Enhanced Acid Production from Cyanobacterial Sludge by Liquid Fermentation Combined with High-Temperature Thermal Hydrolysis: Focus on Source Tracing of Inhibitors

High-value-added utilization of cyanobacterial sludge via anaerobic fermentation is significant for controlling lake algal blooms. However, the impregnable cell structure severely impedes the biotransformation of the cyanobacterial biomass. High-temperature thermal hydrolysis (HTTH) can significantly enhance the bioavailability of cyanobacterial sludge but also introduce some unexpected inhibitors. In this study, an integrated process of HTTH and liquid fermentation was developed to produce volatile fatty acids (VFAs) from cyanobacterial sludge. The findings showed that a hydrolysate containing significant nutrients of up to 111.36 g/L was produced by HTTH at 200 °C, which solubilized roughly 68.1% of the organics in the cyanobacterial sludge. Capillary suction time and viscosity of cyanobacterial sludge were reduced by 99.7%, greatly improving its dewaterability. Fermentative VFA production from the supernatant of hydrolyzed cyanobacterial sludge achieved a concentration as high as 18.7 g/L. However, the generation of inhibitors during HTTH made the conversion rate of cyanobacterial biomass not high enough. Adding the supernatant of hydrolyzed cyanobacterial sludge at an inclusion ratio of 20% decreased VFA production by 48% when glucose was utilized as a substrate. Pyrazines were found to be potential inhibitors in hydrolysate, except for ammonia and microcystins. Fortunately, the inhibitory effect of cyanobacterial sludge hydrolysate could be lessened by seeding sludge acclimation, and after 32 days of acclimatization, the acidification efficiency increased by 40.5%. Findings in this study provide a promising alternative for the high-value utilization of the waste cyanobacterial sludge and warn of HTTH adverse effects.