Effect Of Pretreatments Of Water Hyacinth(Eichhornia Crassipes) And Rice Straw On Biogas Production Via Anaerobic Digestion

Research and development of green and renewable energy have become the focus of the world. Bio-energy will become an important part of sustainable energy systems in the future. Rice straw and water hyacinth are two of the preferred raw materials for energy uses, because of their abundant lignocellulose components. However, Both of them need pretreatment to improve the anaerobic digestion efficiency for biogas production. In this research, the effect of silage pretreatment of water hyacinth (Eichhornia crassipes) and rice straw mixture on anaerobic digestion were studied to maximize the methane yield and the net energy output and to provide a theoretical reference for engineering application.(1) The effects of NaOH hydrothermal pretreatment and three types of biological pretreatments on rice straw were examined using batch anaerobic digesters at mesophilic temperature (35±1℃). A combination of confocal Raman microscopy and transmission electron microscope was used. Results revealed that NaOH hydrothermal pretreatment led to significant increase of cumulative specific biogas and methane production yields, which were77.3%and87.1%higher than that of the untreated rice straw substrate, respectively. Microbial and effluent pretreatments enhanced the concentration of methane faster than other treatments at the beginning of the anaerobic digestion process, however, they had little effect on enhancing the cumulative specific biogas and methane production yield. Pretreatment of rice straw using indigenous microorganisms decreased the cumulative specific biogas and methane yields. The confocal Raman microscopy studies found that the NaOH pretreatment resulted in a loss of dense spatial uniformity structure of tissue, great decreases of the contents of hemicellulose, cellulose and particularly lignin for the rice straw fermentation, and improvement of the cumulative specific biogas and methane yields.(2) Water hyacinth leaves, stems, roots were silage pretreated for0-4days, respectively, before the mesophilic batch anaerobic digestion. Water hyacinth leaf before and after anaerobic digestion and water hyacinth leaf after4days silage pretreatment were studied using slice microscopic analysis. For water hyacinth leaves, the methane yields increased with the increase of the silage time. Water hyacinth leaf pretreated by4days silage resulted in the highest methane yield, which was26.0%higher than the untreated samples. After1-3days silage pretreatment, the cumulative specific methane yields of water hyacinth stems were higher than untreated, and the2days silaged water hyacinth stem led to the highest cumulative specific methane yield. The cumulative specific methane yields of water hyacinth leaf was the highest, followed by the water hyacinth stem, and the water hyacinth root gave the smallest cumulative specific methane yields. Slice microscopic analysis tests showed that silage pretreatment could improve the cell wall degradation of water hyacinth leaf, therefore improving the efficiency of its anaerobic digestion.(3) The effect of25days silage pretreatment on the anaerobic digestion of water hyacinth and rice straw (75:25-100:0ratio mixture, w. b.) at30℃was investigated. Results showed that silage pretreatment improved the methane concentrations and the cumulative specific methane yield of the water hyacinth and rice straw mixture anaerobic digestion. With the increase of the ratio of rice straw, the improvement of specific methane yields compared with untreated was also observed. The cumulative specific methane yields of water hyacinth and rice straw of95:5,85:15and75:25ratio mixture were respectively increased by4.2%,26.8%and33.69%after silage pretreatment than untreated. When silage fermentation juice was added to the water hyacinth silage pretreatment, the VS loss rate was larger (25.75%vs.20.67%). The anaerobic digestion methane concentration of50%-60%was observed for the water hyacinth silage pretreatment with no fermentation juice added. We did not observe significant difference of cumulative specific methane yield (58L/kg VS) for untreated and treated samples. The silage of water hyacinth with fermentation juice did not improve the anaerobic digestive efficiency.The effect of0,7,14,21days silage pretreatment on the anaerobic digestion of water hyacinth and rice straw mixed in1:4TS ratio was studied. The results showed that with the increase of the silage time, the sample’s weights, the TS and the VS decreased, the loss rate of VS increased, the pH value decreased, the methane concentration increased, and the cumulative specific methane yields increased. After21days silage pretreatment, the anaerobic digestion cumulative specific methane yield was61.41±0.66L/kg VS and increased by52.2%than untreated ones.(4) The continuous anaerobic digestion behavior of mixed water hyacinth and rice straw with a ratio of1:4TS (dry basis) was studied. When the organic loading rate increased from0.5to1.5gVS/(L·d), the daily specific biogas yield also increased from0.17to0.28L/g VS, and the methane concentration increased and then decreased, and finally stabilized. When the organic loading rate reached2gVS/(L·d), the methane concentration and pH value decreased very fast. The anaerobic digestion was inhibited, and the daily specific biogas yield was dropped significantly. As a result, the organic loading rate should be controlled below1.5gVS/(L·d).The continuous anaerobic digestion behavior of mixed water hyacinth and rice straw with a ratio of1:4TS (dry basis) after30℃and25days silage was studied. The pH of silage was3.98. The lactic acid content was14.32mg/g. For the continuous anaerobic digestion of silage pretreated water hyacinth and rice straw mixture in1:4TS ratio, the organic loading rate increased from0.5to2gVS/(L·d), the daily specific biogas yields were stable, the methane concentration increased, especially in the early stage of the anaerobic digestion, and and the pH value of the substrate was more stable than untreated. When the hydraulic retention time was reduced, the capacity specific biogas yield increased. The silage pretreatment increased the cumulative specific biogas and methane yields by67.3%and138.5%, respectively, than untreated ones.