Abstract: Cleaning of multi-pore diffusion elements in place with cleaning gases while submerged in liquid media by applying said elements in improved gas cleaning apparatus. Such apparatus may for example include diffusion element retaining rings or other retaining devices, positioned at the peripheries of the respective elements, for securing them in gas-tight relationship with their respective plenums for preventing escape of treating and cleaning gas. Another aspect of the apparatus includes diffusers having diffusion elements with upper and lower surfaces bounded by porous peripheral sides, said diffusers also including apparatus for preventing escape of air through the element sides. In another embodiment of diffusers with diffusion elements having upper and lower surfaces bounded by peripheral sides, there are plenums comprising gas tight enclosures that enclose these lower surfaces and that include upstanding walls facing and adjacent to said sides.