One waste and two products: choosing the best operational temperature and hydraulic retention time to recover hydrogen or 1,3-propanediol from glycerol fermentation.

This study aimed to compare the production of hydrogen and 1,3-propanediol from crude glycerol (10 g/L) in mesophilic (30 °C) and thermophilic (55 °C) anaerobic fluidized bed reactors, namely AFBR30 °C and AFBR55 °C, respectively, at hydraulic retention times (HRT) reduced from 8 to 1 h. In AFBR30 °C, the absence or low hydrogen yields can be attributed to the production of 1,3-propanediol (maximum of 651 mmol/mol glycerol), and the formation of caproic acid (maximum of 1097 mg/L) at HRTs between 8 and 2 h. In AFBR55 °C, the hydrogen yield of 1.20 mol H2/mol glycerol consumed was observed at the HRT of 1 h. The maximum yield of 1,3-propanediol in AFBR55 °C was equal to 804 mmol/mol glycerol at the HRT of 6 h and was concomitant with the production of hydrogen (0.87 mol H2/mol glycerol consumed) and butyric acid (1447 mg/L).

Anaerobic Biodegradation of Biodiesel Industry Wastewater in Mesophilic and Thermophilic Fluidized Bed Reactors: Enhancing Treatment and Methane Recovery.

In the past few years, the extraction of value-added compounds from the anaerobic digestion of glycerol has been an option to add value to this waste because biodiesel production is increasing worldwide. The evolution of research on glycerol valorization by anaerobic digestion has reached the use of high-rate reactors. However, no study has evaluated glycerol digestion in an anaerobic fluidized bed reactor (AFBR), a configuration with potential advantages in methane production. Still, the best operating temperature for high-rate glycerol digestion remains unclear. To clarify these gaps, the present study aimed to compare glycerol digestion in mesophilic AFBR (30 °C) and thermophilic AFBR (55 °C). In both reactors, glycerol concentration was increased from 1.0 to 7.0 g L-1 at a fixed hydraulic retention time of 24 h, resulting in an increase at the organic loading rate from 1.2 to 7.6 kg COD m-3 day-1. Thermophilic digestion of glycerol achieved superior removals of organic matter (67.7-94.2%) and methane yield (330.8 mL CH4 g-1 COD) than the mesophilic digestion (48.6-93.0% and 266.6 mL CH4 g-1 COD). Additionally, the application of the kinetic model of substrate utilization (modified Stover-Kincannon model) indicated a higher substrate utilization coefficient in the thermophilic AFBR (23.09 g L-1 day-1) than the mesophilic AFBR (7.14 g L-1 day-1). Therefore, the application of glycerol concentrations higher than 7.0 g L-1 in thermophilic AFBR should be further investigated. Also, given only operational results, the application of the AFBR in the two-stage anaerobic digestion of glycerol is recommended.