Preface:  There have been institutional tendencies to apply aeration to just the first (or early) stages of RBC trains to remove growth from those units that typically exhibit the heaviest biomass loads.  This circumstance is an example where Mooers Products can share feedback with designers and operators from decades of experiences and scores of installations the unintended consequences of this scheme.

Considerations of Partial or Intermittent RBC Aeration

Criteria for sizing RBCs typically assumes that SS entering an RBC system exit the system as those same SS while SBOD is converted to biomass on the media, sloughed off, settled and removed from the system.  But three decades of experience has clearly demonstrated that as the biomass forms on the media the organisms adsorb the organic SS contributing to that mass (Concentrations of these SS vary by characteristics of raw WW and efficiency of primary treatment.  Fine screens for primary solids removal, for example, allow a large fraction of SS to escape to the RBCs, exacerbating this problem).

As the flow exiting early-stage aerated RBCs transports bio sludge (comprising both active biomass and the generous food source in those primary SS) scoured off the media, into an unaerated following stage, these active SS adhere to that media simply transferring the problem downstream.  In cases where those later stages are Hi-Density media the effect of this phenomena is multiplied by the comparative media area.

It follows then that the entire RBC systems be continuously aerated to (1) preclude conditions that invite mechanical overload problems and (2) increase the treatment capacity of the plant by, according to some reports, up to 25%.There are cases where aeration was mistakenly regarded simply as a method to periodically knock excess biomass load off the RBC media to (1) minimize risk of mechanical failure of media, attachments or shaft, or (2) temporarily rid the RBCs of nasty biomass that interferes with healthy growth.  A regimen of only INTERMITTENT AIR SCOUR, a marginal compromise forced by inadequate blowers or air piping capacity, deprives a plant of the advantages of a continuous supply of PROCESS AIR.

When outfitting a new or upgraded facility with SideCar RBC Aeration, considering the loss of plant treatment capacity and reliability, there are NO SAVINGS and a huge downside in under sizing blowers and air piping.  There is no more economical or effective in-tank aeration option to handle continuous or hi-flow scour than SideCar and the installed cost of optimum size vs undersize or marginal pipe is minimal. And, as discussed above, experience has confirmed that installing only partial aeration for RBC systems creates operating complications and serious hazards of biomass overload on unaerated RBCs.