Bio-Film Control
The main target compound in a biological filter in a recirculation system is ammonia and consequently nitrite. It then becomes imperative to provide the nitrifying bacteria with good conditions.
One of the main factors effecting nitrification is the thickness of the bio film, why in modern biological wastewater treatment emphasis is on bio film control.
Having bio film control, and as the words indicate, is associated with stable performance of a bio filter.
When a bio film starts building up on a carrier surface, more beneficial bacteria become available and it becomes more efficient in its nitrification capacity – until a certain stage.
Effect of Bio-Film control in Salmon Smolt Plant
Retention Time: App. 10 min/passAverage Stocking Density: 70 kg/m3
NH4 - N: < 0.5 mg/LNH4 - N: < 0.3 mg/L
NO2 - N : < 0.2 mg/LNO2 - N : < 0.1 mg/L
BOD5 : 8 mg/LBOD5 : 3 mg/L
CO2 : < 12 mg/LCO2 : < 6 mg/L
TSS : 5-10 mg/LTSS : 2 mg/L
Turbidity : < 1 NTUTurbidity : < 1 NTU

When build up or thickness goes beyond a certain stage, a bio film becomes less efficient and less stable. The build-up is a mix of new bacteria and the decay of older ones. This means we are adding organic matter to the film by its own decay.
This decay of organic matter will use inappropriately much oxygen and leave little left for the nitrifying bacteria, i.e. we are creating excess heterotrophic activity. This means we are not providing the nitrifying bacteria with good conditions as we set out to do.
As further build up takes place, diffusion limitations will occur, especially of oxygen. Oxygen is used rapidly by the heterotrophic activity and apart from leaving very little left for the nitrifying bacteria will also mean that we create anaerobic conditions with cavity formations in the inner parts of the film and it will subsequently fall off. This phenomena is especially related to submerged fixed bed bio filters, a reason they have to be backwashed frequently.   
In a proper designed moving bed bio filter, the movement and agitation of the bio media with the aeration will, apart from supplying plenty of oxygen,  prevent  excessive build up of a bio film.
See further in download “Bio Filtration – Bio film Control”
Clearwater MBBRReferencesIndustrial AquacultureServices