The biodegradability, rate of hydrolysis, physicochemical properties including composition, organic solid contents and chemical analysis of food waste (Korean food waste separately collected) are examined in this study. Subsequently the characterized food waste was tested in a semi-continuous fed and mixed reactor(SCFMR) type anaerobic digester to obtain reactor performance parameters for the design and operation of a food waste biogas plant. The economic feasibility of food waste digestion for biogas was studied, and economically feasible food waste resources available in Korea was also estimated to provide information for biogas development using SCFMR.
The physicochemical properties of food waste and food waste leachate of food waste collected for the treatment in biogas plant were analysed for one year. In case of food waste, average pH 4.92, TS 16.25%, VS 13.90%, TCODCr 145,100mg/L, SCODCr 58,600mg/L, T-N 4,665mg/L, T-P 823mg/L were observed for the period . Food waste leachate showed in average pH 4.40, TS 6.83%, VS 6.17%, TCODCr 80.635mg/L, SCODCr 40,700mg/L, T-N 3,747mg/L, T-P 389mg/L for the period. These basic data can provide very useful informations for the design of biogas plant through lab-scale or pilot scale experiments. However, the properties can be different by region or periods in a year, continuous monitoring of the food waste properties are desirable for the proper management of it.
Ultimate biodegradability of food waste was estimated by graphical statistic analysis method with typical samples collected. 74.3∼82.9% of total volatile solid(TVS) was biologically degradable(BVS, biodegradable volatile solid). And the ultimate biodegradability of food waste leachate was around 84.6∼89.5%. These result shows that the two wastes are highly biodegradable and hence are a very good substrate for biogas production. Meanwhile, minimum hydraulic retention time(HRT) estimated from the cross point of and kinetics curves for food waste and food waste leachate was 16 and 15 days respectively. Actual HRT for biogas plants are generally longer than these considering safety factor into consideration.
Hydrolysis rate of food wastes at various temperature and total solid were studied using multiple reaction rate analysis method. The optimum hydrolysis rate was obtained in 2 days with the ultimate hydrolysis of 32∼64% of total substrate samples of different kind. The result shows, when hydrolysis is done separately, the optimum hydrolysis method including agitation method, temperature, HRT and the optimum TS concentration in the reactor.
With the preliminary search for food waste and food waste leachate characteristics, biodegradability, hydrolysis rate, a semi-continuous fed and mixed reactor(SCFMR) was operated and monitored for biogas production performance with various substrates at varying TS and HRT. Mixed waste (food wasre + food waste leachate) of TS 11.5% digested with 30 days HRT, food waste of TS 10% with 30 days HRT, and food waste of TS 15% with 40 days HRT were tested for their specific methane production rate, and shown to be 0.37Nm3, 0.38Nm3, 0.34Nm3 respectively. However, data analysis showed that the SCFMR was operated in slightly over-loaded state. Hence it is recommended that the reactor should have been operated at higher HRT for stabilized reactor operation and for higher methane production rate.
With the same set of SCFMR operation test the total volatile solid(TVS) removal of each set of reactor test were 58.6%, 68.7%, 64.3% respectively. The TVS removal was calculated from the proportion of input TVS deduced by biogas production for accuracy(accumulation in reactor must be prevented). State parameters in SCFMR operations are observed to be the followings. In the reactor pH were around 7.2∼7.4, alkalinity 13,900∼18,500mg/L as CaCO3, NH4-N 2,700∼3,530mg/L, V/A ratio 0.18∼0.23, and P/A ratio 0.13∼0.69. All the parameters are observed to be within the normal operation range of biogas plant and can be used as standard design parameters.
With all the results of this study above mentioned and an economic feasibility study, it is shown that biogas plant operated with food waste mixed with food waste leachate have improved the economic feasibility of biogas plant operation. The theoretical, technical and economic resources of food waste resources available in Korea were estimated to be 440,000, 380,000 and 95,000TOE/year. 70% of economic resources, equivalent to 67,000TOE/year of energy, are available for biogas production in the future.