MBBR media with biofilm. Photo from AAEES.ORG
Activated sludge systems operate by moving a suspended biomass (MLSS) through an aerated basin and then separating solids for recycle back to the aeration basin. This process was invented in the early 20th century for treating wastewater and many variations on the process exist including contact stabilization, extended aeration, batch reactors, oxidation ditches, and pure oxygen systems. A key operating parameter has been the Food to Microorganisms (F/M) ratio. F/M is traditionally calculated using MLSS or MLVSS as the M and BOD5 as the F. Key is to be consistent and always know what you are using to calculate F/M and identify the best F/M ratio for operations.
Now we move to biofilm type systems versus suspended growth. Here we have tricking filters (old but still good), MBBR, and fixed film media. All relay on a biofilm which is actually just floc grown in attached form. Calculated F/M in this system is difficult since the biomass is attached to media. So we operate any biofilm based system on loading per unit of surface area - this can be BOD5/square meter or Ammonia/square meter. Again just be consistent in calculating.
The key benefit of a biofilm vs suspended growth system is the ability to hold more biomass than comparable activated sludge units which rely totally on solids separation to keep biomass in the aeration tank. Of course this advantage is not present for all influents. For example, systems with significant influent oil & grease can foul (coat) the media which can be a big problem.
Now we move to biofilm type systems versus suspended growth. Here we have tricking filters (old but still good), MBBR, and fixed film media. All relay on a biofilm which is actually just floc grown in attached form. Calculated F/M in this system is difficult since the biomass is attached to media. So we operate any biofilm based system on loading per unit of surface area - this can be BOD5/square meter or Ammonia/square meter. Again just be consistent in calculating.
The key benefit of a biofilm vs suspended growth system is the ability to hold more biomass than comparable activated sludge units which rely totally on solids separation to keep biomass in the aeration tank. Of course this advantage is not present for all influents. For example, systems with significant influent oil & grease can foul (coat) the media which can be a big problem.
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