# Influences of Temperature and Substrate Particle Content on Granular Sludge Bed Anaerobic Digestion
## 1 Introduction
### Temperature Effect on Particle Hydrolysis  
Various authors studied the temperature effect on biogas production. Their findings indicate that there is a positive correlation between temperature and biogas production. Increase in temperature leads to an increase in the maximum substrate utilization rate of microorganisms, as well as the specific growth rate [10]. Anaerobic digestion is generally classified into three categories based on temperature. The categories are psychrophilic (assumed to be <20 \({}^{\circ}\)C [11; 12]), mesophilic (20-42 \({}^{\circ}\)C), and thermophilic (42-60 \({}^{\circ}\)C [13] but typically 50-55 \({}^{\circ}\)C). Psychrophilic AD is probably the least studied of the three categories. The psychrophilic reaction rates are slow, and the microbial growth is limited. In addition, hydrolysis of suspended solid is near zero, and effective application of low-temperature AD is restricted to very-low-strength wastewater with little or no suspended solids [14]. However, recent studies by [PERSON] et al. [15], [PERSON] et al. [16], and [PERSON] et al. [17] demonstrated that psychrophilic anaerobic digestion may have a potential for digestion of substrates with higher solid contents. [PERSON] et al. [14] showed that expanded granular sludge bed reactor (EGSB) might be feasible for soluble pre-acidified wastewater at temperatures of 5-10 \({}^{\circ}\)C. On the other hand, mesophilic AD provides optimum conditions and stable process for biogas production. As a result, large-scale anaerobic digesters often run on operational temperatures in the mesophilic range. Wide ranges of microorganisms thrive in the mesophilic temperature range with the optimum temperature often cited as 35 \({}^{\circ}\)C. Microbial diversity, in turn, contributes to the stability and shock tolerance of the anaerobic process. Thermophilic anaerobic digestion has benefits such as higher biogas production and better digestate quality [18]. However, it is less stable than mesophilic AD due to high risk of ammonia or volatile fatty acid (VFA) inhibition. In addition, it is highly sensitive to temperature fluctuations (unlike thermophilic microorganisms, mesophilic microorganisms may tolerate fluctuations of up to \(\pm\)3 \({}^{\circ}\)C). When the removal of pathogens from the digestate is a requirement, thermophilic AD is the best option, as the high operating temperature kills most of the microorganisms present in the digestate. High temperature could also contribute to higher solid disintegration and hydrolysis (solid removal) compared to mesophilic AD [19]. In general, a temperature increase leads to an increase in the hydrolysis rate [20]. If enzyme concentration is not rate-limiting, the hydrolysis rate as a function of temperature is described by the Arrhenius equation as follows:  
\[\mathrm{K_{h}}\ =\mathrm{K_{\infty}}\ \mathrm{e^{\frac{\mathrm{A\Sigma}}{ \mathrm{T}}}}, \tag{1}\]  
where \(\mathrm{K_{h}}\) is hydrolysis rate constant, \(\mathrm{K_{\infty}}\) is the specific rate constant in day\({}^{-1}\), \(\mathrm{\Delta E}\) is the activation energy in J-mol\({}^{-1}\), T is the temperature in K, and R is the ideal gas constant in J-mol\({}^{-1}\)-K\({}^{-1}\).