3.2. Integrated membrane operations
Despite RO being the most established desalination technology, other pressure driven membrane processes including microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF) also have their potential use in the production of irrigation water.MF and UF can be suitable for pre-treatment to RO. NF can serve as a desalination unit, though the salinity of the permeate is higher than that of RO. The choice between
NF and RO treatment will be determined by several factors including salinity tolerance of the crops grown, the soil and the excess of water applied for irrigation. In this regard, Ghermandi and Messalem [45] have simulated the gain of biomass when irrigated with brackish water, NF permeate or RO permeate, respectively. They created two different scenarios where in the first one irrigation water was applied in excess — at twice the potential plant evapotranspiration rate (based on, at that time, practice in the study area in Israel). The second scenario assumes that irrigation water is applied 10% above the potential plant evapotranspiration rate. Desalinated water gave a higher yield of biomass as compared to irrigationwith brackishwater. In the first scenario,
irrigation with RO or NF permeate did not show any particular difference in yield, despite the difference in salinity. The reason is the high volume applied,which reduces the salt accumulation. In the second scenario the RO permeate gave a higher yield because of the lower volume of irrigation water applied, thus the difference in salinity level between RO and NF permeate had a more significant effect. Ghermandi and Messalem [45] quantified that a transformation from irrigation with brackish water to desalinated water would result in a 45% reduction in the volume of water required for crop production. This kind of change in water management could lead to a 34% saving in groundwater extracted. Moreover, the crop yield could increase by 24% when irrigated
with RO permeate and 18% when irrigated with NF permeate with respect to brackish water irrigation [45]. The increase in yield is cause by the lower salinity level of the irrigation water applied. Another solution could be mixing of NF retentate and RO permeate to obtain low sodium and chloride content and a high number of bivalent
ions. This type of system was proposed by Birnhack et al. [46] with an additional UF step to remove contaminants (pathogens) initially present in seawater and, therefore, also in the NF brine. It was estimate that the cost of this process is approximately one order of magnitude lower than that where magnesium is added either as MgCl2/MgSO4·7H2O, or through calcite/dolomite dissolution or ion exchange [46]. However, the Mg/Ca ratio has to be lower than 1 as previously discussed in this paper. Sundrop Farms in Australia has been re-thinking crop production in a new and sustainable way. The principle behind the concept is shown in Fig. 5, where greenhouses are provided with fresh water through solar powered desalination. This innovative approach for crop production has some significant benefits including no use of natural fresh water resources, no use of fossil fuels, salt and nutrients are collected during desalination and used as fertilizer or sold to other agricultural producers, greenhouse location can be in arid regions and on land not suitable for crop production and with the possibility of minimizing seasonal changes [47]. Despite the technology being MED- and not membrane-based, it is still an interesting solution to future agricultural practices and might be improved further by combining it with membrane technology which can reduce energy requirements.