Study Of Anaerobic Digestion Of Spartina Alterniflora's Biogas Residue

As a perennial salt marsh grass, S. alterniflora is native to the Atlantic and Gulf Coasts of North America.The average net primary production (NPP) of S. alterniflora exceeds 3154.8g dry mass m-2year-1 which become an abundant biomass resource.Considering the growing shortage of firewood and rising cost of fossil fuels, anaerobic digestion is an attractive technique for biosolids treatment in order to get the byproducts biogas and biofertilizer. Early study showed that anaerobic digestion of S. alterniflora was feasible,but for the high content lignocellulose inhibitory factor, the biotransformation rate was only 40% under mesophilic conditions.Most of organic substances were still remained to biogas residue.To improve digestion efficiency and biogas yield, digestion of biogas residue was researched in this paper. The pH value, theVFA (volatile fatty acid), and the biogas yield have been analyzed under the complete mixture anaerobic digestion condition.In order to improve the gas yield of Spartina alterniflora anaerobic digestion and confirm the ideal dosage of NaOH, the NaOH solutions in different concentration of 4%,6%,8% were used to treat the fermented Spartina alterniflora. The results showed that the anaerobic digestion capability of the Spartina alterniflora was enhanced remarkably by NaOH treatment,the rate of the second anaerobic digestion were 262mL/gTS,276 mL/gTS,282 mL/gTS and the methane content were 72%,72%,69% respectively. Considered the economic factor, NaOH concentration of 6% was selected.After Spartina alterniflora's biogas residue being treated by NaOH solution of 6% for 48h at room temperature, the effect of different loading rates for secondary anaerobic digestion was studied. Efficient biogas production was observed at TS loading rates of 8%,10%,12% at (35±1)℃.The peaks of daily gas production were 10,14,13 mL/g and the rates of cumulate gas production were 217,227,228 mL/g respectively. With the increasing of loading rate, the cumulate gas production improved. The methane content exceeded 65%. The lowest pH value was 7.04, acid accumulation was not observed for fermented material as non-degradation organic compound decomposing slowly. For anaerobic digestion of Spartina alterniflora's biogas residue, TS loading rate can increase to over 12%.Secondary anaerobic digestion were performed for mono-digestion of fermented S. alterniflora and co-digestion at fermented S. alterniflora to potato ratio (SA:P) of 6:1,4:1 and 3:1 (based on TS) after NaOH solution treatment. Co-digestion at SA:P of 4:1 without NaOH solution treatment was also evaluated. Optimal conditions for gas production of secondary anaerobic digestion were combined NaOH solution treatment and potato addition at SA:P of 4:1 while the highest gas yield was 288mL/gTS.The results showed that combined NaOH treatment and addition of potato to fermented S. alterniflora followed by secondary anaerobic digestion break complex organic material into simpler and more biodegradable constituents to enhance biogas production.Readily biodegradable organic matter was decomposed by microorganisms in first anaerobic digestion while the secondary fermentation of raw materials was Spartina's biogas residue which was inbiodegradable organic matter. After secondary fermentation, the amount of residue was decreased and reduced the difficulty of subsequent processing, biotransformation was greatly increased. Gas production of first and secondary anaerobic anaerobic digestion was about the same while CH4 concentration of the secondary anaerobic digestion was slightly higher. Secondary anaerobic digestion is of great significance to engineering applications.