Total VFA concentrations increased significantly as
Total VFA concentrations increased significantly as operating temperature increased (Fig. 1). At the end of fermentation, the concentrations of VFAs were 1.195, 1.625, and 3.856 g COD/L at 35, 45, and 55 °C, respectively, indicating that the VFAs produced at 55 °C were 3.2 times higher than that at 35 °C, while slight difference in the performance of VFAs accumulation at 35 and 45 °C was observed. The results showed that there were significant differences (P < 0.05) between the operating temperature and the VFAs concentration. The ratio of VFAs to TCOD showed the degree of conversion from organic substances in microalgal biomass to VFAs ( Fig. 1) and the ratio increased with temperature, which is similar to the pattern of VFAs concentration. During anaerobic fermentation, monosaccharides and Z-DQMD-FMK are transformed into VFAs by the activity of acidogenic bacteria. Therefore, the hydrolysis of microalgal biomass, mainly containing protein, carbohydrate, and lipid, is important because hydrolysis is the rate limiting step during anaerobic fermentation process ( Gonzalez-Fernandez et al., 2013). The improved microalgae hydrolysis at higher operating temperature resulted in augmented soluble protein and carbohydrate concentrations ( Fig. 2), and relatively sufficient soluble substrates at 55 °C were converted into VFAs efficiently by higher activities of fermentative bacteria at 55 °C compared to mesophilic temperatures. The result is consistent with previous findings that acidification activity was higher at thermophilic temperature (60 °C) than at mesophilic temperatures (21 and 35 °C) when fermenting excess sludge composed of organic particulates that include cell walls ( Mengmeng et al., 2009). VFA yields obtained from acquired immunodeficiency syndrome (AIDS) study were 0.10 ± 0.017, 0.12 ± 0.008, and 0.34 ± 0.009 g VFA/g VS at 35, 45, and 55 °C, respectively. The maximum VFA yield at 55 °C was 1.26-fold higher than that reported by Suresh et al. (2013) who attained a yield of 0.27 g VFA/g VS with alkali-sonicated Ettlia sp. and 2.43-fold higher than that reported by Yang et al. (2011) who attained 0.14 g VFA/g VS with thermo-alkali pretreated Scenedesmus sp. However, the results reported were performed at mesophilic temperatures (35–37 °C) without investigating the effect of thermophilic temperatures on VFAs production.