A new post-treatment process for attaining Ca2+, Mg2+, SO42− and alkalinity criteria in desalinated water

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41 (2007) 3989 – 3997

Available at www.sciencedirect.com

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A new post-treatment process for attaining Ca2+, Mg2+, SO2 4 and alkalinity criteria in desalinated water Liat Birnhack, Ori Lahav Faculty of Civil and Environmental Engineering, Technion, Haifa 32000, Israel

art i cle info

ab st rac t

Article history:

A novel post-treatment approach for desalinated water, aimed at supplying a balanced

Received 27 January 2007

concentration of alkalinity, Ca2+, Mg2+ and SO2 4 , is introduced.

Received in revised form

The process is based on replacing excess Ca2+ ions generated in the common H2SO4-

16 May 2007

based calcite dissolution post-treatment process with Mg2+ ions originating from seawater.

Accepted 8 June 2007

In the first step, Mg2+ ions are separated from seawater by means of a specific ion exchange

Available online 12 June 2007

resin that has high affinity toward divalent cations (Mg2+ and Ca2+) and an extremely low

Keywords: Post-treatment Mg2+ Ion exchange Desalination Cost analysis

affinity toward monovalent cations (namely Na+ and K+). In the second step, the Mg2+loaded resin is contacted with the effluent of the calcite dissolution reactor and Mg2+ and Ca2+ are exchanged. Consequently, the excess Ca2+ concentration in the water decreases while the Mg2+ concentration increases. The process is stopped at a predetermined Ca2+ to Mg2+ ratio. All water streams used in the process are internal and form a part of the desalination plant sequence, regardless of the additional ion exchange component. The proposed process allows for the supply of cheap Mg2+ ions, while at the same time enables the application of the cheap H2SO4-based calcite dissolution process, thus resulting in higher quality water at a cost-effective price. A case study is presented in which additional cost of supplying a Mg2+ concentration of 12 mg/L using the process is estimated at $0.004/m3 product water. & 2007 Elsevier Ltd. All rights reserved.

1.

Introduction

Desalinated water provides an increasing portion of the total fresh water supply in a growing number of countries. In Israel, in a few years time, desalinated water will provide over 25% of the overall drinking water volume. A large percentage of this water will be produced by two 100,000,000 m3/year desalination plants: the plant in Ashkelon, operative since September 2005, and the Hadera plant, whose bid was concluded in September 2006 and is expected to start producing water in 2009. In 2006, the Committee for Updating Water Quality Regulations (appointed by the Israeli Ministry of Health) adopted the following set of water quality criteria for desalinated water, to

Corresponding author. Tel.: +972 4 8292191; fax: +972 4 8228898.

E-mail address: [email protected] (O. Lahav). 0043-1354/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2007.06.007

be met prior to the release of the water to the distribution system (Lahav and Birnhack, 2007): alkalinity 480, 80o[Ca2+]o120, calcium carbonate precipitation potential (CCPP) 43 and o10 (all concentrations in mg/L as CaCO3) and pHo8.5. Three main groups of post-treatment processes currently exist for stabilizing reverse osmosis (RO) effluents: (1) processes that are based on direct dosage of chemicals (e.g. dosage of Ca(OH)2 followed by CO2(g)) (Delion et al., 2004; Withers, 2005); (2) processes that are based on mixing the desalinated water with other water sources, with or without further adjustment of water quality parameters (Glueckstern et al., 2005); (3) processes that center around dissolving CaCO3(s) (typically calcite) for alkalinity and Ca2+ supply,

