Every year, over $2.3\times10^7$ ton of MSW(Municipal Solid Waste) is generated in Korea, of which 31.4% is food waste discharged from restaurant, dinning halls, and households, etc.. At present, food waste is collected and landfilled together with other MSW, causing various problems such as odor, leachate, public sanitation as well as shortage of landfill life. Therefore, innovative processes to treat and stabilize food waste have to be developed.
Anaerobic digestion of food waste could be a viable approach for energy recovery as well as for safe disposal of the waste. Anaerobic digestion of organic waste has been actively studied since the sixties and various commercial technologies, such as VALORGA process(France), SOLIDIGEST process(Italy), DRANCO process(Belgium), etc., have been developed. But lack of field experience and low VS removal ratio of only about 50% necessitates a novel further research.
In this study, a novel anaerobic composting technology of food waste was tried using MUlti-step Sequential batch Two-phase Anaerobic Composting (MUSTAC).
The MUSTAC process, composed of three leaching beds for acidification of the waste and post-treatment of the acidified residue, and upflow anaerobic sludge blanket(UASB) for the recovery of methane gas, was used to treat food waste. The leaching beds were operated by multi-step mode while each leaching bed was operated by sequential batch mode with a time interval of two days.
In this process, rumen microorganisms having high cellulolytic activity were seeded to improve efficiency of hydrolysis and acidification in the leaching beds. The reaction constraints in the leaching bed, such as pH drop and VFA accumulation caused by rapid acidification of easily degradable components in the begining, and pH increase and low microorganism reproduction by decrease of degradable substrate afterward, were minimized by adjusting dilution rate considering the extent of acidification of the waste as $3.4-3.6d^{-1}$ in the first two days, $0.9-1.1d^{-1}$ in the following two days, at alkalinity of 7,000mg/L. The post-treatment including stabilization and drying of the acidified residue was made by air injection of 90-100LPM through the bottom of the leaching bed in the last two days. It is the specific feature of this composting system that both acidification of the waste and the following stabilization of the acidified residue can be carried out in the same reactor. The treatment of leachate containing concentrated acidified products was performed in UASB. A part of effluent from UASB was recycled to the leaching beds as dilution water.
The MUSTAC process produced gas at a rate of 6㎥/㎥ day, of which methane content was 60% in average three to four days after start up. And VS reduction of 78% was obtained. The product from the post-treatment by air injection was a humus-like material with 57% TS that could be a soil conditioner. The MUSTAC process could be used for anaerobic composting of the waste and final stabilization of the acidified residue. It ensured high performance and simple operation.