Effect of inoculation time on the bio-drying performance of combined hydrolytic–aerobic process
Introduction
The high water content of municipal solid waste (MSW) will reduce the efficiency of its energy recovery and the feasibility of mechanical separation for beneficial utilization. The bio-drying technology is regarded as a good solution to reduce water content of MSW (Adani et al., 2002, Choi et al., 2001, Rada et al., 2005, Sugni et al., 2005). The microbial metabolism for the bio-drying process was similar to that for the composting process. However, the former aimed at the water removal, while the latter focused on the bio-stabilization and maturity of composted materials. Therefore, the strategy of controlling air-flow rate at a relatively fixed value was usually taken for the former, while feedback control based on O2 content or temperature was used commonly for the latter. Nowadays, a combined hydrolytic–aerobic bio-drying process has become of great interest (Bezama et al., 2007, Zhang et al., 2008). The combined process is characterized by supplementing a hydrolytic stage prior to the aerobic degradation, so that the cell wall or membrane can be destructed with less organics consumption. By this way, the ratio of water content to biodegradable organics is expected to be lowered and thus be favorable for water evaporation during the aerobic stage.
Inocula are usually added at the initial degradation stage and can intensify the aerobic degradation of substrates (Bolta et al., 2003, Vargas-García et al., 2007, Wei et al., 2007, Xi et al., 2005). For the combined bio-drying process, two stages (the hydrolytic and aerobic stages) are involved and the shift between these two stages may lead to the replacement of the dominant microorganisms. Besides, with the evolution of bio-drying, not only the quantity of specific microorganisms and their abilities to degrade substrates, but also the availability of substrates and environmental conditions for microbial growth are different. Therefore, the inoculation time would affect the degradation rate of total organics. Consequently, the performance of water removal and water content reduction may also depend on the inoculation time. Until now, it is unknown how the inoculation time influences the performance of water removal or water content reduction for combined hydrolytic–aerobic process.
This study investigated the effects of inoculation time on the performance of water removal and water content reduction, in order to optimize the inoculation operation of the combined hydrolytic–aerobic process. The activities of extracellular enzymes and the quantity of microorganisms during bio-drying were monitored to explain the bio-drying performance from the viewpoints of enzymolysis and microbiology.
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Characteristics of the MSW feedstock
The MSW was sampled from a residential area in Shanghai, China. It comprised 60% (w/w, in wet weight, the same below) of kitchen waste, 23% (w/w) of paper, 11% (w/w) of plastics and 6% (w/w) of other components. The initial water content was 72% (w/w). The biochemical composition and microorganism numbers of the feedstock with plastics, glasses and metals removed are given in Table 1.
Characteristics of the inoculating material
The inoculating material (Table 1) was collected from the products of the bio-drying process, with all plastics,
Evolution of the O2 concentration and the temperature during bio-drying
In this study, the air-flow rate was controlled to a fixed value. The daily air-flow rate could also remain reasonably constant in real scale, since the daily quantity of feed material was not expected to change rapidly. The O2 concentration in the free space of the columns just before ventilation could indicate oxic degradation level of organics under given ventilation interval and air-flow rate, to some extent. The temporal evolution of the O2 concentrations before ventilation is shown in
Discussion
The performance of water removal and water content reduction corresponded to the organics hydrolysis rate during the hydrolytic stage and degradation rate during the aerobic stage. The hydrolysis or degradation rate was related to the quantity of specific microorganisms and their ability to degrade corresponding substrates as well as to substrates availability and environmental conditions (such as pH, O2 and water content) for microbial growth (Haug, 1993, Ye et al., 2007), which varied with
Conclusion
For combined hydrolytic–aerobic bio-drying process, the addition of inoculating material at different time influenced the total organics degradation rate and thus affected the performance of water removal and water content reduction. The inoculation at the beginning of the aerobic stage (day 5) significantly enhanced the bio-drying performance. Therefore, the beginning of the aerobic stage was suggested to be the optimal time for inoculation. The addition of inoculating material could
Acknowledgements
This work was financially supported by National Key Technology R&D Program (2006BAC06B04, 2006BAC02A03) and Key Grant Project of Shanghai Committee of Science and Technology (06dz12308).
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