Submerged aerated filters (SAF) or submerged aerated fixed film reactors as they are also known, combine the high biomass concentration (mixed liquor suspended solids) of fixed film processes with high oxygen transfer efficiency through forced aeration. With deep beds, SAF typically requires 25-40% less tank volume than an activated sludge plant for the same duty. Modular construction make SAFs a good solution for smaller operations.

Filter media provide a large protected surface area and large voidage for heterotrophic and nitrifying bacteria to grow and because there is no limitation on air flow through the media, it is possible to use media with higher surface area per unit volume than in trickling filters.