Anaerobic digestion of microalgal (Chlorella vulgaris) biomass as a source of biogas and biofertilizer

Abstract

Increased energy demand of the world poses risk because of the limitation of fossil fuel resources. Moreover, global warming has been reached to an alarming level because of carbon dioxide emissions originated from different anthropogenic activities. Therefore, alternative energy sources are needed, and in this sense, anaerobic digestion of microalgal biomass for biomethane production might be one attractive approach. In this study, biogas production from biomass of Chlorella vulgaris grown on wastewater with air as nutrient was investigated at mesophilic and thermophilic conditions. Moreover, the digestate of anaerobic reactors was tested for its biofertilizer potential. Experimental results indicated that the average biogas and biomethane production per gram of volatile solids (VSs) was 238 and 99 mL/g VS, respectively. The removal efficiency of chemical oxygen demand (COD) was 59% and 46% for mesophilic and thermophilic reactors, respectively. Acetic and isobutyric acids were the main volatile fatty acids detected in the reactor effluent. They ranged between 2.5–5.0 and 5–15 mM, respectively. When the fertilizer quality of the digestate of the anaerobic reactors fed with microalgal biomass is considered, it was observed that it does not correspond to any significant value as a fertilizer and soil conditioner. However, the fertilizer potential of the digestate of the reactors can be improved by increasing the influent COD concentration and the loading rate. Then, it can be used as an ingredient to produce commercial fertilizers. Coupling wastewater treatment with renewable energy production from the produced microalgal biomass not only decreases the sludge to be handled and associated costs but also provides a sustainable means of wastewater treatment. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 936–941, 2016