Evaluation of the 5 and 8 pH point titration methods for monitoring anaerobic digesters treating solid waste

Abstract

Simple titration methods certainly deserve consideration for on-site routine monitoring of volatile fatty acid (VFA) concentration and alkalinity during anaerobic digestion (AD), because of their simplicity, speed and cost-effectiveness. In this study, the 5 and 8 pH point titration methods for measuring the VFA concentration and carbonate system alkalinity (H₂CO₃*-alkalinity) were assessed and compared. For this purpose, synthetic solutions with known H₂CO₃*-alkalinity and VFA concentration as well as samples from anaerobic digesters treating three different kind of solid wastes were analysed. The results of these two related titration methods were verified with photometric and high-pressure liquid chromatography measurements. It was shown that photometric measurements lead to overestimations of the VFA concentration in the case of coloured samples. In contrast, the 5 pH point titration method provides an accurate estimation of the VFA concentration, clearly corresponding with the true value. Concerning the H₂CO₃*-alkalinity, the most accurate and precise estimations, showing very similar results for repeated measurements, were obtained using the 8 pH point titration. Overall, it was concluded that the 5 pH point titration method is the preferred method for the practical monitoring of AD of solid wastes due to its robustness, cost efficiency and user-friendliness.

Keywords:

• alkalinity
• anaerobic digestion
• monitoring
• multiple pH-point titration
• volatile fatty acids

Acknowledgements

Thomas Vannecke is supported by the Research Foundation-Flanders (FWO) through a Ph.D. fellowship. Mark van der Ham and Stefan Blankenborg from Enki Energy B.V. are acknowledged for providing us with the samples of the anaerobic digesters. The Biomath research group and the Department of Forest and Water Management of Ghent University are acknowledged for the use of laboratory equipment.

Supplemental data

Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/09593330.2014.964334