Freeze-casted nanofiber based sponges or aerogels exhibit a hierarchical porous structure. Pore formation is only partially understood. Therefore, we studied the underlying solid templating mechanism. We were able to tailor the secondary pore size between 9.5 and 123 mm while retaining the smaller primary pores known from electrospun nanofiber membranes. To understand the effect of microstructure on the sponges' bulk properties, mass flow through the pores and interaction with the sponges' internal surface were investigated. By solely altering the sponges' microstructure we indeed found tunability in permeability by a factor 7 and in filtration efficiency by a factor of 220. Hence, pore architecture of nanofiber based sponges is a key element for their performance. The selected pullulan/PVA polymer blends and aqueous electrospinning conditions are benign and allow the facile adaptation of these ultralight highly porous sponges for a large number of applications.