Agricultural waste utilization for biomethane and algae-based fertilizer production for circular economy

After obtaining of biogas during the anaerobic digestion processes, the liquid fraction can be applied directly as fertilizer (if it meets the relevant sanitary and hygienic norms), or the rich in mineral composition digestate to be used as a medium for development of microalgae. The algal biomass thus obtained could be applied as fertilizer, and the unused part of it could be re-utilized as raw material in other methanation processes, alone or in combination with other raw materials or wastes. Microalgae can easily colonize different environments. These microorganisms represent promising sources for new products and applications. The obtained liquid phase of anaerobic digestate was utilized as medium for enhanced growth of green microalgae. The ability of microalgae to photosynthetically fix carbon dioxide with production of valuable substances, their short growth cycle and easy biomass accummulation was used. In this study, good growth and development was observed for the microalga Scenedesmus obliquus in digestate after a clarification process, based on the macro and micronutrients present therein. This approach may lead to reducing the production costs along with environmental impacts.