A lab-scale biofilm reactor for simultaneous removal of phosphorus and nitrate was operated for one and a half years. Despite using only well defined synthetic wastewater and well defined operation, the activity varied significantly over the months. It was speculated that microbial population shifts were causing this phenomenon. This could also explain a sudden break down of the process following a slight change in the operation. Over shorter periods of time (time-scale:days), the biofilm could be considered stable enough to perform series of comparable batch experiments. Batch experiments with different start concentrations of acetate, nitrate or phosphate were conducted. These verified 0.5 and 0 order removal rates in the bulk water depending on the concentration. This was taken as an indication of a zonation of the biofilm. Due to the measured variability in the activity and due to the importance of the history of the bacteria when considering biological P removal, on-line measurements are strongly recommended for research on this subject. Microbial characterisation methods are recommended as an assisting tool in further research.

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