Chapter 11 - Evaluation and optimization of biogas production from de-oiled microalgae Botryococcus braunii grown in microbial fuel cell

Abstract

Microbial fuel cells coupled with microalgae are an efficient system to produce value-added products. Botryococcus braunii can be used as carbon-capture cells, as an electrode-assisted system and sediments. In this research, B. braunii is used as a microbial cathode electrode. It is found that the usage of an algae-assisted cathode compartment doesn’t required external oxygen supplement, and increased the Coulombic efficiency by 20% because the oxygen production by the algae during the light period is utilized for the reactions. Thus, higher the light intensity, the higher the biomass concentrations. The harvested algae biomass was used for biogas production. Biogas production from anaerobic method has been used intensively over the past several decades. Microalgae biomass has been extensively used for producing biodiesel recently, from which the disposal of waste leads to environment issues; so after extraction of lipid, waste biomass is used as a substrate for biogas production. Microalgae biomass has thick cell walls, which results in less degradation, resulting in less biogas production; hence, to enhance biogas production, pretreatment is done. Thermal pretreatment is used commercially for disrupting the cells due to being a low temperature and less expensive method, requiring less energy input than other methods, such as acid, alkaline, ultrasonic, microwave-assisted pretreatment, and biological methods. De-oiled B. braunii was evaluated as a substrate for biogas production. Batch assays in a thermophilic digestion environment were utilized by optimizing the pretreatment variables such as temperature and heating time of 50°C to 90°C with an interval of 10°C and 6 to 18h with an interval of 3h for biogas production. As microalgae has a low carbon/nitrogen ratio, cornhusk is used as a co-digestion substrate to balance the carbon/nitrogen ratio of de-oiled microalgae and 1g of inoculum is added in the reactor. Response surface methodology by central composite design is used to justify the effects and their interactions between the variables on biogas yield. The response surface plots reported the optimal conditions of pretreated biomass on yield are pretreatment temperature of 70.5°C and reaction time of 12.2h. The regression coefficient (R2) showed 96%, predicting that the model was fit to the experimental parameters and high degree of correlation was observed with less lack of fit and less residual errors.