# Underestimation of Dry Matter of Anaerobic Media with High Bcarbonate Concentration
## 1 Introduction  
Anaerobic digestion (AD) is one promising biogas-producing process that performs wastewater or solid waste treatment. The characterization of the feedstock and digestate in anaerobic digesters is the key to a successful realization of AD. In the anaerobic digestion world, the dry matter (DM) and organic matter (OM) contents of anaerobic samples are the major data for the design, handling, and maintenance of the processing of biogas plants [1]. Many national and international standards and technical guidelines emphasize the importance of DM and OM measurement for the characterization of substrates, such as French norm NF EN 15934 [2], German standard VDI 4630 [3], and ISO 20675:2018 [4]. The determination of DM and OM in biowaste, such as sewage sludge, wastewater, animal slurries, and digestate, in wastewater treatment and biogas plants is usually achieved according to standard protocols [2, 5]. Dry matter content (also put as total solids) is, therefore, determined by drying the samples at 105 \({}^{\circ}\)C until constant weight, usually expressed in the percentage of fresh weight. Organic matter content (also put as Volatile solids) is usually determined by burning dried samples in an oven at 550 \({}^{\circ}\)C until constant weight, expressed in the percentage of dry matter. Both DM and OM contents are used to realize the mass balance check in anaerobic digesters [6]. In addition, the biochemical methane potential (BMP) of one specific substrate is always reported and expressed in thevolume of methane produced by one gram, kilogram, or tonne of DM or OM [7; 8; 9; 10; 11]. They are among the first and most important parameters to be analyzed in biogas reactors at the industrial scale [12; 13]. Hence, an inaccurate measure of DM or OM could inevitably lead to the wrongful interpretation of the experimental results and, therefore, a potential failure of anaerobic digestion. However, these protocols were developed for sludge, biowaste, and other substrates containing relatively few substances that could interfere with the measurement. For instance, the French NF EN 15934 standard, related to \"Sludge, treated biowaste, soil and waste--Calculation of dry matter content by determination of dry residue--or water content\", states that: \"Samples may change during drying, e.g., by absorption of carbon dioxide in the case of alkaline samples or oxygen by reducing substances. Volatile compounds evaporating at 105 \({}^{\circ}\)C are measured as water according to the present procedure.\" Obviously, loss of volatile compounds during DM determination may lead to important errors in mass balances. However, the standard protocols, as mentioned above, do not specify any potential volatile compounds influencing DM determination. The literature focused on ammonia, volatile acids, and alcohols. The proteinaceous biomass, made up of proteins such as shellfish sub-products and manures, will generate ammonium ions, which can be transformed into volatile ammonia. Considering the acidity constant of ammonium to ammonia (pK\({}_{\text{a}}\) = 9.24 at 25 \({}^{\circ}\)C), the latter can be considered insignificant for pH below 8.0. However, as early as 1994, [PERSON] et al. [14] showed that all ammonia is lost for pH above 8.0 for pig, cattle, and poultry manure. Volatile acids, whose pK\({}_{\text{a}}\) are close to 4.8, can also affect the dry matter measurement. However, considering that volatility only concerns non-ionised fraction, a neutral pH would prevent their interference in the dry matter measurements. Finally, the degradation of organics will generate carbonate and bicarbonate ions, which can be transformed into carbon dioxide. For samples highly charged of bicarbonate content, the thermal degradation of bicarbonate, during drying at 105 \({}^{\circ}\)C, could not be neglected. To our knowledge, no publication in the literature was interested in the possible interference of bicarbonate ions with DM measurements, which is the main topic of this work.