Simultaneous biogas upgrading and centrate treatment in an outdoors pilot scale high rate algal pond

The bioconversion of biogas to biomethane coupled to centrate treatment was evaluated in an outdoors pilot scale high rate algal pond interconnected to an external CO₂-H₂S absorption column (AC) via settled broth recirculation. CO₂-removal efficiencies ranged from 50 to 95% depending on the alkalinity of the cultivation broth and environmental conditions, while a complete H₂S removal was achieved regardless of the operational conditions. A maximum CH₄ concentration of 94% with a limited O₂ and N₂ stripping was recorded in the upgraded biogas at recycling liquid/biogas ratios in the AC of 1 and 2. Process operation at a constant biomass productivity of 15gm^-2d^-1 and the minimization of effluent generation supported high carbon and nutrient recoveries in the harvested biomass (C=66±8%, N=54±18%, P≈100% and S=16±3%). Finally, a low diversity in the structure of the microalgae population was promoted by the environmental and operational conditions imposed.