HEPA vs. water filter in vacuum cleaning What is the difference between air being filtered with HEPA or water? Which one performs better within a non-allergic home environment? Does water filter have a meaning still when common vacuum cleaners usually have HEPA filters nowadays?
I am asking because I want to buy a new vacuum cleaner and need to decide between traditional paper bag with HEPA filter and one with water filter (like Zelmer 829.0 SP Aquos or Zelmer 819.0 SP Aquario).
 A: A HEPA filter is defined as one which removes 99.97 of all particulates larger than 3 micrometers it does so by creating a pressure differential between it's input and output between which is placed a fine filter with as large a surface area to trap particulates as to fit its specification.
Depends on your setup, Air extracted from the atmosphere compressed into a set of fine bubbles of air released under pressure in a water column that is being agitated in such a way as to avoid laminar flow of the fluid (nebulising), agitating the bubbles - in order to increase the possibility for particulates to break the surface tension and be "wetted" and therefore filtered before theair components are released back into the atmosphere containing hydrophylic particulates may well work quite efficiently. The same setup in an atmosphere containing hydrophobic particulates - they may well simply be released back into the air as the bubbles burst at the exit air/water interface.
Adding a surfactant or mixture of surfactants (main ingredient in detergents) to the water may alleviate the problem. To avoid bubbling over, an anti-foaming agent may help out. 
I know of no experimental data to support any conclusion as to how efficient it could be made as a system, however it seems to make sense that it may have potential to work well.
