Journal of Membrane Science 301. High-recovery reverse osmosis desalination using intermediate chemical demineralization

                                                                Journal of Membrane Science 301 (2007) 131–141                                                                                                   

High-recovery reverse osmosis desalination using intermediate chemical demineralization

Christopher J. Gabelich^a, Mark D. Williams^b, Anditya Rahardianto^c, John C. Franklin^d, Yoram Cohen^c,∗

^a Metropolitan Water District of Southern California, 700 Moreno Avenue, La Verne, CA, USA ^b Williams-McCaron Inc., Long Beach, CA, USA

^c Water Technology Research Center, Chemical and Biomolecular Engineering Department,

University of California, Los Angeles, CA, USA ^d United States Bureau of Reclamation, Yuma, AZ, USA

Received 8 August 2006; received in revised form 7 June 2007; accepted 10 June 2007

Available online 14 June 2007

Abstract

A two-stage reverse osmosis (RO) process for high water recovery (up to 95%) desalination of Colorado River water was evaluated and demonstrated at the pilot scale. The two-stage process involved intermediate chemical demineralization (ICD) of the concentrate stream from a primary RO (PRO) process followed by secondary RO (SRO) desalting. Using alkaline-induced precipitation in a pilot-scale solids contact reactor (SCR), the membrane scaling propensity of the PRO desalting step was reduced, allowing for further recovery of product water via SRO of the demineralized PRO concentrate stream. When operating the SCR effluent above pH 10, the removal of Ca²⁺, Ba²⁺, Sr²⁺, and silica – the principal scale-forming precursors – were attained via ICD at levels upwards of 94%, 97%, 88%, and 67%, respectively. Despite significant process variations, 95% overall water recovery desalination was demonstrated to be feasible in the pilot scale via the PRO–ICD–SRO approach, with SRO desalting operated at specific permeate flux varying by ≤7% and at stable normalized salt passage of 2.9±0.7% over extended periods of time. This study suggests that long-term implementation of the PRO–ICD–SRO process to achieve overall water recovery ≥95% would require robust on-line pH control strategy to attain consistent SCR effluent pH, along with appropriate selection and use of make-up antiscalants for the secondary RO desalting step, primarily to control membrane scaling by silicates and gypsum.

© 2007 Elsevier B.V. All rights reserved.

Keywords: Precipitation softening; Chemical demineralization; Reverse osmosis; Fouling; High recovery RO desalination

1. Introduction

The unique properties of reverse osmosis (RO) membranes to reject inorganic species, while passing relatively pure water, has led to the widespread use of membrane processes to treat various water sources. With increased product water recovery, the concentration of inorganic species near the membrane surface rises and, beyond a critical recovery level, can exceed the solubility of sparingly soluble mineral salts (e.g., calcium carbonate (CaCO₃), calcium sulfate dihydrate (denoted simply as CaSO₄) and barium sulfate (BaSO₄)) [1]. As a consequence,

membrane scaling by mineral salts reduces membrane productivity and forces operation at lower than optimal recovery level, thereby increasing concentrate production, an unwanted byproduct. Concentrate minimization is essential for many inland locations that do not have access to cost-effective and/or environmentally acceptable disposal methods.

To operate a membrane desalination process at a high level of product water recovery, the concentration of mineral scale forming ions must be reduced below the critical membrane scaling threshold. A two-stage process that integrates RO desalting with chemical demineralization is one of the technically viable solutions for achieving this goal. In the first step, primary RO (PRO) is operated up to a product recovery level below the critical membrane scaling thresholds dictated by the source water quality,operatingpH,andtypeanddoseofantiscalants.Theconcentration of mineral scale-forming ions in the PRO concentrate stream is then lowered using an intermediate chemical demineralization (ICD) process, followed by a secondary RO (SRO) of the treated primary RO concentrate (i.e., PRO–ICD–SRO approach). Accordingly, an important objective of this study was to evaluate and demonstrate the integration of an ICD process to enhance overall product water recovery of RO desalting up to 95%. The proposed process was evaluated for desalting Colorado River water (CRW) – a primary water source for many communities in the southwestern United States – and involved inducing and accelerating mineral precipitation of the PRO concentrate by chemical dosing, followed by solid–liquid separation, stabilization of the filtrate by acid dosing, and subsequent SRO desalting. The concentrate recovery system was evaluated in a pilot plant system in terms of reduction of scale forming precursors (e.g., Ba²⁺, Ca²⁺, silica, and Sr²⁺) and the achievable recovery in the SRO step. The specific goals of this project were to (1) evaluate the effects of pH on the efficiency of membranemineralscaleprecursors’removalfromPROconcentrate by alkaline-induced mineral precipitation in a pilot-scale solids contact reactor (SCR); (2) demonstrate the integration of ICD with SRO at the pilot scale to achieve up to 95% total system water recovery; and (3) demonstrate the impact of process and seasonal water quality variations on the performance of the PRO–ICD–SRO approach at the pilot scale.