The Role of Anaerobic Digestion in Algal Biorefineries: Clean Energy Production, Organic Waste Treatment, and Nutrient Loop Closure

Abstract

This book chapter deals with the production of biogas coupled with the use of waste effluents for microalgae biomass growth. Biogas production through anaerobic digestion of microalgae uses the whole organic content of microalgae to produce energy. Furthermore, biogas generation seems to be the least complex of the different energy conversion routes since anaerobic digestion avoids energy-intensive steps such as biomass drying and extraction. Biogas can be produced as the main product from microalgae (direct anaerobic digestion of the whole biomass) or can be a coproduct of an industry culturing microalgae for different purposes. The integration of different technologies in a biorefinery aims at maximizing benefits while reducing the environmental impact. The future of algae biorefineries would include the extraction of several components from microalgae. Waste biomass can be treated by anaerobic digestion, reducing the pollutant load while producing energy. Additionally, there is a synergy between anaerobic digestion and microalgae growth. Biogas contains a high percentage of CO2, and if it is combusted in CHP units, CH4 is converted to CO2. The digestate produced after anaerobic digestion is a liquid medium where most of the nutrients of the organic substrate are mineralized. Therefore, the two main products of anaerobic digestion could serve as sources of nutrients for microalgae growth. If the nutrient loop is closed, profitable processes can be achieved. Consequently, biofuels and high-value products would be obtained at the same time from microalgal biomass, reducing environmental impact and increasing profits.