Downscaling the chemical oxygen demand test

Abstract

The usefulness of the standard chemical oxygen demand (COD) test for water characterization is offset to some extent by its requirement for highly toxic or expensive Cr, Ag, and Hg species. In addition, oxidation of the target samples by chromate requires a 2–3 h heating step. We have downscaled this method to obtain a reduction of up to ca. 80% in the use and generation of toxic residues and a time reduction of up to ca. 67%. This also translates into considerable energy savings by reducing the time required for heating as well as costly labour time. Such reductions can be especially important for analytical laboratories with heavy loads of COD analyses. Numerical results obtained with the standard COD method for laboratory KHP samples (potassium hydrogen phthalate) show an average relative error of 1.41% vs. an average of 2.14% obtained with the downsized or small-scale version. The average % standard deviation when using the former is 2.16% vs. 3.24% obtained with the latter. When analysing municipal wastewater samples, the relative error is smaller for the proposed small-scale method than for the standard method (0.05 vs. 0.58, respectively), and the % std. dev. is 1.25% vs. 1.06%. The results obtained with various industrial wastewaters show good agreement with those obtained using the standard method. Chloride ions do not interfere at concentrations below 2000 mg_NaCl/L. This highly encouraging proof-of-concept offers a potentially alternative greener approach to COD analysis.

Keywords:

• COD
• green chemistry
• small scale
• waste reduction
• water analysis

Acknowledgements

We thank the Universidad Autonoma del Estado de Mexico (Facultad de Quimica), the Centro Conjunto de Investigaciones en Quimica Sustentable, and CONACYT (Mexico) for equipment usage and funding, as well as major assistance from Dr Veronica Martinez of Centro Interamericano para la Calidad del Agua (CIRA).