Innovative Treatment of Digestate and Biogas Upgrade Using Chlorella Vulgaris

In the era of energy transition, the research efforts are devoted to find sustainable solutions to enable the transition to a decarbonised energy and production system, by renewable energy sources promoting products circularity, green technologies and safer processes. Anaerobic digestion is a bioprocess involving organic substrate breakdown by various microbial species in the oxygen absence. It yields two valuable products: digestate and biogas. Digestate can be used as fertilizer after stabilization and reduction of its polluting load. Through an upgrading process, biogas can be converted into biomethane, a widely utilized resource in energy and transportation. In this study, a non-conventional method has been investigated to achieve simultaneous biomethane production and reduction of digestate polluting power using Chlorella vulgaris, resulting in two valuable products. A 6-liter anaerobic digester was fed with simulated municipal organic waste every 3 days. Biogas was fed into a photobioreactor, where C. vulgaris was cultivated under mixotrophic conditions, utilizing CO2 from biogas as a carbon source. Biogas was converted into biomethane with over 90% methane content, while digestate was treated in the same photobioreactor, reducing its chemical oxygen demand (COD) by up to 80%. Using CO2 from biogas, maximum cell concentration of 1.332 g/L, maximum specific growth rate of 0.091 day-1 and biomass productivity of 0.057 gBS/L d were obtained at 70 µmol/m2 s. Biogas was analyzed by gas chromatography, and digestate was assessed for suspended solids, total solids, and COD. After cultivation, biomass was harvested, dehydrated, and characterized for total lipids and calorific value. Combining both approaches transforms waste into valuable biomethane and microalgal biomass, supporting the zero-waste objective