3.1. Reverse osmosis
The motivation to use membrane operations for desalination arose from the slogan, first raised in the US, of making the desert green. Expensive desalination techniques practised at that time were not viable to achieve the sentiment behind this slogan. At present, reverse osmosis is the most utilized desalination process [20]. The attractiveness of RO
can be outlined in the following points: (i) RO uses only one type of energy (electrical),where MSF needs two (electrical and thermal); and (ii) RO does not have a phase change as seen with MSF. Therefore RO requires relatively less energy in super seding traditional distillation techniques. The main sources for membrane desalination are seawater and brackish water. Owing to the lower amount of total dissolved solids (TDS) in brackish waters, RO systems work better with this feed in
terms of reduced pressure requirements, improved membrane lifetime and higher recovery factors (Table 6) [24]. However, the management of brines from inland desalination plants is a major challenge because these plants are placed far from the coast. Moreover, full exploitation of brackish water resources could lead to salt water intrusion into the aquifers causing a negative environmental impact. Several countries have already adapted desalination for agricultural use. Spain is one of the most productive agricultural regions in Europe and one of the countries where both brackish water and seawater desalination are successfully implemented for agricultural irrigation due to the fact that the majority of soils suitable for agriculture are in regions
with less water available [16]. Consequentially, different methodologies have been attempted to reduce water stress in those zones. Long distance water transfer was suggested in the Spanish National Hydrological Plan [27,28]. However, Albiac et al. [27] claimed that instead of long distance water transfer, with the adverse effect of water cost increase, desalination would be a less economic burden for the farmers
[27]. In 2004, a new water management plan Programa AGUA with increased focus on desalinationwas proposed [28]. Today some of the desalination plants in Spain are mainly providing fresh water for the agricultural industry and less for domestic use (Table 7). A BWRO plant (Almeria Cuevas de Almanzora) supplying water for agriculture was designed
to treat seawater instead of brackish water, therefore if the risk of seawater intrusion becomes a reality, the plant is also able to treat this water source. Moreover, this plant is distinctive in terms of production of different water qualities with respect to end-user requirements including agricultural irrigation, golf course irrigation or even drinking water through the blending of permeate and raw water [29].