Effects of Hydrodynamic Cavitation-Assisted NaOH Pretreatment on Biofuel Production from Cyanobacteria: Promising Approach

Eukaryotic microalgae and prokaryotic cyanobacteria can grow in various water and wastewater types, and both can grow biomass by taking nutrients and converting atmospheric CO2 into useful products. Biofuels obtained by processing this landless grown biomass are defined as "third-generation biofuels". In this study, the effects of hydrodynamic cavitation (HC)-assisted NaOH pretreatment on methane production from cyanobacteria were investigated. Cyanobacterial biomass was isolated from thermal springs located in the southwest of Turkey (Denizli-Turkey) and identified as Desertifilum tharense. Desertifilum tharense biomass was grown on a laboratory scale, and along with its compositional characteristics, culture-specific parameters were determined. HC-assisted NaOH pretreatment was applied to evaluate optimum process conditions for enhancing methane production from D. tharense. In the experimental design, process parameters of cavitation number (Cv: 0.3-0.7), NaOH concentration (0-4%), solid content (1.5%), reaction time (4h), and reaction temperature (30 degrees C) were combined to reveal the parameter-specific impact of HC pretreatment. The effect of the HC-assisted NaOH pretreatment was further investigated with molecular-bond and surface structure characterization. Along with the energy equivalent of obtained biofuel, energy requirements for cultivation, harvesting, pretreatment, and anaerobic digestion (AD) were calculated to determine the process's overall energy efficiency. Kinetic parameters of raw and pretreated D. tharense were determined by first-order, cone, modified Gompertz, and reaction curve models. The results revealed that by the application of pretreatment, a 2-35.3% soluble COD increase was achieved, whereas methane production was increased from 241.5 to 290.6 mLCH(4) gVS(-1). Application of HC with a low Cv of 0.3 boosted methane production up to 20.3% compared to the raw D. tharense.