (cylinder shaped filter of required length) to get the best flow possible. I suppose all I need to know is the micron size needed to do it (if it can be done at all).
I know there are better ways of doing it (centrifugal force) but for the application I'm looking for, it can't be done any other way.