Design and performance test of rolling type mass and heat transfer reactor for dry anaerobic fermentation of kitchen waste

A large amount of kitchen wastes were produced every day, and they will pose a great threat to the environment and human health if it could not be disposed properly. Nowadays, most of the kitchen wastes are disposed in landfills or incinerated, which associates with different issues, such as rising costs, lack of space, leaching, public environmental concern and emission of toxic and greenhouse effect gases. Kitchen wastes contain a large amount of organic matters like lipid and protein. Anaerobic fermentation is considered as the most attractive method for the treatment and recycle of kitchen wastes since they reduce waste volume, generate an energy-rich gas in the form of methane, and yield nutrient-containing final products. Compared to wet anaerobic fermentation, dry anaerobic fermentation is favorable in terms of a low energy requirement for heating, and the residues can be composted directly because it contains less digester effluent. However, the efficiency of dry anaerobic fermentation system is usually very low due to the retarded mass and heat transfer rates caused by high solid content. In order to improve mass and heat transfer efficiency in dry anaerobic fermentation system, in this study we designed a rolling type mass and heat transfer reactor which made substrates be freely mixed. The whole equipment consists of two parts, a pre-fermenter and a fermenter. The effective volume of pre-fermenter and fermenter was 10 L. Substrates were added to the pre-fermenter firstly and the air was removed by adding nitrogen gas. Then the air-removed substrates were discharged into the fermenter through sealed pipe. By this way less oxygen was brought into the fermenter. The performance of the reactor was investigated under mesophilic conditions (37 °C) and the microbial community structure was investigated by analyzing 16S rRNA genes. The methane content in the rolling type mass and heat transfer reactor was over 50% after 5 days, the maximum methane content was 74.89%. The main accumulated volatile fatty acids were propionate, followed by acetate. Propionate is hard to be utilized in common anaerobic fermentation systems and leads to acidification finally. However, the propionate showed less effect on gas production and pH value in the system of our designed reactor. The removal rate of volatile solid was 68.74% after 30 days' fermentation, and the anaerobic fermentation system was stable. The microbial community structure had dynamic changes at different periods of anaerobic fermentation. At phylum level, Firmicutes was the absolute dominant bacteria and Euryarchaeota was the dominant archaea in the system. For methanogens, Methanosarcina which can utilize all three known pathways for methane production was the absolute dominant one except in the ferment substance sampled on the first day, indicating that it is important for efficient methanogenesis in the fermentation system. Methanoculleus and Methanobacterium were the dominant genera only in the ferment material sampled on the first day, implying that they originated from inoculated sludge. The reactor we designed and the experimental data obtained in this study will be useful for the development of efficient dry anaerobic digestion system.