Massive utilization of petroleum and natural gas caused fossil fuel shortages. Consequently, a large amount of carbon dioxide and other pollutants are produced and induced environmental impact. Hydrogen is considered a clean and alternative energy source. It contains relatively high amount of energy compared with other fuels and by-product is water. In this study, the combination of ultrasonic mechanical and biological effects is utilized to increase biohydrogen production from dark fermentation bacteria. The hydrogen production is affected by many process conditions. For obtaining the optimal result, experimental design is planned using the Taguchi Method. Four controlling factors, the ultrasonic frequency, energy, exposure time, and starch concentration, are considered to calculate the highest hydrogen production by the Taguchi Method. Under the best operating conditions, the biohydrogen production efficiency of dark fermentation increases by 19.11%. Results have shown that the combination of ultrasound and biological reactors for dark fermentation hydrogen production outperforms the traditional biohydrogen production method. The ultrasonic mechanical effects in this research always own different significances on biohydrogen production.
Because hydrogen is clean and sustainable and has high thermal energy, research on hydrogen energy has recently been emphasized in academia and industry. Traditional hydrogen production methods include the thermochemical and electrochemical methods, which are highly energy intensive, resulting in expensive and polluting production. However, the biological method applies anaerobic microorganism (primarily clostridium bacteria) to wastewater treatment, simultaneously converting the organic matter in wastewater into usable hydrogen. This method not only solves the problem of environmental pollution but also develops clean hydrogen energy and is an economic and competitive method of hydrogen production.
Reviewing the previous development of destructive ultrasound biological effects, Coakley et al. [
Regarding biohydrogen production, Lin [
Research on the effects of ultrasound exposing bacillaceae has not been discussed. This study thus attempted to develop a suitable medium composition and perform dark fermentation hydrogen production with biological reactors and ultrasonic mechanical effect shown on Figure
Sketch map of biohydrogen production with biomechatronics.
In response to depleting fossil fuel energy and to meet pollution reduction targets, it is necessary to develop new energy technologies that match modern demand. Among all emerging energy sources, hydrogen appears to have the fewest side effects and pose the least environmental harm. Looking at traditional thermochemical methods and electrochemical methods reveals that hydrogen production exhibits strengths and weaknesses in stability, security, and production; however, biohydrogen production represents a natural and potential hydrogen production method. Generally, anaerobe (primarily bacillaceae-clostridium) can transform organic matter into usable hydrogen during wastewater treatment via the biological method and also contributes to reducing pollution. Biological dark fermentation hydrogen production thus is considered a favorable choice as anaerobes can decompose organic compounds to generate hydrogen without a light source and are easily incorporated into waste-water treatment so that a practical form of renewable energy can be developed while processing environmental pollution. When the Taguchi Method is applied to optimize hydrogen production it becomes an economical hydrogen production method. Focused on ultrasound, biological effect, and the Taguchi Method, this chapter further discusses the correlations in this study. Figure
Flowchart of the biohydrogen production experiment.
Ultrasound has been applied to real world applications in recent decades and has been utilized to improve human life. Compared with certain animals, such as bats and whales, people still learn from the biosphere to improve production and life quality. Research on animal behavior models could provide improvements in mechanical design and logistic processing models. Consequently, it becomes necessary to understand the effects of ultrasound on organisms. Ultrasound communicates in transmission media and can generate a series of effects and influences, including organism-related thermal and nonthermal effects. The latter are further divided into mechanical effects and empty-hole effects.
When transmitting through viscous substances, ultrasonic energy is partially absorbed by friction and relaxation among molecules or lattices. The energy thus absorbed is transformed into heat and increases the temperature of the substance involved. As seen for physiological tissues, heat effects can boost metabolism but harm physiological tissues with these different effects depending on ultrasonic intensity and exposure time on physiological tissues. Thermal effect refers to micromassage causing tissues to generate ultrasonic efficacy and thus produce different heat energy and also refers to the increased heat production by the body after eating owing to the metabolic energy cost of digestion. It is considered diet-induced thermogenesis.
The nonthermal effects of ultrasound include radiation pressure, radiation force, acoustic torque, acoustic streaming effect, and cavitation. The basic source of these effects is the peripheral pressure change caused by ultrasound. The cavitation produces the largest effect and thus can break the cell structure, collapse aerosol cells, or tear solid physiological tissues. Rayleigh-Plesset announced the mathematical movement model for the inner cavitation vibration of incompressible liquid in 1949 [
Ultrasound can increase cell membrane permeability (permeability is the ability of water to pass through a material; water passes more easily through materials with good permeability and cannot pass easily through those with poor permeability). The experimental findings showed that constant and pulse ultrasound change tissue internal and external osmotic pressure (osmosis refers to the ability of a matter to pass through the membrane or not) and that cell stimulation needs to be changed to enhance the metabolism and reaction process and change the pH of the human body.
