The organic fraction of MSW (municipal solid waste) amounts to over 40%, which becomes over 60% if the recycling is excluded in the estimation. Especially, food waste occupies over 25% of MSW, and becomes the main source of decay, odor and leachate in collection and transportation due to the high volatile solids (VS) and moisture content. Most food waste has been landfilled together with other wastes, resulting in various problems such as emanating odor, attracting vermin, emitting toxic gases, contaminating groundwater and wasting landfill capacity. Since landfilling of food waste will be prohibited in 2005, the research on its recycling technology is, therefore, a major field of waste management.
This study was performed to assess the newly devised process, called MUlti-step Sequential batch Two-phase Anaerobic Composting (MUSTAC). The MUSTAC process was developed as an ideal method for treating organic waste. The configuration and the operating method of the process are suggested to enhance the performance and to simplify the operation based on the rate-limiting step approach.
Firstly, the enhanced acidogenic fermentation of food waste was examined in a leaching bed by controlling fermentation constraints. The fermentation efficiency is affected by the fermentation constraints such as the biodegradability of substrate, the degrading capability of microorganisms and the environmental conditions. The key factors were, therefore, investigated to control the fermentation constraints, such as the effect of seed inoculation, The effect of adjusting dilution (D) rate and the effect of waste components. Acidogenic fermentation of food waste employing rumen microorganisms resulted in the enhanced efficiency (71.2%) as compared with that (59.8%) employing mesophilic acidogens. In addition, the fermentation efficiency increased from 71.2 to 82.0% by adjusting D rate from 3.0 to $1.0d^{-1}$ depending on the state of the fermentation. The main component of the acidified product was shifted from butyric to acetic acid, indicating that the increase of the fermentation efficiency was mainly caused by the enhanced degradation of vegetables and meats. When each component was spiked to be 80% of food waste, the fermentation efficiency was kept over 62% regardless of waste components. The control of fermentation constraints was, therefore, very effective in improving the fermentation efficiency of food waste.
Secondly, the performance of the UASB reactor treating leachate from acidogenic fermenter was examined. The COD removal efficiency was consistently over 96% at the loading rates up to 15.8 g COD/L·d. The methane production rate increased to 5.5 L/L·d. Of all the COD removed, 92% was converted to methane and the remaining presumably to biomass. At the loading rates over 18.7 g COD/L·d, the COD removal efficiency decreased due to sludge flotation and washout in the reactor, which resulted from short HRT of less than 10.6 h. The residual propionate concentration was the highest among the VFA in the effluent. The SMA (specific methanogenic activity) analysis showed that the VFA-degrading activity of granules were the highest for butyrate, and the lowest for propionate. Typical granules were found to be mainly composed of microcolonies of Methanosaeta. The size distribution of sludge particles indicated that partially granulated sludge could maintain the original structure of granular sludge and continue to gain size in the UASB reactor treating leachate from acidogenic fermenter.
Lastly, the MUSTAC process was suggested and evaluated to overcome the shortcomings of the existing anaerobic digestion processes. The MUSTAC process consisted of five leaching beds for hydrolysis, acidification and post-treatment, and a UASB reactor for methane recovery. Acidified products in leachate from the leaching beds are converted to methane in the UASB reactor. The effluent from the UASB reactor recirculates through the leaching beds as dilution water. A portion of the effluent is exchanged periodically so that inhibitory materials may not concentrate in dilution water. The MUSTAC process treated food waste with a high-rate anaerobic composting technique based on the rate-limiting step approach. Rumen microorganisms exhibiting enhanced cellulolytic activity were inoculated to improve the low efficiency of acidogenic fermentation. Both two-phase anaerobic digestion and sequential batch operation were used to control environmental constraints in anaerobic degradation. The MUSTAC process demonstrated excellent performance as it resulted in high VS reduction (84.7%) and methane conversion (84.4%) efficiencies at high organic loading rates (10.8 kg VS/㎥ -d) in a short SRT (10 days). Methane yield was 0.27㎥/kg VS, while methane gas production rate was 2.27㎥/㎥ -d. The output from the post-treatment could be used as a soil amendment, which was produced at the same acidogenic fermenter without troublesome moving. The main advantages of the MUSTAC process were simple operation and high efficiency. The MUSTAC process proved stable, reliable and effective in resource recovery as well as waste stabilization.
