PhD Defence Harmen Zwijnenberg | Resource Recovery using Donnan Dialysis - Investigating Mass Transport Limitations and Selectivity

Resource Recovery using Donnan Dialysis - Investigating Mass Transport Limitations and Selectivity

Harmen Zwijnenberg is a PhD student in the Department of Membrane Surface Science. Promotors are dr. J.A. Wood and prof.dr.ir. W.G.J. van der Meer from the Faculty of Science & Technology.

Ammonium in wastewater poses an environmental and energy problem. Its breakdown is associated with high costs, soil acidification, and nitrous oxide emissions. Recovering ammonium as raw material is more sustainable but is hampered by the low concentrations in wastewater and most other streams in which it is present. Our research focuses on Donnan dialysis as a continuous ion exchange technology to recover and concentrate ammonium.

The thesis covers the problem definition and gives a comparison of possible alternative membrane solutions. A main limitation for Donnan dialysis is found in the capital cost of the traditional membrane plate and frame stacks and a possible approach could lie in using more compact and cheaper hollow fiber modules. Comparison of the removal efficiency of these 2 types and development of mass transport relations was carried out for a binary ion exchange. This model was further extended including other ions, especially divalent cations, like calcium, which show a high interaction with the used materials and can cause unwanted precipitation of calcium carbonate in installations, i.e. 'scaling'. Both experiments and correlating model results show indeed a detrimental effect of calcium that decreases the efficiency of the ammonium removal. To mitigate this, a selective coating was applied to the membranes, mostly blocking the divalent calcium transport and still allowing ammonium to pass. Both in electrodialysis and Donnan dialysis this layer increased the selectivity of the process a factor 20-25. The characterization of the ion transport of both processes through liquid boundary layers, bulk membrane material and selective coating was investigated. It showed the effect of the coating could be well separated and the results described nicely using a stochastic barrier model. Taking a similar material as the coating, a saloplastic flat film membrane was made using simple melting, suitable for mass fabrication by an extrusion/pressing method and characterized. Its behavior was different from commercial membranes and showed opposite selectivity between sodium and potassium. This implies that these membranes can also be used, for instance, in recovery of greenhouse irrigation water.