Background

Inland brackish water sources often contain precursors of sparingly soluble mineral salts, which upon concentration of the reverse osmosis (RO) retentate as a consequence of the desalting process can result in mineral scaling of RO membranes. Membrane scaling impairs plant productivity (i.e., permeate flux) and product water quality. Therefore, the development of cost-effective scale-control strategies is imperative, including the optimization of conventional techniques of membrane scaling mitigation (e.g., feed pH adjustment and antiscalant dosing).

As water is passed through successive RO modules, mineral salt scale precursor ions (e.g., Ba2+, Ca2+, SO;j_, CO3", etc.) are concentrated in the membrane retentate side (i.e., feed-side) to levels that can exceed the solubility limit of various sparingly water-soluble mineral scalants such as calcium sulfate dihydrate (gypsum), calcium carbonate (e.g., calcite), and barium sulfate (barite), in addition to metal hydroxides and silica [4]. Under such conditions, sparingly soluble mineral salts may crystallize directly onto the membrane surface or form in the bulk and subsequently deposit on the membrane [5,6] forming mineral surface scales. This leads to water-permeate flux decline and potential damage to the membrane, thereby shortening its useful lifetime.

One simple solution to alleviating the mineral scaling problem is to operate at low water recoveries (i.e., lower the fraction of product water produced per volume of feedwater) in order to reduce concentration of mineral salts at the membrane surface (i.e., also known as concentration polarization). However, this approach that limits the recovery leads to increased volume of generated concentrate and introduces a serious concentrate management dilemma, which can result in significant increased process cost or even elimination of membrane desalting as a water desalination option.

In order to maximize water recovery, a majority of RO facilities use pH adjustment and antiscalants to control for these rate-limiting salts [7]. Both of these treatment techniques essentially shift the salt solubility such that scaling does not occur within the RO unit. However, even with the above processes, water recovery is limited (~ 50-85%) with a significant concentration volume that requires further costly processing or disposal. For many inland locations, cost-effective disposal methods are limited or unavailable. Therefore, there is a need to minimize the volume of the concentrate stream in order to reduce the concentrate management challenge.

Concentrate minimization often requires integration of one or more treatment units in combination with RO or other desalination processes. The technologies fall into four basic categories: (1) cation control, (2) anion control, (3) physical separation of anions and cations, and (4) thermal processes. Cation and anion control includes intermediate chemical demineralization (ICD) [8,9] and biological sulfate control [10], respectively. Electrodialysis (ED) and electrodialysis reversal (EDR) [11], along with capacitive deionization (CDI) [12,13], have been proposed as charge separation technologies. Examples of thermal processes are vapor compression (VC) [14], membrane distillation (MD) [15], and freeze desalination [16]. Several newer technologies such as forward osmosis (FO) [17,18], dewvaporation [19], and vibratory shear-enhanced process (VSEP) [20] have been proposed as methods to treat RO concentrate due to their potentially lower fouling propensity and/or energy usage. However, each of the aforementioned technologies is in various stages of development. While ICD has undergone pilot and demonstration testing [5,21], most of the others [e.g., biological sulfate reduction (BSR), FO, MD, and CDI] have not progressed past the bench level. These technologies could be the first step toward achieving a zero-liquid discharge (ZLD) or near-ZLD facility. Each of these processes will be discussed briefly in the following sections.

The limitation to high water recovery RO desalting (and thus RO concentrate minimization) can be illustrated by considering the factor (CF) by which the reject (i.e., retentate) stream is concentrated, relative to the where CC and CF are the retentate and feed concentrations, respectively, RS is the nominal salt rejection (RS — 1 — CP/CF, where CP is the

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