Nitrification kinetics are important for process design, optimization and capacity rating of activated sludge wastewater treatment plants. Assessment of nitrification behaviour historically has focused on measuring the nitrifier maximum specific growth rate, μAUT. Very little attention has been directed at the importance of nitrifier organism decay rate, bAUT (also referred to as kD), and in many instances the decay rate has been assumed negligible. However, incorrect assessment of decay rate leads to errors in the μAUT estimate; the magnitude of the error depends on the μAUT measurement method employed. This paper illustrates why decay rate is important when measuring μAUT, and that the decay rate is significant. The paper also explains why measurement methods for nitrifier decay may have underestimated the decay rate. Results from an experiment incorporating improvements to previously suggested methods and data analysis are presented.

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