Integration of anaerobic digestion and microalgal cultivation for digestate bioremediation and biogas upgrading

Biogas is the gaseous byproduct obtained during anaerobic digestion which is rich in methane, along with a significant amount of other gases like CO₂. The removal of CO₂ is essential to upgrade the biogas to biomethane (> 95% methane content). High CO₂ tolerant microalgae can be employed as a biological CO₂ scrubbing agent for biogas upgrading. Many microalgal strains tolerant to the levels of CO₂ and CH₄ seen in biogas have been reported. A CO₂ removal efficiency of 50–99% can be attained based on the microalgae used and the cultivation conditions applied. Nutrient-rich liquid digestate obtained from anaerobic digestion can also be used as the cultivation medium for microalgae, performing biogas upgrading and digestate bioremediation simultaneously. Mixotrophic cultivation enables microalgae to utilize the organic carbon present in the liquid digestate along with nitrogen and phosphorus. Microalgae appears to be a potential biological CO₂ scrubbing agent for efficient biogas upgrading.