Determination of Chemical Oxygen Demand in Heterogeneous Solid or Semisolid Samples Using a Novel Method Combining Solid Dilutions as a Preparation Step Followed by Optimized Closed Reflux and Colorimetric Measurement Joan Noguerol-Arias,* ,† Angela Rodríguez-Abalde, † Eva Romero-Merino, † and Xavier Flotats †,‡ † IRTA, GIRO Joint Research Unit IRTA-UPC, Torre Marimon, E-08140 Caldes de Montbui, Barcelona, Spain ‡ Department of Agrifood Engineering and Biotechnology, Universitat Polite ̀ cnica de Catalunya (UPC), Parc Mediterrani de la Tecnologia Edifici D-4, E-08860 Castelldefels, Barcelona, Spain * S Supporting Information ABSTRACT: This paper reports the development of an innovative sample preparation method for the determination of the chemical oxygen demand (COD) in heterogeneous solid or semisolid samples, with high suspended solids and COD concentrations, using an optimized closed reflux colorimetric method. The novel method, named solid dilution (SD), is based on a different technique of sample preparation, diluting the sample with magnesium sulfate (MgSO 4 ) previous to COD determination. With this, it is possible to obtain a solid homogeneous mixture much more easily analyzable. Besides, a modification of concentration and ratio of reagents was optimized to make the closed reflux colorimetric method suitable for complex substrates with COD levels ranging from 5 to 2500 g O 2 kg -1 TS. The optimized method has been tested with potassium hydrogen phthalate (KHP) as primary solid standard and using different solid or semiliquid substrates like pig slaughterhouse waste and sewage sludge, among others. Finally, the optimized method (SD/SM-CRC) was intensively tested in comparison to the standard titrimetric method (SM-ORT) using different certified reference materials (CRM). The developed method was found to give higher accuracy, 1.4% relative standard deviation (RSD) vs 10.4%, and bias of 2.8% vs 8.0%, in comparison to the standard open reflux titrimetric method. C hemical oxygen demand (COD) is defined as the amount of oxygen in the form of a strong oxidizing agent consumed in the oxidation of organic compounds, and it is used as one of the main water quality parameters at wastewater treatment facilities. Mathematical models such as the ASM for activated sludge processes 1 and the ADM1 for the anaerobic digestion processes 2 use COD as a key measure for organic components variables. The increasing interest of the anaerobic digestion process for treatment of a wide range of organic waste types has led to the necessity to obtain reliable measures of the organic matter content of the waste substrates and its transformed products, in order to characterize the process efficiency. COD is the best unit to express organic matter, since it is conservative along the biological processes and, therefore, its measure is a limiting factor for having good substrate and process characterizations. While the COD determination for waters is currently performed without significant problems, these appear when this measure is applied to solid or semisolid heterogeneous waste with high fat contents, such as manure or slaughterhouse solid waste, obtaining results with large confidence intervals. The reliable measure of COD of solid substrates has become an issue of concern for many laboratories dealing with solid organic matter characterization, but there is a lack of references about the adequate procedures. 3-5 Recently, some researchers have applied the APHA- AWWA-WPCF, Standard Methods for the Examination of Water and Wastewater, 6 for COD determination of solid samples 7,8 or high suspended solid content samples, such as poultry and cattle manure, 9 pig manure, 10 olive mill waste- water, 11 or municipal waste sludge, 12 among others. Since the APHA-AWWA-WPCF method of COD determination has been mainly used for water and wastewater samples (values below 1 g O 2 L -1 ), it seems to be inappropriate for solid or semiliquid samples having a high percentage of suspended solids, due to the heterogeneity of the substrate. To solve this problem, several fold dilutions of the original sample were used, with the corresponding loss of accuracy and repeatability in the COD values of replicates, as reported by Liao 13 and Yadvika. 4 On the other hand, Raposo 5 demonstrated that their modified and optimized method for determining COD, based on the DIN 38414-S9, 14 was applicable to solid substrates and Received: February 15, 2012 Accepted: June 5, 2012 Published: June 5, 2012 Article pubs.acs.org/ac © 2012 American Chemical Society 5548 dx.doi.org/10.1021/ac3003566 | Anal. Chem. 2012, 84, 5548-5555