## 4 Emerging Biogas Upgrading Technologies
### Biological Technologies
#### 4.4.1 Photosynthetic Biogas Upgrading: Microalgae
A process of biological biogas upgrading, like any other biological process, requires that the biogas components are not inhibitors or toxic to the biological microorganisms (in this case, microalgae) [138]. Methane is the main component of biogas, and according to [PERSON] et al. [138], the same species can reduce their growth rates when exposed to biogas containing 80% methane. CO2 is the other major component of biogas, and it has been found that high concentrations of this gas may have inhibitory consequences. Traditionally, concentrations of 5% of CO2 were considered inhibitory for microalgae growth, but the research carried out for the treatment of flue gases has led to the discovery of species that can tolerate CO2 concentrations of up to 60% [52]. The presence of H2S can inhibit microalgae growth. However, the presence of H2S-oxidizing bacteria and the chemical oxidation of H2S that takes place in the photobioreactor (operating at high dissolved oxygen concentrations under non-sterile conditions) result in the oxidation of H2S into sulfate, which presents no inhibitory effects on microalgae [52]. The fixation of CO2 results in the release of oxygen, impacting the ultimate gas quality [75]. In the process, 1 mol of oxygen is produced for each mol of captured CO2 [139]. If the biogas is directly injected into a closed photobioreactor, oxygen will be released into the biogas, achieving high concentrations (5-25%) [52]. Mixtures of oxygen and methane may be explosive; for this reason, its concentration must be diminished. Furthermore, most standards for biomethane quality require a concentration of oxygen lower than 1% [139]. In Spain, the oxygen content in biomethane for grid injection must be lower than 0.3 mol% (if it complies with certain specifications; if not, the maximum oxygen content is 0.01 mol%) [140]. Thus, efforts have been made to decouple oxygen desorption from CO2 capture. An alternative is to separate the process into two stages, consisting of biogas scrubbing in an external column that is interconnected with the photobioreactor via variable microalgae broth recycling. Using the two-stage method, concentrations of oxygen below 1% have been achieved, and methane losses have been estimated to be \(<\)1% because of the low aqueous solubility of methane [52]. Mass transfer and the solubility of CO2 in microalgae culture media are a limitation [141]. So is the high methane loss due to its solubility in a large volume of microalgae culture media and the difficulty of collecting the final biogas. The lack of microalgal species tolerant to elevated CO2 and the introduction of oxygen from microalgal photosynthesis into the final biogas are also challenges [20]. Different species and configurations obtain different compositions of the upgraded gas. For example, _Chlorella_ sp. _MB-9_, _Arthospira platensis_, and _Spiulina platensis_ obtained methane concentrations of up to 90%, 82%, and 74% with CO2 assimilation of 85%, 100%, and 86%, respectively [75]. H2S can be removed via this upgrading method. Through the symbiosis of microalgae and fungi, an improvement in biogas was also demonstrated, with a CO2 removal rate of 61.08% using _G. lucidum/C. vulgaris_ pellets. Additionally, the use of vitamin B12 significantly enhanced the photosynthetic performance of the pellets [142]. These findings contribute to advancing the development and application of symbiotic systems, particularly in wastewater treatment and biogas enhancement [143].