Organic Leachate and Biogas Utilization in Outdoor Microalgae Cultivation Under Alkaline Conditions at Pilot-Scale

Abstract

AbstractThe performance of an outdoor pilot-scale system consisting of a high rate algal pond interconnected to an absorption bubble column and seeded with a microalgal-bacterial consortium is presented. Biogas and organic leachate (from food waste anaerobic hydrolysis) were used as sources of CO2 and nutrients for the consortium, respectively. The removal efficiencies for CO2 and H2S from biogas were 80.0% and 99.9%, respectively, provoking a methane content in the utilized biogas of approximately 55 vol% for anaerobically-produced biogas and 90 vol% for a prepared gaseous mixture. The molecular oxygen produced by microalgae photosynthesis induced undesirable cumulation of O2 in utilized biogas up to 5%vol. The organic leachate was diluted in an alkaline mineral medium (15:1 v/v), and this rich-nutrient mixture was fed to the system at a hydraulic retention time of 138 d. The biomass productivity applying organic leachate and anaerobically-produced biogas was 2.6 g m−3 d−1, nine times lower than biomass productivity achieved when only mineral salt medium was fed, suggesting the existence of limited nutrient bioavailability. Microalgal-bacterial community shifts evaluated through molecular and morphological methods showed that a robust and stable photosynthetic population was maintained throughout the entire operation of the system, formed mainly by Picochlorum sp., Pseudanabaena sp., Spirulina sp., and Nitzschia sp., as well as heterotrophic bacteria and some microalgae predators. The valued by-products after waste utilization were clean and highly calorific biogas besides microalgae biomass, where the commercialization of the latter would determine the economic viability of biogas and leachate utilization. Graphical Abstract