The desalination of an Arthrospira platensis feed solution by electrodialysis and reverse osmosis

For long-term space missions, the algae Arthrospira platensis (previously Spirulina platensis) can be used as a food component. A. platensis can be cultured in a Zarrouk medium (further called “medium”) which contains several salts. As a result, the cultured cells have to be washed to make them edible. In space these salts have to be recovered to maintain the medium and consequently reduce the uploaded mass. Reverse osmosis (RO) and electrodialysis (ED) were tested for the desalination of the pure medium as the worst-case scenario. Separation performance, energy requirements and potential for application in space were compared for both processes. Three RO membranes were tested while ED tests were performed at different voltages and at varying diluate/concentrate ratios. Energy calculations for the ED revealed the effects of voltage levels and processing time required for desalination. The energy requirement was calculated to be slightly higher for ED than for RO; however, the performance of both techniques is comparable. For space applications other system aspects such as weight and footprint have to be taken into account. When RO and ED are tested, ED is a better candidate for this application since RO needs a heavy, high-pressure pump (weight disadvantage). Moreover, the pump might cause vibrations which should certainly be avoided for space applications. Another advantage of ED is that the final salinity of the diluate can be adjusted if required.

Table 14 Energy requirements and recovery for the RO process with Dow membranes (Dow SW30-2514 module) Number of modules Recovery, % Energy, kWh/m3 2.9 5.8 8.8 11.7 69.8 34.9 23.3 17.4 The energy calculations were performed for a RO process with one small module. Therefore, the recovery is very low as shown in Table 13. In practice on earth, more modules are connected and larger membrane modules are used, in such a way that recovery becomes higher, resulting in lower energy requirements. Table 14 shows the extrapolated energy requirement for a RO process with a simplification of one or more small Dow SW30-2514 modules connected in series at a pressure of 55 bar. For estimation reasons a simple proportionality is assumed in the calculation. It should be noted that in a spacecraft the available room is limited and therefore a high recovery Christmas tree RO-configuration, with longer pressure tubes and larger membrane diameter, is difficult to realize. Therefore, the energy requirements, as From the results it is clear that both ED and RO can be used for desalination of the medium. For the ED process more energy but less time, and consequently less membrane area, is required when a higher voltage is applied. RO experiments show that a high rejection membrane will give a permeate TDS below an envisaged amount of 300 mg/l. For the RO process the energy requirement decreases when a higher feed pressure is used, which is due to the increase in permeate flux and recovery. The energy calculations are based on test data obtained from a single small Dow SW30-2514 module, evidently resulting in a low recovery value and consequently a high energy requirement. Regarding RO applications on earth, the hydraulic pumping energy could be used more efficiently when larger modules would be used in successive stages. In space missions, however, larger (Christmas-tree) RO designs are improbable in view of volume and mass involved. Energy requirements are fairly similar for ED but can be somewhat higher when high voltages are applied. ED and RO can achieve similar separation efficiencies in batch operation. No salts are lost during the process. For the ED process the final salt concentration can be adjusted easily, if required, which is less evident for the RO process. For space applications, RO also has a major disadvantage; it needs a heavy, high- pressure pump and materials that can withstand these high pressures. This will lead to a higher mass for the RO process, which is a critical factor for space missions. In terms of safety, the high operating pressure and the vibrations caused by the high-pressure pump in the RO process might also be a problem. Taking all these considerations into account, ED seems a much more suitable technique when compared to RO for the desalination of the culture medium during a space mission.