## 5 Discussion and Future Perspectives
### Economic Challenges  
Due to the high operating costs associated with biogas upgrading, improvement technologies face significant economic challenges. Most studies indicate that upgrading technologies are expensive when it comes to commercialization. Therefore, they focus on reducing operational costs. WS is currently the most-applied technology due to its low capital and operation costs. However, MS has undergone the highest growth in recent years [20].  
Absorption is a common process for capturing CO2 in industry, and several solvents are available (amines, sodium hydroxide, aqueous ammonia, ionic liquids, hot potassium carbonate, etc.). The solvent regeneration step is a classic example of a costly process that consumes high amounts of energy and may jeopardize the economic viability of biomethane facilities [175]. Therefore, most research in this area has focused on alkali solvents' regeneration. Indeed, alkali solvents are usually cheaper than other solvents such as amines, but their poor regeneration limits their deployment. Hence, the challenge lies in being able to regenerate and recycle the reactive solvent.  
Other studies have focused on waste as a raw material to produce low-cost solvents, which allows higher carbon utilization. For instance, [PERSON] et al. [181] investigated human urine to produce an \"original\" solvent for biogas upgrading. In any case, economic evaluations are required to identify the comparative cost-effectiveness of these alternatives versus the existing ones (namely pressurized WS and amine-based absorption amine scrubbing, which are the most widespread technologies for biogas upgrading).  
Several effects of biochar additions and other additives on the AD process have been reported in the literature; the material addition to AD might complement conventional upgrading technologies to reduce operational costs. Another technology that might complement conventional technologies and significantly reduce operational costs is the utilization of ash as a CO2 sorbent in a fixed-bed adsorption column.  
Currently, methanation is not economically viable; however, the prospect of feasibility is anticipated in the future when a system with a significant proportion of intermittent renewables becomes available. The direct methanation of biogas poses both a challenge and an opportunity by avoiding the CO\({}_{2}\) separation step, which is crucial for economic sustainability. While CO\({}_{2}\) methanation has advanced in catalyst development, the process design still needs adjustments for economic viability and adaptations to smaller decentralized plants [110]. Although the direct methanation of biogas with adiabatic and cooled reactors can produce SNG, the technical design and applications are highly relevant in terms of their costs compared to methanation. The total cost of SNG remains high, but it is very sensitive to the prices of biogas and electricity [182]. The cost and wear of the catalyst, reactor cooling, and post-treatment costs are still being analyzed in theoretical studies and early demonstration plants [107].  
Biological technologies rely on the availability of hydrogen, which can be obtained through electrolysis. Although they are in a developmental stage, a substantial cost reduction is expected with the availability of standardized electrolyzers [106].