Ultrasound enhances tissue regeneration and the effects of peripheral nerves and significantly influences the conduction velocity of peripheral nerves. The major factor is the diathermanous effect of ultrasound. Higher intensity ultrasound generates a transmission area that proves the sensibility of nervous tissues toward ultrasound.
Generally, based on nutrient category and the distinct formula, hydrogen production methods can be classified as thermochemical, electrochemical, and biological. The thermochemical and electrochemical methods have some weaknesses, such as large energy consumption and instability. Biohydrogen production can transform organic waste into energy via biodegradation and biotransformation. This method not only can solve environmental pollution and generate hydrogen energy but is also considered the best method of energy production for achieving sustainable environmental protection and commercial value. Biohydrogen production can use numerous microorganisms, for example, green algae, cyanobacteria, photosynthetic bacteria, and fermentation bacteria. The biological method includes photosynthesis, light fermentation, and dark fermentation. The dark fermentation applied in this study utilizes anaerobic bacteria to decompose organic matters, which is considered the traditional anaerobic fermentation method.
Regarding the organic matter in wastewater, the anaerobic bacteria contain hydrolytic bacteria, hydrogen production acetic acid bacteria, acetic acid synthetic hydrogen production bacteria, and methane bacteria. In the traditional anaerobic digestion process, the organic matter converts into methane and carbon dioxide, and hydrogen is generated using acid production metabolism, but acetic acid bacteria and methane bacteria consume hydrogen so that hydrogen becomes the intermediary in the metabolic pathway. In this case, the concentration of hydrogen accumulated was lower than in the traditional anaerobic digestion process. In terms of anaerobic hydrogen production bacteria, clostridium bacteria is common and merely needs preprocessing sludge to inhibit or destroy methane bacteria activity, which can be achieved through heat or acid-alkali treatment. Having glucose as the substrate, the equation for hydrogen production of anaerobic bacteria is as follows:
The traditional anaerobic digestion process hydrolyzed complex organisms (cellulose polysaccharide or protein) into small molecules (carbohydrate amino acid) which were converted into hydrogen, carbon dioxide, acetate, propionate, butyrate, and alcohols through fermentation. The propionate, butyrate, and alcohols were converted into hydrogen, carbon dioxide, and acetate through fermentation, while the hydrogen and carbon dioxide were converted into acetate by acetic bacteria and the acetate was further converted into gas by methane bacteria. The process using anaerobic bacteria yields a better hydrogen rate if the main metabolite is organic acid than if it is alcohols or ketones. Electrons are transferred to higher reduced form outcomes such as alcohols or ketones, thus resulting in a low hydrogen rate.
Short stemmed bacillaceae is one of the bacteria that are best able to produce hydrogen. Under anaerobe, the best production occurs at 25~60°C and neutral pH, and it forms endospore and becomes dormant in an unsuitable environment. Bacillaceae is spore-producing with a particular structure and grows in thalli. Each thallium can grow a spore at its end that is wider at the end and also differs from other thalli.
Spore formation is a severe physiological and biochemical change for thalli. Both thalli and spores exhibit two nuclei and form two cell membranes, as the precursor spores. When the cells take the first step, they merely form endospores only when the thick wall of precursor forms is complete. The entire process lasts about 8~10 hours.
According to Wang [
Taguchi’s Orthogonal Array describes the creation of a pair of parameters from all levels at two column intervals in the experimental table, where each composition exhibits the same frequency. The Classical Orthogonal Array is named after
This study combined the mechanical effect generated by ultrasound and biohydrogen production technology and applied the Taguchi Method to design the experiment and analyze the parameter relations to optimize the effect. In the process, dark fermentation was utilized for hydrogen production based on several advantages as follows. Fermented strains have rapid hydrogen metabolism. It could rapidly and continuously produce hydrogen from organic matter. It shows a favorable growth rate on general microbial systems.
For these reasons, dark fermentation hydrogen production appears to obtain better benefits in terms of microorganism quality and quantity than does photosynthesis bacteria hydrogen production. Regarding the mechanical effect of ultrasound on dark fermentation, frequency, energy, number of exposures, and duration are the changeable parameters. Additionally, pH, temperature, nutrient content, starch, and stir speed can be adjusted to match specific organisms. The experiment designed three parameters for ultrasonic mechanical effect using the Taguchi Method, namely, frequency, intensity, and exposure time, to observe and analyze the effects on biohydrogen production.
The ultrasonic system and biohydrogen production equipment utilized for this experiment are described as follows.
