Potential drawbacks associated with agricultural irrigation with treated wastewaters from desalinated water origin and possible remedies

Over 90% of the water supplied in the coastal region in Israel in 2013 (600 Mm³ y⁻¹) will be from desalination plants. The wastewater generated from this water (>400 Mm³ y⁻¹) is planned, after proper treatment, to be reused for agricultural irrigation, making this low-salinity water the main agricultural-sector future water source. In this respect both the Mg^2 + concentration and the Sodium Adsorption Ratio value of the water are of concern. We show that the typical Na⁺ concentration addition to wastewater (between ∼100 and ∼165 mg L⁻¹) is much higher than the combined addition of Ca^2 + and Mg^2 + (between 0 and several mg L⁻¹). Since desalinated water is typically supplied with low Ca^2 + and Mg^2 + concentrations (∼35 and 0 mg L⁻¹ respectively), the treated wastewater is characterized by very low Mg^2 + concentrations, low salinity and very high SAR values, typically >6 and up to 10 (meq L⁻¹)^0.5. SAR values can be lowered by adding either Ca^2 + or Mg^2 + to desalinated water. Adding Mg^2 + is preferable from both health (minimizing cardiovascular disease hazards) and agriculture (inexpensive Mg fertilization) aspects. The low cost of Mg^2 + addition at the post-treatment stage of desalination plants corroborates the request for Mg^2 + addition in regions where treated wastewater from desalinated water origin is planned to be reused for irrigation.