Bacterial Communities in Various Conditions of the Composting Reactor Revealed by 16S rDNA Clone Analysis and DGGE - Sustainable Biotechnology: Sources of Renewable Energy - page 169

2.3.3 Small-Scale and Large-Scale Reactor

As mentioned above, a variety of bacterial community structures in various com-
posting reactors have been reported. Nevertheless, knowledge of the microbial
community in large-scale, completely-mixed composting reactors is still lacking.
Therefore, this study was conducted to clarify the bacterial community succession
during the start-up period of a large-scale, completely-mixed composting reactor by
using 16S rDNA clone and DGGE analysis, and to compare it with the bacterial
community in a small-scale reactor.

3 Technical Details-Materials and Methods

3.1 Operation of the Reactors

Small- and large-scale, complete-mixing, composting reactors were used in
this study. Three composting reactors for household use (“Namagomi-eater”
TK400-H, Matsushita Electric Works, Japan) were used as the small-scale, FBC
reactors. The working volume was 15 L. The biomass carrier (or bulking agent)
comprised about 5 L of wood chips with a size range of 0.5-2.0 mm, or plastic
bottle flakes with a size range of 2.0-10.0 mm. An artificial organic waste sample,
made up of 500 g wet wt dog food (VITA-ONE, Nihon Pet Food, Japan) containing
about 90% water, was loaded daily into each reactor. The contents in the reactors
were gently mixed by automated paddles for 1 min each hour. Mechanical heating
was used to maintain the temperature in all the reactors above 35 ◦ C to accelerate
biodegradation. In the small-scale reactor, three experimental conditions (reactor
A, B, and C) were used. In reactor A and C, a high decomposition rate of organic
materials was maintained by “partial washing” [14, 16] as follows. Approximately
10% of the contents (0.75 L) were taken out every three days, mixed with 10 L
of water, and then filtered on a 35

m mesh filter. Upon filtration, the solid part
retained on the mesh filter was dried in an oven at 60 ◦ C for 48 h, and then re-loaded
into the reactor. This process prevents not only a decrease in decomposition rate but
also aggregation of the contents in the FBC reactor [14]. In reactor B, there was
no maintenance, except for moisture content, where spontaneous aggregation was
allowed in the decomposition process. The moisture content in each reactor was
kept at 40-50% by the addition of distilled water. Samples were obtained from each
small-scale reactor after 60 days of operation.
To compare the difference of the scale of the reactor, large-scale composting
reactor was also used (O-1, which we constructed). The working volume was 4 m 3 .
The bulking agent comprised about 2 m 3 of plastic bottle flakes with a size range of
2-10 mm. 600 kg wet weight food waste derived from a school cafeteria with 73%
moisture content, was loaded into the reactor at the start of the experiment. This is
termed batch operating. The contents of the reactor were gently mixed by automated
paddles at 1.5 r.p.m. for 30 min once a day. Temperature was not regulated. Samples
were obtained from the large-scale reactor, once a day, for 25 days. In the small-scale

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