Growth of Microalgae-Bacteria Flocs for Nutrient Recycling from Digestate and Liquid Slurry and Methane Production by Anaerobic Digestion

Abstract

Biogas production by anaerobic digestion from different wastes represents a growing interest in the panel of renewable energy. Digestate has already been a subject of numerous studies as part of microalgal culturing because it is still rich in nutrients. This study wants to use it as a reference to investigate the possibility to exploit Slurry for the same applications. The first part of this research aims to evaluate microalgae-bacterial flocs growth for nutrient recycling from liquid digestate and slurry, working at three different dilutions (10%, 30%, and 50%) of these two substrates, in order to determine the best value for nutrients and pollutants removal (ammonia and chemical oxygen demand removal rate) and microalgae-bacterial biomass production (autotrophic index). The best dilutions were 30% for digestate and 10% for slurry, allowing the highest ammonia and chemical oxygen demand removal rates. The second part evaluated methane production during anaerobic digestion at different ratios of substrate/inoculum (0.2, 0.5, and 0.8), using microalgae-bacterial flocs as a substrate and digestate or slurry as the inoculum. After 30 days, the anaerobic digestion without flocs showed the best performance compared to digestion with flocs (726.7 mL CH4·g−1 slurry, 245.6 mL CH4·g−1 digestate), whereas, for flocs digestion, the best ratio for both inocula was 0.2 substrate/inoculum with 317.2 mL CH4·g−1 slurry and 165.7 mL CH4·g−1 digestate. All solid masses are expressed in terms of volatile solids (VS).