## 5 Discussion and Future Perspectives
### Process Complexity  
Some biogas upgrading technologies still require significant improvements before they can be commercialized. Technological limitations vary among methodologies. For example, regarding WS technology, a challenge to address is determining the biogas production yield under thermophilic conditions, as studies have predominantly been conducted under psychrophilic and mesophilic conditions until now [155]. \"In-situ enrichment\" is a suitable technology for biogas produced in AD plants but not for landfill biogas. The principle is based on aqueous CO\({}_{2}\) equilibrium [156]. Indeed, dissolved CO\({}_{2}\) in the sludge is desorbed in a bubbled column. Methane enrichment is obtained because CO\({}_{2}\)-free sludge can re-adsorb CO\({}_{2}\) from biogas in digester headspace. But some difficulties have been found in laboratory tests, mainly due to pH variations that may disrupt the anaerobic process (e.g., ammonia inhibition), and such performance has not been significant. Furthermore, other studies have demonstrated a positive effect of CO\({}_{2}\) on AD. [PERSON] et al. [157] have shown the capacity of AD to transform the additional CO\({}_{2}\) injected into a laboratory-scale digester into methane. They concluded that their findings could result in the on-site valorization of concentrated CO\({}_{2}\) streams to potentially increase methane production. As an \"in-situ enrichment\", the utilization of \"biochar\" is only suitable for AD plants. In recent years, biochar has been attracting increasing interest as an additive in AD, and many studies have shown that biochar improves methanogenesis in the AD process. As a result, the methane production rate is significantly enhanced [158; 159]. Biochar can act as a support for cell immobilization and microbial growth (promoting the formation of a biofilm on biochar) [160]. It is also an electron conductor that can stimulate direct interspecies electron transfer (DIET) [159], and it improves the buffering capacity [161], etc. Research is still currently very active. The main drawback is that biochar is not a commercial product (the biochar market does not exist) [162], and consequently, industrialization would be not possible. Also, other additives for AD are being studied, such as nutrient complementation [163], ash [164], nanoparticles [165], iron salts [166], magnetite-doped activated carbon [167], etc. Polymer resins have not been investigated enough. Studies such as those by [PERSON] and [PERSON] [168] demonstrate that research efforts continue towards the synthesis of highly porous polymers for the capture and conversion of CO\({}_{2}\) on a laboratory scale. In recent years, research has also been interested in CO\({}_{2}\) capture via clathrate hydrate formation [169]. Gas hydrate technology is a very attractive technology, but up to now, its separation efficiency has remained limited, and further research is needed. Ecological or industrial lung (also called \"Biomineralization\", \"Enzymatic Carbon Dioxide Capture\", or merely \"Carbonic Anhydrase\") is an example of biomimicry. Current research efforts are directed at Carbon Capture and Storage (CCS), but findings can be applied to biogas upgrading. Indeed, this enzyme could be used to improve gas separation efficiency in membranes [170] or amine scrubbers [149]. Nevertheless, more research needs to be conducted to overcome several technical barriers (the identification and characterization of supports for CA immobilization, the characterization of new and different CAs, the design of continuously operating reactors, media regeneration, et cetera) and achieve a cost-effective biomimetic technology [171; 172; 173]. Ash filter technology is suitable for small-scale AD plants and landfill biogas. Therefore, this technology is probably the easiest to industrialize of all emerging technologies presented herein.