|Ph.D Thesis||Department of Civil and Environmental Engineering|
|Supervisor:||Assoc. Prof. Lahav Ori|
|Full Thesis text|
Desalination permeates are slightly acidic, poorly buffered and very soft. Thus, post-treatment is invariably required. Despite this, no unambiguous quantitative criteria had been proposed to address its required quality, following the post-treatment stage. Consequently, the first step in the current work focused on developing the Israeli quality criteria for desalinated water. The following set was decided upon and serves now as the Israeli criteria: Alkalinity > 80, 80 < [Ca2] < 120, 3 < CCPP < 10 (all concentrations in mg/L as CaCO3), and pH < 8.5. Subsequently, the work focused on development and modeling of a novel post-treatment process, aimed at cost-effectively complying with the new criteria, while also supplying Mg2 ions, which are required for both health and agricultural reasons. The investigated process is based on (a) dissolving calcite using H2SO4, or CO2 or a combination of both acids; and (b) replacing a portion of the Ca2 ions generated in the calcite dissolution process with Mg2 ions originating from seawater or brackish water. This is achieved by first separating Mg2 ions from seawater or brackish water by means of a specific ion-exchange resin having a high affinity towards divalent cations (Mg2 and Ca2) and an extremely low affinity towards monovalent cations. Thereafter, the Mg2-loaded resin is contacted with the calcite dissolution reactor effluent and Mg2 and Ca2 are exchanged. Consequently, the Ca2 concentration in the water decreases while the Mg2 increases. Eleven case studies were experimentally investigated.
A generic mathematic model simulating the entire process was developed in order to extend the problem into a wider range of case studies. Using the model, the process’s operational parameters, resultant water quality and its cost can be evaluated for almost any operational conditions.
Dolomite dissolution was also investigated as a competitive means of adding Mg2, Ca2 and alkalinity to desalinated water. The results showed that dolomite dissolution per se is not feasible for post-treatment purposes, because dolomite stops dissolving at a relatively low pH. Therefore, three combined dolomite-calcite dissolution alternatives were also investigated. The results showed that the most promising method is to dissolve dolomite and then use the CO2(aq) rich effluent of the dolomite dissolution reactor to further dissolve calcite.
The additional operational costs of supplying 1 kg Mg2 using any of these post-treatments were estimated at <1$, i.e. more than one order of magnitude cheaper than the cost of supplying Mg2 through fertigation or fertilization.