Process
Fixed-Film Secondary Treatment
Attached-growth processes grow the treatment biomass as a biofilm on a support medium — a robust, energy-resilient alternative or complement to suspended-growth activated sludge.
Suspended vs attached growth
In suspended-growth systems such as activated sludge, biomass floats freely in the mixed liquor. In fixed-film (attached-growth) systems, the same biology grows as a biofilm attached to media. The biofilm is retained in the reactor independent of settling, which makes fixed-film processes resistant to sludge washout and to the bulking problems that can affect activated sludge.
Common Fixed-Film Systems
Trickling Filter
Wastewater is distributed over a bed of rock or structured plastic media. Biofilm on the media removes BOD as water trickles down and air moves through the void space. Sloughed biofilm is separated in a downstream clarifier.
Biological Aerated Filter (BAF)
A submerged fixed-film reactor with fine media that provides both biological treatment and filtration in one unit. Air is supplied by diffusers. Compact and well suited to nitrification and to sites with limited footprint.
Rotating Biological Contactor (RBC)
Discs partially submerged in wastewater rotate slowly, alternately exposing the attached biofilm to wastewater and air. Low energy and simple to operate; capacity is set by total disc surface area.
Moving Bed (MBBR)
Free-floating plastic carriers keep biofilm suspended in an aerated tank, combining attached-growth robustness with the flexibility of a mixed reactor. Covered in detail on the dedicated MBBR page.
Engineering Note
Frequently Asked Questions
Fixed-film processes grow the treatment biomass as a biofilm attached to a support medium, rather than suspended in the liquid. Examples include trickling filters, biological aerated filters (BAF) and rotating biological contactors (RBC). Wastewater passes over or through the media and the biofilm removes BOD and, with sufficient area, ammonia.
In a trickling filter, wastewater is distributed over a bed of media (rock or structured plastic). A biofilm grows on the media surface; as wastewater trickles down, organisms remove organic matter while air moves through the void space to supply oxygen. Excess biofilm sloughs off and is separated in a downstream clarifier.
Fixed-film processes are attractive where low operating complexity and energy resilience are priorities, where sludge settling problems (bulking) must be avoided, or as a roughing stage ahead of activated sludge. They tolerate load variation well but can be less flexible for tight nitrogen removal than suspended-growth BNR systems.
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