# Underestimation of Dry Matter of Anaerobic Media with High Bcarbonate Concentration
## 3 Results and Discussion
### Bicarbonate Interference  
#### 3.2.1 Pure Bicarbonate Solutions  
Sodium bicarbonate solutions at 10 g/L, i.e., concentrations in the order of magnitude of those found in anaerobic digesters, were in turn heated to 70 and 105 \({}^{\circ}\)C overnight. Table 2 shows the losses observed at 70 and 105 \({}^{\circ}\)C for the 10 g/L solutions, as well as for 2-5 g of mussel and slipper shells deposited in about 30 g of water. These results show that bicarbonate decomposes, even at 70 \({}^{\circ}\)C, at a ratio of about 36%. This implies that the DM contents of a medium containing bicarbonate ions are correspondingly underestimated, especially for samples highly charged of bicarbonate content. To our knowledge, these surprising results have never been shown in the field of anaerobic digestion for biogas production. In fact, the thermal decomposition of bicarbonate is well known and corresponds to the following stoichiometry:  
\[2\ \mathrm{NaHCO_{3}}<=>\ \mathrm{Na_{2}CO_{3}}+\mathrm{CO_{2}}+\mathrm{H_{2}O}.\]  
This decomposition starts at 50 \({}^{\circ}\)C and increases from 70 \({}^{\circ}\)C [20, 21]. It is at the origin of the outgassing of CO\({}_{2}\), for example, during the baking of bread. Numerically, two moles of \(\mathrm{NaHCO_{3}}\), i.e., 168 g, will generate a loss of one mole of water and one mole of CO\({}_{2}\), i.e., a total of 62 g, or 36.9% of the initial mass. In anaerobic digestion media, bicarbonate is not systematically associated with the sodium ion; therefore, the loss of mass may be different, depending on the cations, but it will be of the same order of magnitude. For instance, the same calculation for potassium bicarbonate gives a mass loss of 31.0% (62 g loss as water and carbon dioxide for two moles of \(\mathrm{KHCO_{3}}\), i.e., 200 g). Sodium carbonate is stable and will be found in the ash. This reaction result is, therefore, an underestimation of the dry matter content of media containing a lot of bicarbonate. Finally, the results in Table 2 show that mussel or slipper shells do not interfere with the DM measurement. Solid calcium carbonate (the main constituent of the shell) is not affected by drying. As the decomposition reaction is identical at 70 and 105 \({}^{\circ}\)C, only the latter temperature was used for the next tests. These tests were carried out with concentrations of 10 g/L, what about more concentrated solutions and, moreover, how to be sure that the mechanism is temperature-dependent? To answer those questions, new tests were conducted with an aliquot of a 50 g/L bicarbonate solution, heated to 105 \({}^{\circ}\)C overnight and freeze-dried in parallel. The freeze-dried bicarbonate solution was then heated to 105 \({}^{\circ}\)C overnight (Table 3). The results, using a five-more concentrated pure sodium bicarbonate solution, are identical to the previous ones, with an estimated loss of 37.4%. Conversely, freeze-dried \begin{table}
\begin{tabular}{c c c} \hline
**Samples Tested** & **Loss at 70 \({}^{\circ}\)C** & **Loss at 105 \({}^{\circ}\)C** \\ \hline Sodium bicarbonate solution (10 g/L) & 35.9 \(\pm\) 0.5\% & 35.9 \(\pm\) 0.1\% \\ Mussel shell & 0.9 \(\pm\) 0.1\% & 0.8 \(\pm\) 0.3\% \\ Slipper shell & 0.7 \(\pm\) 0.05\% & 0.7 \(\pm\) 0.3\% \\ \hline \end{tabular}
\end{table} 
Table 2: Bicarbonate and shell losses after drying overnight at 70 and 105 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c} \hline
**Samples Tested** & **Loss at 105 \({}^{\circ}\)C** & **Loss after Freeze-Drying** \\ \hline Bicarbonate solution (50 g/L) & 37.4 \(\pm\) 0.0\% & 3.9 \(\pm\) 0.3\% \\ Bicarbonate (50 g/L) solution & 35.0 \(\pm\) 0.3\% & – \\ after freeze-drying & & \\ \hline \end{tabular}
\end{table} 
Table 3: Bicarbonate losses of a concentrated solution after heat-drying and lyophilization. aliquots of this solution exhibited a loss of about 4%, confirming the hypothesis of a purely thermal decomposition of the bicarbonate, concomitant with the dehydration of the sample.