Biogas yield by mixed anaerobic fermentation of kelp residue and aquaculture solid waste

Abstract: Above 50% of kelp in weight is left as residue during kelp processing, and 25%-40% of the feed in aquaculture is converted to solid waste. Efficient and cost-effective utilization of kelp residue and aquaculture solid waste is vital for sustainable development of modern fisheries. Experimental studies on biogas yield from two-phase fermentation of kelp residue and aquaculture solid waste were carried out in the condition of (35±1) ℃ medium temperature. The effects of TS concentration and inoculation rate on mixing hydrolytic acidification characteristics of kelp residue and aquaculture solid waste were studied, and sequentially the biogas yield property from acidified liquor in a batch-adding manner was studied as well. Concentrations of VFA (volatile fatty acids) and their main components, COD (chemical oxygen demand), pH value, and so on were measured during mixing hydrolytic acidification process. Accordingly, the items including daily biogas yield, methane content in the biogas, and pH value were recorded during the fermentation process. Results showed that the mixing hydrolytic acidification process of kelp residue and aquaculture solid waste was fast. The highest concentration of acetic acid was measured on the 2nd day of hydrolyzing, while the concentration of propionate acid and butyric acid increased relatively quickly after 5 days' hydrolyzing. Formic acid production during the mixing hydrolytic acidification process was relatively lower. With TS concentration of 6%, 8% and 10%, the acidifying degree of acetic acid and main organic acids (acetic acid + butyric acid + formic acid) reached 42.6%, 50.0%, 49.8% and 61.7%, 68.7%, 62.2%, respectively. Additionally, with inoculation rate of 10%, 20% and 30% for mixing hydrolytic acidification, the acidifying degree of the acetic acid and main organic acids were 50.7%, 44.3%, 40.3% and 69.4%, 57.5%, 58.0%, respectively, after 3 days' hydrolyzing. It was accordingly known that desired acidified liquor could be obtained for further fermentation and biogas production by mixing hydrolytic acidification of kelp residue and aquaculture solid waste at (35±1) ℃ in 2-3 d if TS concentration, inoculation rate and pH value were kept at 8%-10%, 10%-20%, and 7.0－8.0, respectively. Furthermore, the fermentation of acidified liquor demonstrated that the biogas production process started quickly and kept an increasing biogas yield and methane content by batch-adding of acidified liquor and with the quantity ratios of acidified liquor to the inoculation methanogenic sludge of 1:5, 1:7 and 1:9. There was a measureable increase in pH value of liquor during the first 5 days and a little fluctuation in pH value in the ratios of 1:5 and 1:7 during the 11th-12th day, but stable pH value was found after 13 days. Relatively higher daily biogas yield and earlier peak of biogas yield were measured in the liquor of 1:9, followed by the liquor of 1:7 and 1:5. After the fermentation of 13 days, the quantity ratio of acidified liquor to the inoculation methanogenic sludge had no significant influence on the daily biogas yield and methane content in the biogas. Comparatively, with the quantity ratios of acidified liquor to the inoculation methanogenic sludge of 1:7 and 1:9, the biogas production efficiency could reach 489.4-581.5 mL/gVS and the methane content in the biogas reached 82.7%－84.9% after 8-13 days fermentation at (35±1) ℃ and pH value of 6.0－7.0. No excessive accumulation of VFA was observed during the fermentation process of the acidified liquor. Therefore higher efficiency and stability of biogas production may be achieved by mixing hydrolytic acidification and batch-adding fermentation of kelp residue and aquaculture solid waste.