Electrodialysis has been shown to be an effective means for removing ammonium ions from wastewater without use of consumable chemicals and without adding other substances to the treated water. Provided that continuing efforts to develop efficient electrodialysis equipment prove successful, it should be possible to apply this treatment principle to wastewater streams to be recycled in life-support systems for spacecraft and other closed habitats. Effluents from some industrial processes that generate high concentration of ammonium ions may also be treatable by this principle.
In electrodialysis (see figure), an electric potential is applied across a membrane that is selectively permeable by the ions of interest — in this case, ammonium (NH4+). Typically, a membrane suitable for this purpose comprises a polymer matrix, within which ionophores are immobilized. Membranes based on perfluorosulfonic acid and membranes in which nonactin serves as the ionophore have been found to function efficiently as selective transporters of ammonium ions from effluent to concentrated-waste streams.
In an experiment, electrodialysis in an electrolytic cell containing such a membrane reduced the ammonium concentration of a simulated effluent stream from 290 to 2 parts per million. The rate of flow of the stream was 1.152 liters/day. The area of the membrane was 10 cm2. With a potential of 10 V applied, the electric-current density in the cell was 2 mA/cm2. With these parameters, the specific energy consumption amounted to 1.5 ×104 joules per liter of treated water. Because the minimum ammonium concentration was still above the maximum allowable (0.5 parts per million) for potability in a life-support system, further effort would be necessary to develop a practical electrodialysis unit for incorporation into such a system.
An Electrolytic Cell containing a suitable membrane can be used to transfer ammonium ions from one stream to another.
This work was done by Ella F. Spiegel of Eltron Research, Inc., for Johnson Space Center. No further documentation is available. MSC-22818