# Underestimation of Dry Matter of Anaerobic Media with High Bcarbonate Concentration
## 3 Results and Discussion
### Bicarbonate Interference
#### 3.2.5 Influence of Temperature on the Bicarbonate Decomposition  
Finally, one question that remains concerns the temperature at which this decomposition takes place. It has been shown above that the loss of bicarbonate is significant as early as 75 \({}^{\circ}\)C. This concerns the measurement of dry matter; however, if this decomposition were  
Figure 1: Schematic DM and OM balances, considering the bicarbonate decomposition. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **[NaHCO\({}_{3}\)] (g/L)** & **Without Correction** & **With Correction** \\ \hline Influent, over 20 days feeding & average: 9.38 & 48.8 kgDM & 54.0 kgDM \\ Effluent, over 20 days feeding & average: 18.2 & 38.4 kgDM & 48.8 kgDM \\ DM conversion & – & 21.4\% & 10.5\% \\ OM conversion & – & 21.7\% & 21.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mean bicarbonate content and overall DM and OM balances for a mesophilic pilot plant, considering or not the bicarbonate loss. significant, for example at 55 \({}^{\circ}\)C, systematic losses of buffering capacity could be observed in thermophilic reactors. To clarify this point, two complementary techniques were implemented: firstly, a measurement of the mass loss of aqueous solutions of sodium bicarbonate and solid sodium bicarbonate for different temperatures, using a thermo-balance; and, secondly, a measurement of the mass loss of a sample of solid sodium bicarbonate with a TGA/DTA device. Approximately 4 g of solid sodium bicarbonate was placed in the moisture analyzer cup and heated to 50, 60, and 70 \({}^{\circ}\)C for 1 h. Figure 2a shows that no mass loss was observed at 50 and 60 \({}^{\circ}\)C and 10 \(\pm\) 1% was lost at 70 \({}^{\circ}\)C after about one hour. This result is not contradictory to the results of paragraph 3.2.1, in which the solid bicarbonate samples were placed in the oven overnight. These results suggest slow kinetics, which will be checked using TGA/DTA, as discussed below. These experiments were performed in triplicate. Figure 2b shows that at 50 \({}^{\circ}\)C in 40 min, the mass loss is statically negligible, but it is rapid at 70 and 105 \({}^{\circ}\)C. After about 40 min at 70 \({}^{\circ}\)C and 22 min at 105 \({}^{\circ}\)C, the masses of the solutions stabilise at about 0.13 \(\pm\) 0.01 g, which means a loss of bicarbonate of 0.07 \(\pm\) 0.01 g or 35 \(\pm\) 5%. The precision of the instrument does not allow for a better accuracy. For technical reasons, it was unfortunately not possible to test other temperatures between 50 and 70 \({}^{\circ}\)C  
To confirm the temperature at which the decomposition reaction starts significantly, a 117 mg sample of solid sodium bicarbonate was placed in the TGA/DTA cup. Given the maximum mass allowed by the apparatus, a solution could not have been used, as the loss of mass due to decomposition would not have been measurable. The apparatus was programmed to stabilise for one hour at 50 \({}^{\circ}\)C, rise to 55 \({}^{\circ}\)C in 5 min, stabilise again for one hour at 55 \({}^{\circ}\)C, and so on, in successive steps of 5 \({}^{\circ}\)C up to 105 \({}^{\circ}\)C, finally maintained for 2 h (Figure 3). The figure shows that the mass does not vary up to a temperature of 70-75 \({}^{\circ}\)C and kinetics are relatively slow; the loss of mass is 32% at 105 \({}^{\circ}\)C after 14 h of this experiment. These results show that sodium bicarbonate decomposition only manifests itself measurably at temperatures above 70 \({}^{\circ}\)C, meaning that this loss will influence DM measurements by the proper functioning of digesters operating under thermophilic conditions at 55 \({}^{\circ}\)C.  
Figure 2: (**a**) Mass evolution of 4 g of solid sodium bicarbonate in the moisture analyzer, set at 50 \({}^{\circ}\)C (blue line), 60 \({}^{\circ}\)C (orange line), and 70 \({}^{\circ}\)C (green line).