Possible complication regarding phosphorus removal with a continuous flow biofilm system: Diffusion limitation

Diffusion limitation of phosphate possibly constitutes a serious problem re garding the use of a biofilm reactor for enhanced biological phosphorus rem oval. A lab-scale reactor for simultaneous removal of phosphorus and nitrat e was operated in a continuous alternating mode of operation. For a steady- state operation with excess amounts of carbon source (acetate) during the a naerobic phase, the same amount of phosphate was released during the anaero bic phase as was taken up during the anoxic phase. The measured phosphorus content of the biomass that detached during backwash after an anoxic phase was low, 2.4 +/- 0.4% (equal to 24 +/- 4 mg P/g TS). A simplified computer model indicated the reason to be phosphate diffusion limitation and the mod el revealed a delicate balance between the obtainable phosphorus contents o f the biomass and operating parameters, such as backwash interval, biofilm thickness after backwash, and phase lengths. The aspect of diffusion is con sidered of crucial importance when evaluating the performance of a biofilte r for phosphate removal. (C) 2001 John Wiley & Sons, Inc.