Transport characterization and modelling of Donnan dialysis for ammonium recovery from aqueous solutions

Efficient recovery of ammonium, a valuable nutrient, from dilute streams is still an unsolved challenge. One possible approach to separate and simultaneously concentrate ammonium from low concentration streams is Donnan dialysis. To design a process, a mass transport model was utilized for both plate and frame and hollow fiber modules and compared to experiments. The chosen model was based on film theory, accounting for mass transport resistances in the liquid as well as the membrane phase to allow for a complete investigation of mass transfer limitations. Experimental data for the flux of ammonium and potassium using flat sheet Nafion-115 and FujiFilm type-2 and hollow fiber Nafion membranes was predicted very well by the model for several flow conditions and concentrations. The two stage ammonium removal could be predicted accurately from a starting concentration over two orders of magnitude in concentration. For the hollow fiber module case, liquid phase resistances on the draw side were more substantial vs. plate and frame due to the lower velocity. Both plate and frame and hollow fiber modules showed similar transport rates, up to 1mol/(m2·h). In the investigated, non-optimized, laboratory system, an 80% removal of ammonium was achieved with a processing flow of 12.5 L/(m2·h).