Development of On-site Treatment Bioreactor of Food waste through Lab-scale and Pilot-scale Experiments

Abstract

As municipal solid waste (MSW) has been explosively increasing around the world, effective treatment and disposal of MSW are becoming a global problem. About one-third of MSW is food waste (FW), and the amount has been increased to more than 1.3 billion tones every year around the world. Especially, urban food waste in Taiwan, Japan, and South Korea, which accomplished fast economic expansion, the capacity of current facilities in these countries cannot handle all rapidly increasing FW due to the stringency of environmental standards, requiring more treatment facilities. Thus, the on-site FW treatment facilities as alternative methods to reduce excessive burdens of the conventional food waste treatment system. This study aims to develop and evaluate the on-site treatment and reduction bioreactor for FW generated in apartment buildings through lab-scale and pilot-scale experiments. The lab-scale bioreactors operating on the different temperatures of 35, 45, and 55 ℃ with the air-flow rate of 4, were tested for the feasibility of organic matter reduction by utilizing naturally occurring fermentation bacteria. The pilot-scale bioreactor consisting of the biological treatment tank and drying tanks was operated on the different air-flow rate of 4 and 10 L/min and feed rate of 20 kg/day and 5 kg/day and evaluated by the removal efficiencies of the organic matter and mass of FW. Moreover, final products produced by the bioreactor were analyzed for the potential of the by-product to be recycled as an energy resource. Furthermore, the change in the bacterial community of FW and the digested samples from the biological treatment tank was identified at the genus and species level. Operational results of the lab-scale fed-batch reactors operating at 35, 45, and 55 ℃ were observed 18.96 ± 6.67 %, 7.48 ± 2.75 %, and 8.22 ± 3.23 % of volatile solids reduction (VSR). In the pilot-scale experiments, the results of the fed-batch reactor operating at air-flow rate of 10 L/min and feed rate of 20 kg/day were observed 19.17 % of VSR in the biological treatment tank. The average mass reduction and moisture removal ratio were increased from 78.50 and 92.50 % to 83.50 and 99.00 %, when the air-flow rate was changed from 4 L/min into 10 L/min. Also, the average VSR of the reactor operating at the feed rate of 5 kg/day was observed 38.80 % in the biological treatment tank, and the average mass reduction and moisture removal ratio were 91.50 % and 99.90 %. The on-site FW reduction and recycling bioreactor produced two final products; one is the third dried solid (TD), and the other is the effluent. The TD produced by the pilot-scale reactor operating on the feed rate of 20 kg/day and with air-flow rate of 10 L/min was 5.37 % of moisture content and 85.57 % of VS contents. It complied with the official standard requirements for fertilizers except for chlorine and sulfur and for bio-solid refuse fuel (Bio-SRF) except for chlorine, which indicated that the TD could be considered safe when mixed with other fertilizer and biofuel. Also, The effluent produced by the pilot-scale reactor operating on the feed rate of 20 kg/day and the air-flow rate of 10 L/min contained 14.42 g/L of VFAs and 3.67 g/L of lactic acid. At the genus level, Lactobacillus was the most predominant in FW (78.09 %) and DS (76.90 %), which produce mainly lactic acid as the major metabolic end-product of carbohydrate fermentation through a fermentative metabolism. Besides lactic acid, other side products consist of acetate and CO2. Weissella is obligatory fermentative, producing CO2 from carbohydrate metabolism with either a mixture of lactic acid and acetic acid as major end-products, which is increased from 5.56 % to 13.60 % at the genus level. Especially, Lactobacillus sakel, accounting for 98 % of Lactobacillus genus in samples, is generally found in kimchi of traditional Korean food. In summary, our finding of the lab-scale reactors suggested that VS removal of organic matter in FW can be obviously higher efficiency at 35 ℃ (Mesophilic temperature) rather than 45 and 55 ℃ (Thermophilic temperatures). Results of the pilot-scale reactor indicated that the performances of VSR and mass can enhance due to the increase of air-flow rate and change of both HRT and OLR. The by-products produced by the pilot-scale reactor were indicated the possibility of reuse as fertilizers, bio-solid refuse fuel (Bio-SRF), and carbon source. By monitoring the reactor performances and detection of the changes in the bacterial community, the sufficient strategy using only food waste as substrate and inoculum could be sufficient, which was proved. Thus, the on-site FW treatment and reduction bioreactor consisting of the biological treatment tank and drying tanks for apartment buildings and residential complexes in urban areas was developed, which could be alternative methods to reduce excessive burdens of the conventional FW treatment system by the only using the electricity of 24.71 kWh/d.