Continuous CO2 and O2 measurements in the exhaust gas of wastewater treatment plants have been simulated to study their significance for fast process monitoring. More specifically, the question was raised whether the ratio of the carbon dioxide production rate to the oxygen consumption rate (the RQ value) can be used to distinguish C-oxidation from N-removal (nitrification or combined nitrification-denitrification). Although the oxygen uptake rate and carbon dioxide production rate by the micro-organisms are indicative indeed, these rates can not very well be monitored in the gas phase mainly due to the additional CO2 production accompanying alkalinity consumption. Only large changes in nitrification activity can be monitored this way. The RQ is however a strong measure for the COD/TOC ratio of the converted waste. Combination of RQ measurements with TOC measurements can therefore probably replace laborious COD measurements. In plants with combined C-removal and nitrification, the difference in in- and effluent alkalinity is a measure for nitrification. If NH3-removal is determined, the biomass production rate can be calculated from the gas flow rate in combination with the exhaust CO2 and O2 measurements.

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