3.2.3. Relationships between microbial activity and aeration, friction,
collision
Microbial activity occurs in a three-dimensional space; if we
divide the composting pile into countless equal reaction units
and put these tiny units on a flat surface, the composting process
can be understood as a membrane reaction. Fig. 6 illustrates rapid
degradation mechanism of food waste. It can be observed from the
figure that the oxygen microorganisms need to respire was existed
in a gap between two solid particles. The oxygen transfer process
can be divided into three steps: diffusion in the gas phase, dissolution
at phase interface, diffusion in the water phase. All the diffusion
process followed Fick’s law (Sun and Yang, 2008). According
to the law, the driving force was determined by surrounding conditions
and concentration gradient. The concentration of oxygen is
21% at the beginning of the process (volume fraction of oxygen in
the air), which has the maximum driving force. As composting process
proceed, the concentration of dissolved oxygen was continues
to decrease which triggered the diffusion process and a mixed gas film, carbon dioxide as the main component, was formed due to
respiration as showed in Fig. 6(b). The formation of mixed gas film
has a negative effect on oxygen transfer, as composting process
proceed, the film was continues to grow until the next heap turning
comes to destroy the balance and then a new cycle was began.
Fig. 6(c) illustrates degradation mechanism of food waste in DHAF.
It’s obvious that food waste suffered from a series of collision and
friction between different food waste mass under the continuous
effect of stirring, as a result, food waste group was divided into several
small food waste bits. As fermentation process proceeded,
more reaction unit was generated, which keep in consistent with
the conclusion of Fig. 5. Furthermore, carbon dioxide generated
by aerobic respiration of microorganisms was easily removed by
the movement of food waste, merging into the air flow in the upper
part of the reaction chamber, with more waste gas been exhausted
and fresh air from the air inlet entering into the reactor, the loss of
oxygen consumed was rapidly replenished. What’s more, the continuous
collision and friction between different food waste bits can
cause water exchange of particle surface water film. Food waste is
a kind of heterogeneous mixture, every reaction unit may has a
unique substrate with unique elemental composition, as fermentation
process proceeded, the chemical and physical properties of
Fig. 6. Rapid degradation mechanism of food waste.
12 Y. Jiang et al. / Bioresource Technology 197 (2015) 7–14
water film on the surface of substrate changes with different reaction
unit, which lead to different water environment to microorganism.
For example, a rice particle has a high content of starch
and lack of protein, but a pork particle may have a high content
of protein and fat and lack of starch. Accordingly, the composition
of the chemical and physical properties of water film on the surface
of rice particle and pork particle has a great difference. In TSC, the
only driving force for organic matters ‘trading’ between two kinds
of food waste particles is diffusion. So the rate of organic matter
exchange is very slow. As a result, the static environment has a
low buffering capacity, which may lead to emergence of extreme
environment inhabiting microbial activity. However, the extreme
environment can’t appear in DHAF due to water exchange brought
by collision and friction between different food waste bits. The
water exchange can mix two or more different water films with
the fastest speed to avoid the emergence of unbalanced distribution
of organic matters. The strong buffering capacity brought by
water exchange can sustain a moderate water environment for
microorganisms to survival in DHAF.
In conclusion, the relationships between microbial activity and
aeration, friction, collision can be summarized for three aspects:
(1) strong oxygen transfer capacity, (2) great reaction surface area,
(3) strong buffering capacity. With these advantages, food waste
can be rapidly degraded in DHAF process.
3.2.3. Relationships between microbial activity and aeration, friction,collisionMicrobial activity occurs in a three-dimensional space; if wedivide the composting pile into countless equal reaction unitsand put these tiny units on a flat surface, the composting processcan be understood as a membrane reaction. Fig. 6 illustrates rapiddegradation mechanism of food waste. It can be observed from thefigure that the oxygen microorganisms need to respire was existedin a gap between two solid particles. The oxygen transfer processcan be divided into three steps: diffusion in the gas phase, dissolutionat phase interface, diffusion in the water phase. All the diffusionprocess followed Fick’s law (Sun and Yang, 2008). Accordingto the law, the driving force was determined by surrounding conditionsand concentration gradient. The concentration of oxygen is21% at the beginning of the process (volume fraction of oxygen inthe air), which has the maximum driving force. As composting processproceed, the concentration of dissolved oxygen was continuesto decrease which triggered the diffusion process and a mixed gas film, carbon dioxide as the main component, was formed due torespiration as showed in Fig. 6(b). The formation of mixed gas filmhas a negative effect on oxygen transfer, as composting processproceed, the film was continues to grow until the next heap turningcomes to destroy the balance and then a new cycle was began.Fig. 6(c) illustrates degradation mechanism of food waste in DHAF.It’s obvious that food waste suffered from a series of collision andfriction between different food waste mass under the continuouseffect of stirring, as a result, food waste group was divided into severalsmall food waste bits. As fermentation process proceeded,more reaction unit was generated, which keep in consistent withthe conclusion of Fig. 5. Furthermore, carbon dioxide generatedby aerobic respiration of microorganisms was easily removed bythe movement of food waste, merging into the air flow in the upperpart of the reaction chamber, with more waste gas been exhaustedand fresh air from the air inlet entering into the reactor, the loss ofoxygen consumed was rapidly replenished. What’s more, the continuouscollision and friction between different food waste bits cancause water exchange of particle surface water film. Food waste isa kind of heterogeneous mixture, every reaction unit may has aunique substrate with unique elemental composition, as fermentationprocess proceeded, the chemical and physical properties ofFig. 6. Rapid degradation mechanism of food waste.12 Y. Jiang et al. / Bioresource Technology 197 (2015) 7–14water film on the surface of substrate changes with different reactionunit, which lead to different water environment to microorganism.For example, a rice particle has a high content of starchand lack of protein, but a pork particle may have a high contentof protein and fat and lack of starch. Accordingly, the compositionof the chemical and physical properties of water film on the surfaceof rice particle and pork particle has a great difference. In TSC, theonly driving force for organic matters ‘trading’ between two kindsof food waste particles is diffusion. So the rate of organic matterexchange is very slow. As a result, the static environment has alow buffering capacity, which may lead to emergence of extremeenvironment inhabiting microbial activity. However, the extremeenvironment can’t appear in DHAF due to water exchange broughtby collision and friction between different food waste bits. Thewater exchange can mix two or more different water films withthe fastest speed to avoid the emergence of unbalanced distributionof organic matters. The strong buffering capacity brought bywater exchange can sustain a moderate water environment formicroorganisms to survival in DHAF.In conclusion, the relationships between microbial activity andaeration, friction, collision can be summarized for three aspects:(1) strong oxygen transfer capacity, (2) great reaction surface area,(3) strong buffering capacity. With these advantages, food wastecan be rapidly degraded in DHAF process.
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