Chemical Engineering Journal 187 (2012) 275–282 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal j ourna l ho mepage: www.elsevier.com/locate/cej A novel approach for SWRO desalination plants operation, comprising single pass boron removal and reuse of CO 2 in the post treatment step Oded Nir a,∗ , Moshe Herzberg b , Amer Sweity b , Liat Birnhack a , Ori Lahav a,∗ a Faculty of Civil and Environmental Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel b Ben Gurion University, Sde Boqer campus, 84990, Israel a r t i c l e i n f o Article history: Received 22 September 2011 Received in revised form 13 January 2012 Accepted 17 January 2012 Keywords: Boron removal SWRO Desalination Post treatment Stripping towers a b s t r a c t A different approach is presented for the operation of seawater RO desalination plants in which the boron concentration in the product water should not exceed 0.3 mgB/l. The approach is based on strong acid (either H 2 SO 4 or HCl) dosage to the feed water to attain pH ∼ 4.3, followed by CO 2 stripping and subsequently strong base addition to pH 9.00–9.25. At this high pH range, a high B removal efficiency is attained, and since the water is practically devoid of carbonate species, no CaCO 3(s) scaling takes place, and pH elevation is limited by Mg(OH) 2(s) precipitation, expected only at pH > 9.45. The approach enables operation in the absence of antiscalants. Furthermore, CO 2 stripping is effected in stripping towers in two steps: the high CO 2(aq) concentration is first stripped by vacuum-operated stripping towers and the CO 2 -rich air is used for dissolution of calcite in the post treatment stage. The remaining CO 2 mass is stripped to the atmosphere using blower-assisted stripping towers. This paper aims at introducing the new concept and providing “proof of concept”. The paper addresses experimental and theoretical aspects of the proposed process, as well as engineering and economic evaluation. The proposed approach is shown to be both technically feasible and cost effective, as compared with conventional boron removal alternatives. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Removal of boron species (B) from desalinated water is a signif- icant component in the process design of many seawater reverse osmosis (SWRO) desalination plants. At concentrations above ∼1.0 mg B/l, boric acid is known to damage various agricultural crops and plant species used in municipal gardening. For exam- ple, crops such as avocado and most citrus types are sensitive to B at the concentration range 0.5–0.75 mgB/l [1]. Therefore, although the world health organization (WHO) had recently updated up the guidelines for B concentration in drinking water from 0.5 to 2.4 mg/l [2] (due to strictly human health reasons), it is most likely (and also apparent from the results of recent international bids) that the demand for low B concentration (0.3–0.8 mgB/l) in desalinated waters will remain unchanged. Boron exists in natural fresh water as a weak acid with a thermodynamic pK value of 9.23. At pH val- ues lower than the pK, the protonated, neutral, boric acid species (B(OH) 3 ) dominates, while above it the negatively charged borate ion (B(OH) 4 - ) prevails. In seawater, which usually contains ∼5 mg B/l, the apparent pK value is ∼8.6 due to seawater’s ionic com- position [1]. The boric acid species, which dominates in natural ∗ Corresponding author. Tel.: +972 4 8292191; fax: +972 4 8228898. E-mail address: agori@technion.ac.il (O. Lahav). seawater pH of ∼8.2, is poorly rejected by the commonly avail- able RO membranes. While the rejection of charged ions, such as Na + and Cl - , is higher than 99%, practical B rejection using standard SWRO membranes is only ∼65–80%, corresponding to ∼0.9–1.8 mg/l B in the permeate [3]. Thus, either ion-exchange- based post treatment (PT) or the operation of a second (brackish) RO pass is typically implemented in order to meet the sometimes strict B regulations [1]. Application of a BWRO second pass includes dosage of a strong base to the first SWRO pass permeate, in order to elevate pH to 9.5–10.5, prior to its introduction into the membrane. The pH elevation diverts the boric acid species towards the borate ion, whose rejection by RO membranes is much more efficient. Ion exchange (IX) technology utilizes a resin with a high affinity towards B, which adsorbs B(OH) 4 - at basic to neutral conditions. Strong acid is required for the regeneration of the resin and a strong base is required for neutralization thereafter [1]. Several process configurations were developed that make use of these technolo- gies, including combinations of the two [1,3,4]. New boron removal approaches have also been recently suggested [5,6]. Cost approx- imations for B removal from RO permeates at the post-treatment stage [4,7,8], resulted in a roughly similar cost range, i.e. between 0.04$/m 3 and 0.1$/m 3 for either the IX- or BWRO-based methods. While energy consumption is the major cost factor for the operation of BWRO B removal, consumption of chemicals is the most signif- icant cost item associated with the IX approach. Cost evaluations 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.080