Seawater desalination and salt production in a hybrid membrane–thermal process

Several hybrid systems, which are developed now, namely NF-MSF, NF-RO-MSF, NF-RO-MD and others offer promising performance. An integrated system for complete usage of components in seawater is also being considered. In all the aforementioned systems NF as water pre-treatment method is applied and a considerable decrease in scale forming components is achieved. Higher recovery may be then reached when desalting in comparison to traditionally treated water. Very compact NF membranes are applied which results in a very high rejection of divalent ions and relatively high rejection of sodium chloride. A simultaneous decrease of NaCl concentration in NF process is then obtained which causes an increase in the RO recovery since the osmotic pressure value of NF permeate is diminished. If NF-MSF (or NF followed by any thermal method) system is considered a NaCl concentration decrease in the pretreatment is however not essential for the NF step. The salt concentration influence on thermal method performance is very weak in comparison to RO. Furthermore the highly concentrated MSF brine might be used as a possible byproduct, e.g. for obtaining salt. A dual-purpose desalination-salt production system may be then suggested. Two arrangements are presented, namely: UF-NF-MSF-crystallisation and UF-NF-RO-MSF-crystallisation systems. The high rate of water recovery is accompanied by salt obtainment in these systems. If $30/ton of salt is assumed the water cost is equal to $0.71/m3 in UF-NF-MSF-crystallisation system and $0.43/m3 in UF-NF-RO-MSF-crystallisation system. The above data demonstrate that these water costs are competitive compared to those of potable water produced in thermal or RO seawater plants.

In the suggested systems not only desalinated water but salt as well is a valuable product of plant supplied by seawater. From the data presented one can see that by integrating membrane and thermal operations in dual-purpose system it is possible to improve the performance of seawater desalination. Thus 61.3% water recovery is obtained. As there are two valuable products, the cost of each of them is obviously directly related to selling cost of the other. If $30 per 1 t of salt is assumed, the water cost is equal to $0.71/m3 in UF-NF-MSFcrystallisation system and $0.43/m3 in UF-NFRO-MSF-crystallisation system. The above data demonstrate that these water costs are competitive compared to those of potable water produced in thermal or RO seawater plants.