Page 244 - Lignocellulosic Biomass to Liquid Biofuels

200 Lignocellulosic Biomass to Liquid Biofuels

the cultured organism is inoculated to the medium. During the process
the reactor is aerated, though no further additions of medium are made.
Once a production cycle is terminated, the spent medium is removed to
add fresh medium to the cultivation vessel. The cultivation medium is
prepared and sterilized before fermentation run. In this cultivation, micro-
organisms are inoculated into the bioreactor before the process started.
Since there is no fresh media added after the experimental run, the con-
centration of nutrition is decreased continuously. The volume of culture
usually remains constant. Batch fermentation gives characteristics growth
curve with lag, exponential, stationary, and death phases. Finally, the
microbial cells grow and produce the yield.
Advantages: The conversions of substrate occur completely which are
the main advantages of this system. It has properly sterilized and low risk
of infection from microbial strain.
Disadvantages: The disadvantages of this system are as follows: the labor
cost is high. Every batch needed to be sterilized, growth, and cleaning the
system.
2. Fed-batch cultivation: The fed-batch (or semiclosed) system is a cul-
ture where substrates are inoculated to the bioreactor after some interval.
In this system, nutrient media is prepared, and organisms are inoculated to
the broth medium and then incubated. In the course of incubation, nutri-
ents are fed at given intervals. As a result, the volume of the culture is
continuously increased. This technique is applicable to various fermenta-
tion processes when some nutrients, though essential for biomass growth,
may inhibit the microbial growth if their concentration is too high. In
this case, lower initial concentrations of these nutrients can be adopted,
adding them continuously or discontinuously during the fermentation.
The parameters, such as temperature, pH, substrate inoculation interval
time, are needed to be investigated. The growth phase of the microorgan-
isms is monitored enormously.
Advantages: The toxic and concentrated microorganisms are
suitable for this fermentation system.
Disadvantages: More attention should be necessary when toxic micro-
organism is used. Experiment handling is not an easy task. Sometimes
microorganisms are expensive.
3. Continuous cultivation: A continuous (or open) system is a culture
allowing the continuous production of products. This system is feasible
for syngas fermentation, especially for industrial purposes. Fresh sterile
medium is fed continuously to the vessel and spent fermentation medium