Oded Nir

Predicting the Rejection of Major Seawater Ions by Spiral-Wound Nanofiltration Membranes

Seawater nanofiltration (SWNF) generates a softened permeate stream and a retentate stream in which the multivalent ions accumulate, offering opportunities for practical utilization of both streams. This study presents an approach to simulation of SWNF including all major seawater ions (Na(+), Cl(-), Ca(2+), Mg(2+), and SO4(2-)) based on the Nernst-Planck equation, and uses it for permeate and retentate streams composition prediction. The number of degrees of freedom in the system was reduced by assuming a very high ionic permeability for Na(+), which only weakly affected the other parameters in the system. Two alternatives were examined to analyze the importance of concentration dependence of ion permeabilities: The assumption of constant ion permeabilities resulted in a reasonable fit with experimental data. However, for the permeate composition the overall fit was significantly improved (P < 0.0001) when the permeabilities of Ca(2+) and Mg(2+) were allowed to depend on the ratio of their total concentration to Na(+). This type of dependence emphasizes the strong interaction of divalent ions with the membrane and its effect on the membrane fixed charge through screening or charge reversal. When this effect was included, model predictions closely matched the experimental results obtained, corroborating the phenomenological approach proposed in this study.

Modeling pH variation in reverse osmosis

The transport of hydronium and hydroxide ions through reverse osmosis membranes constitutes a unique case of ionic species characterized by uncommonly high permeabilities. Combined with electromigration, this leads to complex behavior of permeate pH, e.g., negative rejection, as often observed for monovalent ions in nanofiltration of salt mixtures. In this work we employed a rigorous phenomenological approach combined with chemical equilibrium to describe the trans-membrane transport of hydronium and hydroxide ions along with salt transport and calculate the resulting permeate pH. Starting from the Nernst-Planck equation, a full non-linear transport equation was derived, for which an approximate solution was proposed based on the analytical solution previously developed for trace ions in a dominant salt. Using the developed approximate equation, transport coefficients were deduced from experimental results obtained using a spiral wound reverse osmosis module operated under varying permeate flux (2-11 μm/s), NaCl feed concentrations (0.04-0.18 M) and feed pH values (5.5-9.0). The approximate equation agreed well with the experimental results, corroborating the finding that diffusion and electromigration, rather than a priori neglected convection, were the major contributors to the transport of hydronium and hydroxide. The approach presented here has the potential to improve the predictive capacity of reverse osmosis transport models for acid-base species, thereby improving process design/control.

A new algorithm for design, operation and cost assessment of struvite (MgNH4PO4) precipitation processes

Deliberate struvite (MgNH4PO4) precipitation from wastewater streams has been the topic of extensive research in the last two decades and is expected to gather worldwide momentum in the near future as a P-reuse technique. A wide range of operational alternatives has been reported for struvite precipitation, including the application of various Mg(II) sources, two pH elevation techniques and several Mg:P ratios and pH values. The choice of each operational parameter within the struvite precipitation process affects process efficiency, the overall cost and also the choice of other operational parameters. Thus, a comprehensive simulation program that takes all these parameters into account is essential for process design. This paper introduces a systematic decision-supporting tool which accepts a wide range of possible operational parameters, including unconventional Mg(II) sources (i.e. seawater and seawater nanofiltration brines). The study is supplied with a free-of-charge computerized tool (http://tx.technion.ac.il/~agrengn/agr/Struvite_Program.zip) which links two computer platforms (Python and PHREEQC) for executing thermodynamic calculations according to predefined kinetic considerations. The model can be (inter alia) used for optimizing the struvite-fluidized bed reactor process operation with respect to P removal efficiency, struvite purity and economic feasibility of the chosen alternative. The paper describes the algorithm and its underlying assumptions, and shows results (i.e. effluent water quality, cost breakdown and P removal efficiency) of several case studies consisting of typical wastewaters treated at various operational conditions.

A new, energy-efficient approach for boron removal from SWRO plants

Abstract A different approach for the operation of seawater reverse osmosis desalination plants, in which the boron concentration in the product water should not exceed 0.3 mgB/L, was recently introduced. The new approach is based on strong acid (either H2SO4 or HCl) dosage to the feed seawater to attain pH ∼ 4.3, followed by almost complete CO2 stripping and subsequently strong base addition to pH 9.0–9.25. At this high pH range, a high B removal efficiency can be attained even by the new generation of ultra-low energy (high-flux) membranes. This paper addresses the energy saving potential stemming from the elimination or size reduction of the 2nd reverse osmosis (RO) pass and from the use of high-flux elements, both made possible by the new approach. Additionally, total dissolved solids removal from the 1st RO pass permeate can be obtained by operating a smaller, more energy efficient, 2nd RO pass.

Acid–base dynamics in seawater reverse osmosis: experimental evaluation of a reactive transport algorithm

Acid–base properties such as pH, alkalinity and boron speciation are essential factors for the design and operation of seawater reverse osmosis desalination, as they influence critical processes such as membrane scaling, biofouling and boron permeation. However, due to the high complexity of acid–base dynamics in both the retentate and permeate streams of reverse osmosis applications, the evolution of acid–base related properties in these streams is not yet completely understood. This often leads to overly simplified models resulting in inaccurate predictions, which may impede optimal design and thus the cost-effectiveness of the process. This paper introduces a unique computerized model dedicated to modeling acid–base dynamics within seawater reverse osmosis processes based on a reactive-transport algorithm, which couples transport and equilibria phenomena. The WATRO (Weak Acid Transport in Reverse Osmosis) simulation program based on this algorithm (freely available in the ESI) is experimentally tested here and shown to extend the predictive capacity and accuracy for these important quantities beyond previous work, and also beyond the capabilities of available commercial software. Implications related to 1st and 2nd pass CaCO3 scaling and boron rejection are exemplified and discussed. From the practical standpoint, the advantages of the suggested approach are particularly prominent for 2nd RO pass modeling and design, mainly due to the passage of hydronium and hydroxide ions through the membrane.