3.1.1. Drawbacks of water production by RO
Regardless of the success of RO in desalination, the process is also associated with several drawbacks including relatively low water recovery factors, scaling and biofouling, and high electrical energy consumptions. In particular for irrigation water production, a major concern in using desalinated water is the associated cost compared to existing water resources, which is less acceptable for irrigation than for potable
uses. One way to apply desalination to agricultural use is minimizing the cost of electrical energy requirements in RO, responsible for 60% of the water cost [39]. There fore, hybrid systems utilizing renewable energy sources, such as solar power and wind have been investigated. One of the disadvantages of renewable energy sources is their unreliability. Instead, a desalination plant should be supplied with energy when needed,
thereby allowing variation of capacity with respect to the producible energy at specific times. There are some other drawbacks associated with RO desalination for agricultural use. The RO membrane rejects normally more than 95% of all the common ions present in seawater and brackish water. Irrigation of crops with a low sodium and chloride content is beneficial, since these ions damage the soil and inhibit plant growth. However, ionssuch as calcium, magnesium and sulfate are essential nutrients for the crops. An example can be found in Table 10 where the characteristics of the desalinated water produced at the Ashkelon desalination plant are compared with those recommended in Israel for drinking and agricultural purposes [40]. In particular, the magnesium and alkalinity contents of the RO permeate obtained are too low and a post-treatment is
required. The latter includes dissolving calcite with H2SO4 to reach a calcium concentration of 40–46 mg/l. Sulfate is also removed almost completely during the desalination process but it is added with sulfuric acid resulting in a final concentration of 20–25 mg S/l [40]. However, further post-treatment could be necessary tomeet the demands of magnesium and there are various possibilities: (i) direct dosage of MgCl2 (which also increases the number of undesirable chloride ions and the
cost of $0.045/m3 for a concentration equal to 10 mg/l magnesium [40]); (ii) dolomite dissolution (with a cost increase of $0.01–0.02/m3, yet dolomite is difficult to dissolve compared to calcite [40,41]); (iii) mixing of desalinated water with natural resources such as seawater or brackish water (although resulting in unreliable water composition with an increase in sodiumand chloride and negative environmental effects
[40–42]); and (iv) use of ion exchange resins to first extract magnesium from seawater and thereafter exchange magnesium with calcium with an additional cost of $0.004/m3 [43]. Dissolution of minerals can eventually meet the quality standards for irrigation water. Nevertheless, nutrients such as phosphorus, nitrogen, and potassium
must still be supplied from elsewhere (fertilizers/manure). Disposal of the RO retentate stream from desalination plants has a negative environmental impact, especially when it comes from inland desalination for which the following disposal options are possible:
• evaporation ponds
• deep-wells
• surface water bodies
• municipal sewers
• concentration into solid salts
• irrigation of crops which is not inhibited by high salinity.
However, each of these above-mentioned disposal techniques has limitations e.g. with an increase in capacity, the area of evaporation ponds must also increase, and when saline water is used for irrigation it can cause destruction of soil and cause plant decay, though some saline tolerant crops could be directly irrigated with RO brine [44]. Despite some associated drawbacks of RO desalination including higher costs than natural resources, it is the most established technology for potable water production. The continuous development of the RO process with water production rate increases means easier access to desalinated water, also for the agricultural industry, is not unlikely. Membrane engineering could also help in minimizing the drawbacks of RO by integrating different membrane operations or replacing existing technologies with novelmembrane operations, such as forward osmosis (FO) or membrane distillation (MD), although these technologies are still not as developed as RO.