Optimization of the determination of chemical oxygen demand in wastewaters

Chemical oxygen demand (COD) is one of the most relevant chemical parameters for the management of wastewater treatment facilities including the control of the quality of an effluent. The adequacy of decisions based on COD values relies on the quality of the measurements. Cost effective management of the minor sources of uncertainty can be applied to the analytical procedure without affecting measurement quality. This work presents a detailed assessment of the determination of COD values in wastewaters, according to ISO6060:1989 standard, which can support reduction of both measurement uncertainty and cost of analysis. This assessment includes the definition of the measurement traceability chain and the validation of the measurement procedure supported on sound and objective criteria. Detailed models of the measurement performance, including uncertainty, developed from the Differential Approach, were successfully validated by proficiency tests. The assumption of the measurement function linearity of the uncertainty propagation law was tested through the comparison with the numerical Kragten method. The gathered information supported the definition of strategies for measurement uncertainty or cost reduction. The developed models are available as electronic supplementary material, in an MS-Excel file, to be updated with the user's data.

Variation of the percentage contribution, Pj, of the major uncertainty components, of the measurement expanded uncertainty (U) and of the target measurement expanded uncertainty [UTg(HS)] with the COD value.Figure optionsDownload as PowerPoint slideHighlights
► Optimization of the determination of COD based on models of uncertainty components.
► Uncertainty propagation law and numerical Kragten method produced equivalent results.
► Uncertainty propagation law allowed optimizing sensitivity coefficient of variables.
► Strategies for uncertainty magnitude and cost reduction were developed.
► ISO6060:1989 standard is adequate for assessment of compliance with Directive 91/271/EEC.