The experimental structure in which ultrasound was used for hydrogen production is shown in Figure
Experimental structure: 1: ultrasonic pulser, 2: reactor, 3: thermostatic bath, 4: thermostat, 5: flowmeter, 6: video camera, 7: computer, 8: sampling tube, 9: gas tube, 10: stirrer, 11: stirring stone, and 12: ultrasonic transducer.
Regarding the nutrient content, potato starch was used as the carbon source, ammonium hydrogen phosphate (NH4H2PO4) as the nitrogen source, and dibasic sodium phosphate as the buffer salt for acid change, and all these materials included some microelements, as shown in Table
Nutritional composition and formula.
Composition | Concentration mg/L |
---|---|
Carbon source | 10 g/L–20 g/L–30 g/L |
NH4H2PO4 | 4500 |
Na2HPO4 | 11867 |
K2HPO4 | 125 |
MgCl2 |
100 |
MnSO4 |
15 |
FeSO4 |
25 |
CuSO4 |
5 |
CoCl2 |
0.125 |
The experimental framework is shown in Figure
Orthogonal Array Ultrasonic exposure energy was first used as the controlling factor to clarify its effects on biohydrogen production. As the ultrasonic pulser could be adjusted to 2, 4, and 8 joules, the three values were the adjustable level numbers. Distinct ultrasonic exposure time periods were selected as the second controlling factor. The exposure time periods were 15 min exposure followed by 30 min rest, 15 min exposure followed by 15 min rest, and continuous exposure for full time. The third controlling factor was frequency of transducer exposure. The exposure frequency was changed to discuss the effects of ultrasound on biohydrogen production, and the adopted frequencies included 0.5 MHz, 1 MHz, and 5 MHz. The effect of various starch concentrations on biohydrogen production was further discussed. Wang [
Experiment planning table.
Factors | Specifics | Level 1 | Level 2 | Level 3 |
---|---|---|---|---|
A | Power energy (joules) | 2 | 4 | 8 |
B | Exposure time | 15/30 | 15/15 | All the time |
(exposure/stop, min/min) | ||||
C | Frequency (MHz) | 0.5 | 1 | 5 |
D | Starch concentration (g/L) | 10 | 20 | 30 |
By following Table Strains were first boiled to delete the competitor and then cooled. Nutrient was prepared by mixing suitable starch, buffer salt, microelements, and 1 L boiled water that was being cooled. Strains and nutrient were placed in the reaction tank in a ratio of 1 : 10, and argon was infused for about 10 minutes to expel the air in the reaction tank. The external temperature of the reaction tank was controlled at 36°C. A stirrer was placed under the reaction tank to drive the stirring rock to evenly mix the reaction tank liquid. Proper control conditions, as listed in Table
To increase the experiment reliability, nine sets of Taguchi’s Orthogonal Arrays
The
The factor from
To select the number of controlling factors and levels using the Taguchi Method, one of the curve diagrams of the experimental results after three repetitions is shown in Figure
The effects of biological H2 production using the Taguchi Method.
Experimental sets |
|
|
|
---|---|---|---|
1 | 889.79 | 118.93 | 24.08 |
2 | 2126.58 | 147.42 | 17.72 |
3 | 2785.89 | 115.27 | 22.12 |
4 | 3141.07 | 119.35 | 12.50 |
5 | 811.02 | 185.28 | 185.28 |
6 | 2087.64 | 164.75 | 20.09 |
7 | 1370.16 | 86.55 | 27.54 |
8 | 3199.82 | 198.66 | 24.16 |
9 | 1079.55 | 79.31 | 18.81 |
The curve diagram showing the first experimental results using the Taguchi Method.
The experimental results in Table
Experiments examining the effect of ultrasonic influences on hydrogen production.
