Thermal pre-treatment: Getting some insights on the formation of recalcitrant compounds and their effects on anaerobic digestion

Formation of Recalcitrant Compounds during Anaerobic Digestion of Thermally Pre-Treated Sludge: A Critical Macromolecular and Structural Study

“…Lu et al (2018) [20] reported a production of 0.258 g of humic acid g VS −1 at the thermal pre-treatment stage, which is very close to the amount obtained in this study. Moreover, the results obtained are in line with our previous research with a mixture of glucose/glycine pre-treated at the same conditions of time and temperature, where around 30% of COD of the original mixture was transformed into recalcitrant compounds that were not degraded in the anaerobic digestion stage [16].…”

“…This was attributed to the occurrence of dehydration and condensation reactions as a part of the intermediate and final stages of Maillard reactions, suggesting an increased aromaticity. This reduction in hydroxyl groups was also observed by Ribeiro et al (2001) [44] when drying humic substances for 1 h at 120 • C, by Yang et al (2019) [14] in its preparation of artificial humic substances, by Vergnoux et al (2011) [46] in the study of humic substances of burned soils, and by us in the thermally treated mixture of glucose and glycine [16].…”

“…This can be correlated with the increase in peaks at 1630 cm −1 , associated with the early stage [30,31], and 1580 cm −1 and 1510 cm −1 , associated with the intermediate and final stages of Maillard reactions [30,[32][33][34][35][36][37][38]. The appearance of these peaks was described Mendez et al (2014) [50] during the pre-treatment of microalgae; by Rodríguez-Abalde et al (2013) [51] when applying thermal pre-treatment to slaughterhouse waste; by Yang et al (2019) [14] during the preparation of artificial humic acid, and in our previous research dealing with the thermal pre-treatment of a synthetic substrate [16].…”

“…However, IR spectroscopy has gained increasing attention. In our previous research, we identified that the melanoidins formed during thermal treatment of a simple mixture of glucose and glycine presented a characteristic increase in the peaks associated with amide II bonds in the IR spectrum [16]. However, it is not clear if the same could be expected for sludge, considering the complex interactions that may occur.…”

Enhancement of Methane Production in Anaerobic Digestion of Food Waste using Thermal Pretreatment

“…Some researchers have reported a slower methane generation after pretreatments using higher temperatures compared to the control. Also, they have observed higher Maillard reaction products when feedstock concentrations are high (Ortega-Martínez et al, 2021).…”