Underestimation of Dry Matter of Anaerobic Media with High Bicarbonate Concentration

Abstract

In the anaerobic digestion world, the dry matter (DM) and organic matter (OM) contents of the samples are major data for the design, handling, and maintenance of the processing of biogas plants. Volatile compounds, such as volatile fatty acids and ammonia, are likely to volatilize during drying at 105 °C and, thus, distort DM values; they have been described at length in the literature. On the other hand, the partial decomposition of bicarbonate, present in the digestion media, has been little described in the scientific literature. In this work, it appears that, in accordance with the stoichiometry of the partial decomposition of bicarbonate into CO2 and H2O, about 37% is volatilized as early as 70 °C and a fortiori at 105 °C, whether in aqueous solutions of pure bicarbonate or in different real digester media. With freeze-drying, no degradation of bicarbonate was observed, confirming the thermo-dependence of this reaction. This decomposition leads to an underestimation of DM measurements and some erroneous DM mass balances. It also led to an indirect overestimation of the OM, when expressed as a percentage of the DM.

Keywords

• anaerobic digestion
• volatile fatty acids
• alkalinity
• total solids