Jun EGUCHI
Mesophilic and Thermophilic Anaerobic digestion of high concentration Sewage Sludge and Rice Straw
Toshiya KOMATSU
Anaerobic digestion of high concentrations of sewage sludge has been paid attention, because the process has a possibility of improving the energy balance in sewage sludge treatment system. In this study, we operated laboratory-scale digesters to digest rice straw with high concentration (TS: 5.0%) sewage sludge in mesophilic (36¡î) and thermophilic (52¡î) conditions. The hydraulic retention times were 20 days. A digester fed sewage sludge alone was operated as a control reactor. Our objectives were to investigate the effects of rice straw on the digestion performance, to determine the effect of different pretreatment methods on the digestibility of rice straw, and to calculate the energy balance of the digestion system.
Two different rice straw pretreatment methods were compared; grinding (ca. 5mm) plus soaking in distilled water (water pretreatment), and grinding plus soaking in an enzyme solution (enzyme pretreatment). As the results, addition of rice straw increased methane production by 41?48% in mesophilic digestion and by 34?41% in thermophilic digestion. Methane production was higher in digesters with enzyme pretreatment. The highest methane production of 19.0 L/L-substrate was observed in enzyme-pretreated thermophilic digestion.
The energy output (produced methane) was compared with the total input to assess the energy balance of the digestion system. The experimental results were used in estimating digester performance, whereas the energy balances were calculated for full-scale systems in which the digester volume was assumed to be 3300m3. It was found that the best net energy output was obtained in the enzyme-pretreated mesophilic digestion system.
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