Recycling of Shellfish Wastes with a Combined Process of Carbonate Production and Methane Fermentation

A new process was proposed for the total recycling of shellfish wastes combining the carbonate production with a high-pressure carbon dioxide aqueous solution and methane fermentation. The shell part of shellfish, mainly composed of calcium carbonate was dissolved into the aqueous solution of carbon dioxide at about 30 bar or acetic acid, and will be separated from the flesh part. On the other hand, the flesh part can be anaerobically fermented to produce biogas mainly composed of methane. Two options can be considered for the treatment of the carbonate solution; recovered as pure calcium carbonate, or disposed of into the ocean as a carbon sequestration process. Process feasibility was examined by obtaining key parameters through laboratory-scale experimental studies. The dissolution kinetics of blue mussel samples was examined with a high-pressure CO2 solution and acetic acid to elucidate the influences of CO2 pressure, temperature, stirring speed, and the sample size on the dissolution kinetics. The biogas production rate was examined with the flesh part of the sample mussel either treated with an acid solution or untreated. The biogas production rate was found to be almost unaffected by the acid exposure. Based on the experimental results, the process design and evaluation was carried out. The cost of the process with CO2 treatment and recovery of calcium carbonate was about 21,000 JPY/t-shellfish waste, that of ocean disposal process was 44,000 JPY/t-shellfish waste, and that of acetic acid treatment process was 16,000 JPY/t-shellfish waste, respectively. The proposed processes are competitive with the conventional waste disposal cost, while being simpler and more environmentally benign.