This paper describes the development and accuracy analysis of a single-stage respirometer which can be used both in the laboratory for wastewater characterization and in the plant as a process instrument. It is based on an accurate model of parasitic aeration, making the two-stage assumption unnecessary. Its operation is supervised by a real-time software, written in Lab View, managing the various measurement procedures and estimating the wastewater characteristics. Its accuracy is assessed through sensitivity and error propagation analysis, proving superior to the conventional model. A laboratory implementation of the instrument was tested with readily degradable substrate, yielding consistent and accurate respirograms.

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