Unlike PRO, RED converts the ionic flux directly into electrical energy
by means of the electrodes at each side of the stack [76]. Irrespective of the great progress in PRO and RED, it is still in the R&D stage and the main prospect is adopting desalination where high concentrated solutions are already present. Salinity gradient
power, if fully developed, can ease the advent of RO desalination for irrigation use through more efficient desalination in terms of decreased water production cost and increased capacity. Like other membrane processes, the need for appropriate membranes means that there is still a gap between the ongoing research and full commercial
use. Lower electrical resistance and better chemical and thermal stability at lower costs are required in ED and related process development of ion-exchange membranes with higher permselectivity. [70].
4. Membrane operations for wastewater reuse
Wastewater treatment and reuse is another opportunity to minimize the pressure on natural fresh water resources. However, people usually accept wastewater reuse for irrigation but not for domestic use even though the reclaimed water meets drinking water standards (Fig. 7) [77]. Untreated wastewater could, potentially, be used directly for irrigation purposes. However, in this case, a strictmanagement of thewastewater has to be practised, including: (i) the wastewater should be applied at an appropriate time before harvest to be sure of the pathogen death on the crops; (ii) appropriate irrigation methods should adopted to avoid unnecessary contact between crop and wastewater; and (iii) consumers should only utilize the crops after cooking [78].
However, to avoid any health risk and to maintain a high production rate rawsewage requires treatment before its use for irrigation. Pedreroet al. [79] reviewed the effect of treated municipal wastewater when utilized for the irrigation of different fruits and vegetables (onion, lettuce, broccoli, melon, and citrus plants). The studies revealed that treated municipalwastewater is well-suited to irrigation because plants and
vegetables appeared luxuriant due to the nutrients present in the wastewater.
The following section highlights different membrane operations studied for reclaimed wastewater used as irrigation water.