Two bench-scale Runs, namely Run A and Run B, have been investigated in order to study the effect of immobilized bacteria on denitrification in anoxic-oxic process. The hollow polypropylene pellets (O.D. 4 mm, I.D. 3 mm, nominal density 1.001 g/cm3) have been dosed into the anoxic reactor in Run A up to 24% of the volume. Run B was operated with no pellets in the anoxic reactor as a control experiment. Both Runs A and B contained the above pellets in the oxic reactor for a total volume of 24%. Run A showed superiority in nitrogen removal (73%) compared to Run B (65%) at a loading of 0.219 kgN/m3.d for a detention time of 4 hours at 15°C. By further investigating into the mechanism of nitrogen removal, it was concluded that the immobilized bacteria in the anoxic reactor in Run A enhanced not only the denitrification but also the ammonification that resulted in higher nitrogen removal efficiency.

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