|M.Sc Student||Hadas Segal|
|Subject||Development of a Second Pass and Establishment|
of Temperature Dependency, as Part of an Energy
Efficient Seawater RO Desalination
Process Aimed at Boron and TDS
|Department||Department of Civil and Environmental Engineering||Supervisor||Full Professor Lahav Ori|
|Full Thesis text|
Boron and TDS removal at single-pass reverse osmosis desalination is a novel, energy efficient process that is being developed by our research group at the Technion. The principle behind this alternative process is to remove boron and TDS at high pH (pH>9) in the first RO pass, while overcoming the risk of CaCO3 scaling encountered at high pH, by applying a decarbonization pretreatment step (via pH decrease and CO2 stripping). A significant increase in recovery ratio and ~10% energy saving may be obtained by using this method with high flux RO membranes, as shown in a previous work for seawater at 250C. Since reverse osmosis desalination is highly temperature depended, the current work tested the feasibility and efficiency of the process at the high- and low-temperature range, using the temperature range in the Mediterranean Sea, i.e. between 15 and 310C. In addition, TDS removal in a single pass process is limited, resulting in permeate concentration of >350 mg/l (high flux membranes), while the TDS permeate quality demand in Israeli seawater desalination tenders is about 30 mg/l. To answer both issues, this work is divided to two sections; first, the feasibility of applying the single pass process was assessed at a wide temperature range, and two pH values at 310C: pH9.3 and pH9.5. Second, the addition of a second pass was examined, assuming the same feed water temperatures, and TDS and boron concentrations as attained in the 1st pass permeate. Finally, the cost of the process was assessed at varying operational conditions.
As expected, the results show a strong correlation between the feed temperatures and the permeate flux and quality. The TDS was 442, 288 and 201 mg/l at 54% recovery ratio and 31, 25 and 150C, respectively. Boron removal was adequate at pH9.5. However, antiscalant dosage is required in the first pass, at 310C and pH9.5, due to the elevation of the propensity for Mg(OH)2 scaling. For feed water at 310C, where antiscalant dosage is avoided, i.e. at pH 9.3, permeate B concentration was slightly above the required concentration.
The TDS and boron concentrations in the second pass brine met the threshold at 90% recovery ratio. The maximal recovery ratio that was achieved in the second pass was 94% with a boron concentration of 0.35 mg/l. The cost of applying this alternative process was lower by about 5$cent/m3 permeate than in the conventional process, when using Tel Aviv’s coast seawater temperatures.
Unlike the second pass brine of conventional processes, the one produced in the developed process can be recycled to the feed stream, since most of the boron was removed in the first pass. The optimum obtained overall recovery ratio for a two pass process was 51-52.5%. Second pass addition increase the operational cost by a ~3$cent/m3, however this alternative is still less costly than the conventional process.
The results of this work aim can be used to provide a design tool for flexible seawater RO operation, in order to meet water quality requirements at varying water feed temperatures.