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.
Highlights
Received: 19 December 2021Anaerobic digestion (AD) is one promising biogas-producing process that performs wastewater or solid waste treatment
Most of the tests were carried out under standard drying conditions, i.e., at 105 ◦ C; what about at a lower temperature? it is often customary to put a lid on crucibles to avoid splashing of material with evaporation; would this be sufficient to limit the loss of VFA? Rather than testing different pH conditions, as Derikx et al [14] did, only two conditions of pH and VFA form were tested, with pHs 2.53 and 9.23
These tests were conducted with pure bicarbonate solutions in water, so four questions arise: firstly, whether this decomposition is observed in real anaerobic digestion effluents; secondly, to what extent is this decomposition of the bicarbonate and, this underestimation of dry matter (DM) detrimental; thirdly, how does this underestimation affect the measurement of organic matter (OM); and, fourthly, at which temperature the decomposition begin?
Summary
Anaerobic digestion (AD) is one promising biogas-producing process that performs wastewater or solid waste treatment. Organic matter content ( put as Volatile solids) is usually determined by burning dried samples in an oven at 550 ◦ 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]. Some authors recommend limiting the temperature to 80 ◦ C and extending the drying time to avoid the loss of volatile compounds in silage and anaerobic media [15,16,17,18,19] These papers are old and related to silage with high acid contents and low pH; they are somewhat marginal to our concerns. The present work deals with the possible interference of bicarbonate and volatile acids in the determination of dry matter content in artificial aqueous solutions and real anaerobic samples. The importance of taking into account the bicarbonate decomposition in calculating DM content has been underlined by a mass balance study in real AD media
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