A novel hybrid respirometric principle is proposed that is particularly suited to sludge and wastewater characterisation in the context of activated sludge process models. Advantages of two respirometric principles are combined and their disadvantages eliminated to increase measuring frequency and precision. Emphasis is put on decreasing the bias in parameter estimates that results from the use of unreliable sensor constants in the calculation of respiration rates. To this end checks for dissolved oxygen probes, aeration systems and pumps are built into the respirometer's operation. Checks are to be run while the respirometric batch experiment is conducted so that between-experiment variation is eliminated and within-experiment variation is minimised. It is also stressed that a combined sensor/process model should be used to estimate the process parameters rather than a sequential procedure in which the sensor constants are first used to calculate respiration rates, that are subsequently used for sludge and wastewater characterisation. Finally, three possible practical implementations of the new principle are discussed in relation to maximising parameter estimation accuracy.

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