A novel method is presented to measure the overall biofilm respiration rate in a submerged fixed bed reactor. The method, named “double gassing-out” is based on the measurement of the oxygen uptake rate under two different conditions: (i) replacing the air flow rate by nitrogen in the biological reactor, ensuring the conservation of the same hydraulic conditions and (ii) measuring the oxygen displacement rate by nitrogen in an identical reactor design with no microorganisms. The difference between these measurements gives the overall biofilm respiration rate. Results obtained in a nitrifying fixed bed reactor are presented, compared to classical techniques and discussed.

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