| Summary: | 碩士 === 國立屏東科技大學 === 環境工程與科學系所 === 106 === In this study, a mobile thermophilic anaerobic digester system was operated for a two-staged digestion study. The first stage was operation of the acidogenesis tank and then the methanogenesis tank. The second stage was an operation of solid state fermentation (SSF). The digestate from the first stage was used to adsorb four dried plant materials (red quinoa, corn stalk, forage grass, and napier grass) to study the potential of further gas production. Different temperatures, moisture contents, and plants were used to investigate the degradability and gas production rate. Objective of this study was to increase the gas production rate and promote the applications of the digestate.
This study continued for 614 days and was classified into 3 sections. The first section was defined as the raw kitchen waste section and was from day 1 to day 114. The second section was defined as the cooked kitchen waste section and was from day 115 to day 446. The third section was defined as the co-digestion section and was from day 505 to day 614. There were two system failures occurred during the whole period of study. In the operation of the mobile thermophilic anaerobic digester system for the raw kitchen waste (Section 1), the volumetric loading rate (VLR) of the methangenesis tank was 1.61kg COD/m3-day and the mean feed concentration was 32.12 g COD/L. Steady state data showed the overall system removal efficiencies of SS, VSS, and TCOD were 56.15 ± 5.80%, 62.18 ± 6.19%, and 61.63 ± 9.38%, respectively. The average gas production rate was 0.72 ± 0.76 m3 gas/day, and the average percentage of CH4 was 62.2 ± 2.6%, and the COD recovery was 54.26%.
In order to increase the gas production the system loading was increased sharply at the early stage of Section 2. This caused the daily gas production decreased to 2.93 ± 2.44 m3 at the 140th day of operation. The average VFAs concentration was 11,725 mg/L as acetic acid from day 184 to day 199, which indicated that the acid production rate was higher than the methane formation. From day 163 the CO2 concentration was found to be higher than the CH4 concentration in the gas production, another indication of system failure. The system was then reseeded to restart, and system loading was increased gradually to avoid system failure. The ratio of cooked kitchen waste to dilution water was increased slowly from 30kg:170L to 65kg:135L. In Section 2, VLR was 6.61kg COD/m3-day and the mean feed concentration was 132.26g COD/L. Steady state data showed the overall system removal efficiencies of SS, VSS, and TCOD were 80.26 ± 4.75 %, 84.33 ± 3.67 %, and 71.78 ± 5.56 %, respectively. The average gas production rate was 3.19±1.20 m3 gas/day, and the average percentage of CH4 was 59.7 ± 2.2 %, with a COD recovery of 55.26 %. Later of this stage the daily gas production was decreased to 1~2 m3 (～day 370), and the VFAs concentration was found to be 11,575 mg /L as acetic acid (from day 367 to day 373). At day 436, the CH4 concentration was down to 30% and the CO2 was 28.6 %. At day 422 the CO2 concentration was found to be higher than the CH4 concentration, and the second system failure occurred.
The co-digestion stage (Section 3) was operated under the feed COD ratio of cooked kitchen waste: pig manure: forage grass = 5:4:1. The VLR was 2.46kg COD/m3-day and the mean feed concentration was 49.22 g COD/L. Steady state data showed the overall system removal efficiencies of SS, VSS, and TCOD were 71.48±11.51 %, 78.20±8.95 %, and 77.32± 10.26 %, respectively. The average gas production was 3.19 ± 0.92 m3 gas/day, and the average percentage of CH4 was 58.3 ± 4.6%, with a COD recovery of 73.08%. Due to a thin tubbing problem in the lab-scale system, it was found that some grasses were clogged in the wheel of the effluent pump, and some grasses were also found at the supernatant in the methanogenesis tank. This could lead to a false higher COD removal efficiency and a lower COD recovery.
The SSF test was conducted for 5 batches using 4 dried plant materials (red quinoa, corn stalk, forage grass and napier grass). Results showed that napier grass had the highest gas production at the moisture content of 85.7% and specific gas production rates of 0.71 L/L-day (10 days) and 0.57 L/L-day (15 days). All the tests showed that the napier grass had the highest specific gas production rate. Therefore, the napier grass is a good candidate for the choice of energy crop in pilot scale test and the moisture content can be set at 85.7%. It was also found that the activeness of the bacteria in the digestate play an important role in the degradability and gas production of the SSF. The temperature of both stages (two-phased digestion and SSF) should be identical, i.e., both are mesophilic or thermophilic. This is to ensure a good performance of the bacteria in the digestate.
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