**9. Conclusions**

In general, owing to its simplicity, ease of use, low cost and availability and the possibility of simultaneously removing other undesirable biogas components, adsorption on activated carbon is most commonly used to remove VMSs from biogas. Considering the regeneration di fficulties, its use seems to be justified in the case of low VMSs concentrations (<1 mg/m3) or when an economic analysis—covering the periodic replacement of the spent adsorbent and waste management—shows the simultaneous removal of hydrogen sulfide in the same apparatus to be cost-e ffective. The use of adsorption on activated carbon as the final biogas cleaning stage can be considered after the prior drying and removal of some VMSs, e.g., by absorption using SelexolTM [46] or oils. The main disadvantage of activated carbon in the considered application is the promotion of VMSs polymerization on its active surface, which quickly deactivates the adsorbent and practically prevents its regeneration. Moreover, the non-selectivity of activated carbon, due to the high diversity of its pores, results in quick bed saturation. Together with siloxanes other volatile compounds, including halides, hydrogen sulfide, ammonia, water vapor and even a small amount of methane, are adsorbed. All this entails additional expenditures on biogas pretreatment (drying, dedusting, the possible removal of competing impurities, and cooling) and frequent bed replacements. Another significant disadvantage of removing VMSs by means of activated carbon is the displacement of the previously adsorbed lighter and more volatile VMSs, e.g., L2, by heavier cyclic VMSs or by other VOCs. This can result in a sudden breakthrough of the bed. Although this technology is widespread, the e ffects of di fferent ways of activating and impregnating activated carbons and of biogas composition and parameters on the e fficiency of VMSs adsorption are still poorly understood. It is also unclear what conditions favor the polymerization of VMSs. Further research, especially on developing new methods, e.g., chemical methods (oxidation), of regenerating spent carbon and possible ways of its environmentally safe utilization, is needed. In the latter respect, chemically and biologically inert silica gels and zeolites, exhibiting similar VMSs adsorption properties as activated carbons, seem safer. Moreover, they are mechanically and thermally stronger and more easily regenerable, while their price is similar. Since in the case of silica gel it is necessary to deep dry biogas, zeolites seem to have greater potential as they can also be used for biogas desulphurization. They also show less a ffinity for methane. On the other hand, adsorbents based on alumina and polymer resins are most amenable to regeneration. According to the latest research results, polymer resins are particularly promising as they have the greatest potential for adsorbing VMSs and can be easily regenerated at relatively low temperatures.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.
