## 4 Emerging Biogas Upgrading Technologies
### Biological Technologies
#### 4.4.1 Photosynthetic Biogas Upgrading: Microalgae  
Photosynthetic biogas upgrading takes advantage of the ability of microalgae to bioconvert CO2, in the presence of water, light, and nutrients, into algae biomass, oxygen, and heat [75]. The process is called oxygenic photosynthesis, and it is described in Equation (12) [52]. \[\text{CO}_{2}+\text{H}_{2}\text{O}+\text{photons}+\text{nutrients}\to\text{O }_{2}+\text{CH}_{1.63}\text{N}_{0\text{-}14}\text{O}_{0\text{-}43}\text{P}_{0 \text{-}006}\text{S}_{0.005}+\text{waste heat} \tag{12}\]  
The main advantages of this process are that wastewater or residual nutrients from an anaerobic digester can be used as the source of nutrients for microalgae, enabling the integration of biogas upgrading with wastewater treatment and reducing the eutrophication potential of AD. The process is held at ambient temperature and has a low demand for reagents, which means a reduction in operational costs compared to other technologies. The CO2 from the biogas is not released into the atmosphere. Additionally, in the process, apart from upgraded biogas, microalgae biomass is obtained, which can be used as feedstock for the production of biofuels [52; 90]. The potential methane recovery through the photoautotrophic biogas upgrading process is 97%, with the simultaneous removal of H2S [70]. Eukaryotic microalgae and prokaryotic cyanobacteria remove CO2 from the raw biogas using water, nutrients, and solar radiation. The removal of VMSs, VFAs, and siloxanes from biogas could eventually be carried out through biotechnologies since they are biodegradable molecules, which would support a priori a better environmental and economic performance [137]. Among biological methods, the photosynthetic removal of H2S is attracting significant attention due to its simultaneous occurrence during CO2 capture in algal-bacterial photobioreactors, which will drastically reduce the operational cost of biogas enhancement. Other biotechnology for removing H2S includes biotrickling filtration [136]. Moreover, the biological disposal of H2S does not require the use of chemicals and has a lower cost. Wastewater or digestate from an anaerobic digester can be used as a source of nutrients for microalgae. The most-used microalgae species are _Chlorella, Arthospira_, and _Spirulina_[52]. They can be used in closed or open photobioreactors [75]. Closed systems achieve higher photosynthetic performance and lower land and water needs, but their investment costs and energy demand are higher. On the other hand, open photobioreactors have lower \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multicolumn{2}{c}{**Operating Conditions**} & \multirow{2}{*}{**Promoter**} & \multicolumn{2}{c}{**Biogas**} & \multicolumn{2}{c}{**CH4 in**} & \multicolumn{2}{c}{**CO2 in**} & \multirow{2}{*}{**Case/**(Reference)**} \\  & **T (K)** & & & **Composition** & & & **Upgraded Gas** \\  & **T (K)** & **\(p\) (MPa)** & & **(\%vol)** & & **(\%Vol)** & **(\%Vol)** \\ \hline CO2 & \multirow{2}{*}{278.15} & \multirow{2}{*}{6} & \multirow{2}{*}{\begin{tabular}{c} 60\% CH4 \\ 40\% CO2 \\ 300 ppm \\ 55\% CH4, \\ 5\% H2 \\ \end{tabular} } & \multirow{2}{*}{70} & \multirow{2}{*}{30} & \multirow{2}{*}{\begin{tabular}{c} 1/[130] \\ (up-scaled \\ apparatus) \\ \end{tabular} } \\  & & & SDS & & & & \\ \hline CO2 & \multirow{2}{*}{275.1} & \multirow{2}{*}{4.5} & \multirow{2}{*}{\
\begin{tabular}{c} 300 ppm \\ 37.5\% CO2, \\ 5\% N2, 2\% O2, \\ 0.5\% H2 \\ \end{tabular} } & \multirow{2}{*}{70.1} & \multirow{2}{*}{22.4} & \multirow{2}{*}{
\begin{tabular}{c} 2/[135] \\ (up-scaled \\ apparatus) \\ \end{tabular} } \\  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Experimental results for biogas upgrading via hydrate formation. investment and operational costs, but their photosynthetic performance is poorer, and they need more water and land.