Exp. | A | B | C | D | Production efficiency | Production rate | Starch surplus ratio |
|
||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Average |
|
Average |
|
Average |
|
Average |
|
|||||
(%) | (dB) | (mL/h) | (dB) | (%) | (dB) | (h) | (dB) | |||||
1 | 1 | 1 | 1 | 1 | 18.94 | 18.94 | 85.40 | 37.81 | 10.27 |
|
22.54 |
|
2 | 1 | 2 | 2 | 2 | 22.66 | 26.50 | 131.47 | 42.10 | 7.77 |
|
16.35 |
|
3 | 1 | 3 | 3 | 3 | 23.02 | 27.10 | 164.77 | 43.60 | 8.32 |
|
22.03 |
|
4 | 2 | 1 | 2 | 3 | 27.11 | 28.46 | 142.18 | 42.87 | 7.43 |
|
15.68 |
|
5 | 2 | 2 | 3 | 1 | 21.82 | 26.67 | 127.01 | 40.10 | 9.40 |
|
30.18 |
|
6 | 2 | 3 | 1 | 2 | 26.76 | 28.17 | 149.62 | 43.40 | 8.38 |
|
17.32 |
|
7 | 3 | 1 | 3 | 2 | 21.12 | 25.23 | 129.84 | 40.20 | 7.73 |
|
24.47 |
|
8 | 3 | 2 | 1 | 3 | 26.68 | 28.34 | 218.88 | 46.70 | 9.04 |
|
22.10 |
|
9 | 3 | 3 | 2 | 1 | 20.23 | 25.82 | 66.23 | 36.04 | 9.50 |
|
20.69 |
|
Hydrogen production efficiency for different
A | B | C | D | |
---|---|---|---|---|
Level 1 | 26.26 | 26.29 | 27.23 | 25.89 |
Level 2 | 27.77 | 27.17 | 26.93 | 26.63 |
Level 3 | 26.46 | 27.03 | 26.33 | 27.97 |
Range | 1.50 | 0.88 | 0.90 | 2.07 |
Rank | 2 | 4 | 3 | 1 |
Hydrogen production rate for different
A | B | C | D | |
---|---|---|---|---|
Level 1 | 41.17 | 40.29 | 42.64 | 37.98 |
Level 2 | 42.12 | 42.97 | 40.34 | 41.90 |
Level 3 | 40.98 | 41.01 | 41.30 | 44.39 |
Range | 1.14 | 2.67 | 2.30 | 6.41 |
Rank | 4 | 2 | 3 | 1 |
By analyzing the starch surplus ratio, Table
Starch surplus ratio for different
A | B | C | D | |
---|---|---|---|---|
Level 1 | −18.89 | −18.68 | −19.38 | −19.84 |
Level 2 | −18.56 | −18.92 | −18.34 | −18.19 |
Level 3 | −19.03 | −18.89 | −18.76 | −18.45 |
Range | 0.47 | 0.25 | 1.05 | 1.65 |
Rank | 3 | 4 | 2 | 1 |
Lag time for different
A | B | C | D | |
---|---|---|---|---|
Level 1 | −26.27 | −26.53 | −26.49 | −27.99 |
Level 2 | −26.26 | −27.00 | −25.11 | −25.88 |
Level 3 | −27.29 | −26.29 | −28.22 | −25.95 |
Range | 1.03 | 0.71 | 3.11 | 2.11 |
Rank | 3 | 4 | 1 | 2 |
According to Tables
The Response Diagram of hydrogen production efficiency for different levels.
The Response Diagram of hydrogen production rate for different levels.
The determined parameters corresponded to Tables
The results of the confirmation experiment.
Quality indicators | Orthogonal Array | Optimal condition | Improvement (%) |
---|---|---|---|
Production efficiency (%) | 27.11 | 32.29 | 19.11 |
Production rate (mL/h) | 218.88 | 271.01 | 23.82 |
Introducing the Taguchi Method, the number of experiments could be significantly reduced by the Orthogonal Array, which is determined from controlling factors and parameters. Two steps in optimization process are accomplished. The variability of experiment is reduced and then the hydrogen production efficiency and rate are maximized. For verifying the analyzed results, the experiment is performed as follows. The optimal hydrogen production efficiency is 32.29%, which is 19.11% higher than the maximum in Taguchi’s Orthogonal Array. The optimal hydrogen production rate is 271.01 mL/h, which is 23.82% higher than in Taguchi’s Orthogonal Array. Hydrogen production efficiency and hydrogen production rate are optimized given ultrasonic energy 4 joules, 15 min exposure followed by 15 min rest, transducer 0.5 MHz, and starch concentration 30 g/L. Within the four controlling factors, starch concentration most strongly affects the experimental results. For hydrogen production efficiency, exposure intensity ranks second, while for exposure time hydrogen production rate ranks second. The ultrasonic mechanical effects always own different significances on biohydrogen production. For example, the ultrasonic exposure intensity possesses 72.46% significance when compared with the starch application on the consideration of hydrogen production efficiency. At the same time, the ultrasonic frequency shows the strongest effect on the concentration of the lag time.
Besides developing a suitable medium composition, this study also combines ultrasonic mechanical effects with biological reactors to perform dark fermentation hydrogen production. The Taguchi Method is also utilized to discuss the relationship among parameters in the hydrogen production process and is expected to optimize the hydrogen production conditions and further understand the effects of ultrasound on microorganisms.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was partially supported by the National Science Council, Taiwan, under Grant NSC-99-2221-E-244-004. Ted Knoy is appreciated for his editorial assistance.