In terms of bed capacity, dry gas—0% moisture, or as close as you can get—is generally optimal. Section 12.4 of Air Pollution Control (Cooper & Alley, 4th ed.) characterizes the impact of several factors on the theoretical equilibrium saturation capacity of a fixed-bed adsorption system:
- Thickness of the adsorption zone (in which the sorbent is not fully saturated);
- Deviation from the isotherm (heat wave);
- Displacement of adsorbate by sorbed water vapor;
- Sustained loss of capacity due to residual moisture in the bed after regeneration.
It seems that the combined effect on bed capacity of the latter two (moisture-related) factors is about as influential as the combined effect of the former two (non-moisture-related) factors if the system is run to complete saturation (which would be abnormal).
In practice, it really depends on how long much demand is placed on the system between regeneration cycles. The text implies that a typical capacity factor might be around 30%. Notably, the effect of the moisture-related factors is proportionally greater near the bed inlet. (This makes sense when you consider that the first factor, adsorption zone thickness, is only relevant as the leading edge of the adsorption zone approaches the outlet of the bed.) So it seems generally reasonable to expect that high moisture content in the gas might reduce the effective capacity of the bed by as much as half.
Obviously, there's a trade-off between the cost of increasing the bed size and the cost of dehumidifying the influent gas stream, and as the gas gets drier and drier it's more costly to remove that last bit of moisture (diminishing returns). So while ideally your filtration system operates at 0% humidity, the optimum level in practical terms depends on the relative cost of dehumidification and increased bed size. Selecta may simply have decided 50% is good enough, from a cost perspective, to sell in the market for which that presentation was created.
I don't see a method of regeneration indicated in the slides you linked but it seems reasonable to assume pressure swing adsorption (PSA) for biogas enhancement (see, e.g., Anaerobic Digestion of Agro-Industrial Wastes, Schomaker et al). Other regeneration methods, if used, might have different implications for residual moisture (but also cost). Something to think about.