88
3 Advanced Technologies (Biological and Thermochemical) …
are specifically efficient for hardwood as the biomass. The ammonia solution may
be recovered and recycled [101].
3.2.3.6 Combined Pre-treatments
Using each of the above-described methods alone might not be much effective
and they all have certain limitations. For example, mechanical methods have high
energy consumption; Chemical methods produce a lot of inhibitors after pretreatments; enzymatic methods use highly specific enzymes, which are costly limiting
their commercial application. Thus, combined pre-treatments are introduced as an
alternate method by many researchers. As an example, combined enzymatic pretreatments with sonication, liquid hot water and ozonation as well as combined sonication, temperature with pressure and acid hydrolysis were performed on microalgal
biomass. Various combined pre-treatment methods produced a higher yield than any
individual pre-treatment method [98].
3.2.3.7 Effects of Operating Conditions on Fermentation
Different factors such as temperature, initial solid loading and initial pH can affect the
bioethanol production. Initial solid loading has a positive effect on fermentation and
increased concentration of solids up to a certain value results in improved bioethanol
concentration and yield. Further increase in solids concentration results in a decrease
in bioethanol production, due to poor solid mixing at high solid loading resulting in
ineffective mass and heat transfer. Moreover, high sugar concentration can cause a
nutrient concentration gradient. This would develop an extracellular osmotic pressure
greater than that of the intracellular environment, resulting in diffusion of water in
cells through a membrane of a hypotonic solution to a hypertonic solution. Thus,
the yeast cells channel their metabolic processes towards survival strategies such as
cell maintenance and biomass growth. On the other hand, increased concentration
of ethanol at high solid concentrations would play an inhibitory effect on yeast
growth and fermentation activity. Production of other inhibitors such as furfurals and
phenolic compounds could seriously weaken the fermentative microbes and lower
volumetric ethanol productivity [97, 107, 108]. Effect of temperature on bioethanol
production follows a similar trend: very low temperatures reduce the enzymatic
saccharification efficiency resulting in a negative effect on the sugar release and
utilization, whereas very high temperatures (higher than 40 °C or 50 °C) lead to the
thermal lysis of yeast cells and thus reduce the bioethanol concentration and yield
[107, 108]. The initial pH of the substrate can significantly affect the final ethanol
concentration. Generally, mildly acidic pH is more favorable for enzyme activity
in yeast cells. At very low pH levels, the production of acids such as acetic acid
plays an inhibitory role in ethanol production. Most of the studies suggest that the
optimum pH range for ethanol production is 4.0–5.5. Higher pH values result in a high
concentration of protons in the fermentation medium that affects the overall charge
3 Advanced Technologies (Biological and Thermochemical) …
are specifically efficient for hardwood as the biomass. The ammonia solution may
be recovered and recycled [101].
3.2.3.6 Combined Pre-treatments
Using each of the above-described methods alone might not be much effective
and they all have certain limitations. For example, mechanical methods have high
energy consumption; Chemical methods produce a lot of inhibitors after pretreatments; enzymatic methods use highly specific enzymes, which are costly limiting
their commercial application. Thus, combined pre-treatments are introduced as an
alternate method by many researchers. As an example, combined enzymatic pretreatments with sonication, liquid hot water and ozonation as well as combined sonication, temperature with pressure and acid hydrolysis were performed on microalgal
biomass. Various combined pre-treatment methods produced a higher yield than any
individual pre-treatment method [98].
3.2.3.7 Effects of Operating Conditions on Fermentation
Different factors such as temperature, initial solid loading and initial pH can affect the
bioethanol production. Initial solid loading has a positive effect on fermentation and
increased concentration of solids up to a certain value results in improved bioethanol
concentration and yield. Further increase in solids concentration results in a decrease
in bioethanol production, due to poor solid mixing at high solid loading resulting in
ineffective mass and heat transfer. Moreover, high sugar concentration can cause a
nutrient concentration gradient. This would develop an extracellular osmotic pressure
greater than that of the intracellular environment, resulting in diffusion of water in
cells through a membrane of a hypotonic solution to a hypertonic solution. Thus,
the yeast cells channel their metabolic processes towards survival strategies such as
cell maintenance and biomass growth. On the other hand, increased concentration
of ethanol at high solid concentrations would play an inhibitory effect on yeast
growth and fermentation activity. Production of other inhibitors such as furfurals and
phenolic compounds could seriously weaken the fermentative microbes and lower
volumetric ethanol productivity [97, 107, 108]. Effect of temperature on bioethanol
production follows a similar trend: very low temperatures reduce the enzymatic
saccharification efficiency resulting in a negative effect on the sugar release and
utilization, whereas very high temperatures (higher than 40 °C or 50 °C) lead to the
thermal lysis of yeast cells and thus reduce the bioethanol concentration and yield
[107, 108]. The initial pH of the substrate can significantly affect the final ethanol
concentration. Generally, mildly acidic pH is more favorable for enzyme activity
in yeast cells. At very low pH levels, the production of acids such as acetic acid
plays an inhibitory role in ethanol production. Most of the studies suggest that the
optimum pH range for ethanol production is 4.0–5.5. Higher pH values result in a high
concentration of protons in the fermentation medium that affects the overall charge