Cost-effective bio-methanation via oxalic acid coupled sonication pretreatment of Gracilaria salicornia

Gracilaria salicornia (macroalgae) can be used for the production of biofuels such as biomethane. Because of the complex cell wall structure in macroalgae, disintegration is crucial for anaerobic fermentation. The main goal of this study is to implement the combination of oxalic acid and ultrasonic pretreatment (SP) techniques over macroalgae Gracilaria salicornia in order to generate energy-efficient biomethane. A combination of ultrasonic pretreatment and oxalic acid (OSP) improves macromolecule availability in anaerobic fermentation, leading to enhanced biomethane production. Ultrasonic pretreatment was initially performed at 20-180 W for 5-60 min. A maximum solubilization of 9.19% was obtained, with a SCOD of 496 mg/L, at optimal conditions (140 W intensity, 30 min). Oxalic acid was added in various dosages from 0.01 to 0.1 N to optimize OSP conditions, enhance disintegration efficiency, and maximize biomethane production. With a SCOD of 914 mg/L and 16.93% SCOD solubilization, the best result was observed with 0.06 N oxalic acid dosage. A maximum biomethane production was achieved in the OSP sample (0.252 L/g COD) compared with the control (0.22 L/g COD) and SD (0.139 L/g COD) samples. As compared to SP, OSP was proved to be more energy-efficient. It was concluded that OSP was an efficient energy source for field applications. Combined OSP was a successful strategy for the production of energy-efficient biomethane.