Effect of physico-chemical treatment on bio- methane production from marine alginate

Abstract

Marine macroalgae are receiving increasing attention as attractive renewable resources for biofuels production with many advantages over biomass from terrestrial biomass. Among the macroalgae, massive brown algae are primarily composed of polysaccharides such as alginate, fucoidan, mannitol, and trace cellulose. The main polysaccharide, alginate, accounts for up to 40% dry wt. in brown algae as a principal material of the cell wall [1, 2]. A high yield of organic acids can be theoretically available by simple decomposition of alginate and maintaining its carboxyl group structures [3, 4]. However fermentative production of organic acids and biomethane from alginate are still challenge due to its low solubility in water. Therefore, the objective of this research was to evaluate the effect of various pretreatments (i.e., hydrothermal, alkali and acid) on bio-methane production from alginate in two-phase anaerobic process. Batch tests were performed with various pretreatments of alginate; hydrothermal (reaction temperature: 130 to 300oC, reaction time: 5, 15, and 30 min), alkali (solvent: NaOH, reaction Temp.: 80 to 120oC, and reaction time: 15 to 60 min), and acid (solvent: H2SO4, reaction Temp.: 120oC, and reaction time: 15 min). After the pretreatments, the pretreated solutions were used to produce volatile fatty acids (VFAs) which are main intermediates for biomethane production in two-phase anaerobic process. The average molecular weight (M.W.) of pretreated samples was analyzed by gel permeation chromatography (GPC). A high performance liquid chromatography was used to quantify VFAs (C2 to C6) and bioalcohols production. The CH4, CO2, and H2 were analyzed by gas chromatograph. In results, the average M.W. of pretreated alginate from hydrothermal, acid, and alkali were the range of 430,000 to 1,200, 2,100 to 1,800, and 270,000 to 2,700 Da, respectively. After pretreatment, fermentative VFAs production was performed using the hydrothermal, acid, and alkali pretreated solutions. The maximum VFAs yield of 91±5.7% was observed in hydrothermal pretreatment (i.e., 200℃ for 15 min) and major components of VFAs were iso-valerate (47.0±4.2%) and iso-butyrate (46.6±0.6%). It suggested that hydrothermal pretreatment could effectively hydrolyze the high molecules of alginate into smaller molecules for VFAs production. The maximum yield of 50.8±1.3% CH4 was also observed in hydrothermal conditions at 140oC for 30 min rather than acid and alkali treatments. During the VFAs production, the maximum VFAs yield was detected at 200oC for 15 min however the maximum yield of biomethane was observed at 140oC for 30 min. This was caused that the individual VFAs composition of each hydrothermal condition distinguished. The acetate composition of two conditions (140oC for 30 min and 200oC for 15 min) were 74.0±3.9% and 3.4±1.3%, respectively. Consequently, the hydrothermal pretreatment of 140oC for 30 min showed the best performance of biomethane production among the various pretreatment methods. Therefore, the further study of this research would be analyzed major inhibitors from hydrothermal treatment for improvement of biomethane process efficiency.