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

Abstract

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 %).

Combination of Dry Milling and Separation Processes with Anaerobic Digestion of Olive Mill Solid Waste: Methane Production and Energy Efficiency

“…Indeed, high temperature and pressure conditions can lead to the formation of recalcitrant compounds, which affect the anaerobic digestion process [31]. However, after treatment at 148 °C, olive pomace produced 70% more methane than for the raw sample [19]. A similar increase in methane yield was observed in the current study with T olive pomace after ultrafine milling.…”

“…Donoso-Bravo et al reported the effect of milling on the kinetic parameter values of raw and 3 mm milled samples. The maximum methane production rate was enhanced, although only a 5% methane yield increase was obtained [19]. Fenton reagent addition and steam explosion pretreatments had a negative impact on methane production, achieving 50% of reduction for steam explosion [19,30].…”

“…The maximum methane production rate was enhanced, although only a 5% methane yield increase was obtained [19]. Fenton reagent addition and steam explosion pretreatments had a negative impact on methane production, achieving 50% of reduction for steam explosion [19,30]. Indeed, high temperature and pressure conditions can lead to the formation of recalcitrant compounds, which affect the anaerobic digestion process [31].…”

“…Electrostatic separation has previously been applied to wheat bran [16], oil cakes [17], and rice straw [18], although it has yet to be combined with anaerobic digestion. In a study on 3 mm screen size milling before anaerobic digestion of OP, the methane maximum production rate was enhanced, although no impact was observed on the methane yield [19]; in addition, the energy requirements in this study were not addressed. More generally, literature concerning the comparison of energy consumption and energy efficiency of chemical, physicochemical, and mechanical treatment of OP biomass still remains scarce.…”

Is anaerobic digestion a feasible alternative to the combustion of olive mill solid waste in terms of energy production? A critical review

“…less than 10% of difference. 18,27,30,37 Otherwise, some authors described methane production values markedly lower than the required, resulting in very low net energy production. For instance, Siciliano, et al 24 reported an unusual methane yield coefficient of 34 mL CH4/g VS, which resulted in a production of 44 kJeq/kg OMSW of thermal energy and a deficit of electricity, i.e.…”