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

Thermal hydrolysis is a common pre-treatment, used before anaerobic digestion processes, to enhance the hydrolysis rate. However, formation of inhibitory compounds and the increase of liquid fraction colour have been identified as potential drawbacks. This study was oriented to study the methane production from simple substrates, subjected to thermal hydrolysis. A mixture of glycine and glucose at different concentrations was prepared, at a ratio similar to proteins and carbohydrates found in activated sludge. Two temperatures were tested. At 120 °C a decrease on biogas production rate was observed. On the other hand, at 165 °C generation of recalcitrant material was observed, causing a decrease in methane potential and COD degradation, when a mixture of glycine and glucose was used as substrate. This was atributed to the formation of recalcitrant compounds via Maillard reaction, hyphothesis supported by FTIR-ATR, which indicated the formation of amide II Bonds.

Impact of milling, enzyme addition, and steam explosion on the solid waste biomethanation of an olive oil production plant.

Anaerobic digestion is a consolidated bioprocess which can be further enhanced by incorporating an upstream pretreatment unit. The olive oil production produces a large amount of solid waste which needs to be properly managed and disposed. Three different pretreatment techniques were evaluated in regard to their impact on the anaerobic biodegradability: manual milling of olive pomace (OP), enzyme maceration, direct enzyme addition, and thermal hydrolysis of two-phase olive mill waste. The Gompertz equation was used to obtain parameters for comparison purposes. A substrate/inoculum ratio 0.5 was found to be the best to be used in anaerobic batch test with olive pomace as substrate. Mechanical pretreatment of OP by milling increases the methane production rate while keeping the maximum methane yield. The enzymatic pretreatment showed different results depending on the chosen pretreatment strategies. After the enzymatic maceration pretreatment, a methane production of 274 ml CH4 g VS added (-1) was achieved, which represents an improvement of 32 and 71 % compared to the blank and control, respectively. The direct enzyme addition pretreatment showed no improvement in both the rate and the maximum methane production. Steam explosion showed no improvement on the anaerobic degradability of two-phase olive mill waste; however, thermal hydrolysis with no rapid depressurization enhanced notoriously both the maximum rate (50 %) and methane yield (70 %).