Approach and Introduction to Hydrogen Production

There are various options for hydrogen production, which are classified into thermochemistry, electrochemistry, and biological processing. The dark fermentation biological process, which effectively transforms organisms into energy through decomposition and biotransformation, was utilized for this experiment because it was regarded as the most suitable way to achieve environmental protection along with economic efficiency and resource recycling.

Anaerobic bacteria were utilized for dark fermentation hydrogen production in this experiment, as they effectively decompose organisms and produce hydrogen. Using glucose as the matrix, the reaction formula of hydrogen production with anaerobic bacteria is

where CH3COOH is acetic acid and C3H7COOH is butyric acid.

Biological Effects of Ultrasound

Sonic waves transmit energy through a flexible and elastic medium by the propagation of waves generated from molecular movement. Sonic waves cannot be transmitted in a vacuum.

The movement of sonic waves is a physical phenomenon, with its properties being described in terms of wave frequency, wavelength, number of waves, and wave speed. Ultrasound is generally defined as high-frequency sound that cannot be heard by human ears. People normally hear sonic waves in the range of 20 Hz to 20 kHz; therefore, ultrasounds are sonic waves > 20 kHz.

Ultrasound transmits energy through particle vibration in a propagation medium and forms an ultrasound field. When ultrasound irradiates biological media with distinct frequencies and intensities, various physical effects appear between ultrasound energy and particles of matter. These effects are divided into thermal and non-thermal effects, and the latter is further divided into mechanical and cavitation effects. Cavitation effects are regarded as non-thermal and have their greatest impacts on biological tissues, as ultrasound transmits with waves of condensation and rarefaction in liquid that result in cavitation effects. Cavitation appears in liquids as tiny bubbles. Tiny bubbles in the liquid experience the processes of ultrasound vibration, growth, contraction, and crashing. When cells are subjected to high shear waves generated by vibrated bubbles or bubble crashing, a series of biological reactions appear.

Rayleigh-Plesset provided the mathematical movement model for inner cavitation vibration in an incompressible liquid [16]. Applying that theory to calculate the appearance of dark fermentation rod hydrogen-producing bacteria, we then can observe the activation effects by setting different pulse intensity for the vibration of the natural frequency. The following equation was used to calculate natural frequency:

where R0 is the radius, a is surface tension, у is the heat capacity ratio, p is density, n is the viscosity coefficient, and P0 is the pressure of the bacteria.

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