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followed by pH (and CCPP) adjustment using NaOH (De Souza et al., 2002; Glade et al., 2005; Hasson and Bendrihem, 2006; Letterman et al., 1991). The first two groups are less commonly practiced because (a) direct dosage of chemicals is usually expensive and (b) when desalinated water is diluted with other water sources further chemical dosage usually becomes unavoidable, if all criteria are to be met (Delion et al., 2004; Glueckstern et al., 2005; Withers, 2005). Therefore, the third process group becomes the most cost effective, particularly in places where CaCO3(s) is readily available (e.g. Israel). CaCO3(s) is a slightly soluble solid at the neutral pH range. In order to expedite dissolution kinetics, the pH must be reduced before the desalinated water is introduced into the calcite reactor. Two acidic substances are typically used to lower pH: H2SO4 and CO2(g). Using a strong acid such as H2SO4 is a cheaper and simpler practice, and therefore this approach, depicted in Fig. 1, was chosen as the post-treatment method in the already operative Ashkelon desalination plant. The main advantage of this approach is that, due to the rapid dissolution rate of calcite at low pH values, only a fraction of the desalted water can be passed through the calcite reactor (between 18% and 30% of the total flow rate of the plant, typically 25%), a fact that renders the reactor considerably cheaper. On the other hand, H2SO4-based calcite dissolution processes result in a dissolved calcium to alkalinity concentration ratio that is always equal to, or higher than, 2–1 (in equivalent units) while the alternative process, i.e. CO2-based calcite dissolution, results in a ratio of approximately 1–1 (Lahav and Birnhack, 2007). The Ashkelon plant was designed and built before the new criteria set, which allows a maximal ratio of 1.5–1 between [Ca2+] and alkalinity, was approved. Following the adoption of the new criteria, the process that is based on dissolving calcite with H2SO4 became impractical for future desalination plants in Israel. As an immediate result, the technology that was proposed by the desalination companies that competed for the Hadera plant bid in 2006

15% CO2 loss

NaOH

was changed to the more expensive alternative of dissolving calcite with CO2(g). As mentioned, the Ashkelon plant was built before the new criteria were published, and thus the water produced by it does not meet the new regulations. The current water quality that the plant produces is as follows: alkalinity ¼ 45–48 mg/L as CaCO3, [Ca2+] ¼ 100–110 mg/L as CaCO3, pHffi8.15 and a CCPP value of between 0.3 and 0.8 mg/L as CaCO3. It can be seen that the actual [Ca2+] to alkalinity ratio in Ashkelon’s final product is around 2.3–1, the deviation from the theoretical 2–1 ratio is attributed to a certain CO2(g) loss from the unsealed calcite reactors. In the year and a half that has passed since the plant started to produce water, it became apparent that the most problematic water quality parameter is the low alkalinity value, which results in a very low buffering capacity. Consequently, although the water is released from the plant with a positive CCPP (or Langelier Saturation Index, LSI) value and can thus be considered chemically stable, a minor change of its chemical quality might result in a significant change in its chemical stability. For example, the compulsory addition of 0.7–1.0 mg/L of fluorine (by the addition of H2SiF6 (fluorosilicic acid)) at the outlet of the plant results in a significant pH drop, the LSI value (and CCPP) becomes negative and the water becomes unstable from a chemical-corrosion standpoint. Therefore, the plant has been requested by the Office of the Israeli Water Commissioner to examine the option of raising the alkalinity value in the water to 465 mg/L as CaCO3. The reader is reminded that attaining such an alkalinity value also means that the calcium concentration would be raised to around 150 mg/L as CaCO3, a value that exceeds the upper limit set in the criteria for calcium. Another drawback that is associated with both calcite dissolution processes is that they (naturally) do not result in the addition of Mg2+ ions to the water. Mg2+ ions, although not included in the current Israeli quality criteria, are very much welcome in desalinated water for both agricultural (Tisdale

product

w flo % lit 82 Sp % 70

CaCO3(s)

H2SO4 Water from RO process (18% - 30%) Fig. 1 – Schematic of a H2SO4-based calcite-dissolution post-treatment process, as practiced in the Ashkelon desalination plant.

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4 1 (200 7) 398 9 – 399 7

et al., 1993) and human health reasons (Kozisek, 2003; WHO, 2006). Regarding the latter, in a recent WHO meeting of experts (held in Washington, DC, in April 2006) on the possible protective effect of hard water against cardiovascular disease, it was reported that low magnesium status in humans had been implicated in a variety of diseases including hypertension, coronary heart disease, type 2 diabetes mellitus and the metabolic syndrome. The importance of Mg2+ in drinking water was emphasized and it was stated that recent studies indicate that benefits of magnesium concentration in the water level off at concentrations of about 10 mg Mg2+/L (WHO, 2006). Another recommendation that emerged from this meeting was that desalination stabilization practices should ensure that the overall process does not significantly reduce the total intake of nutrients such as calcium and magnesium (WHO, 2006). The Ministry of Agriculture and the Israeli Water Commission Office are currently seeking a cost-effective process for the addition of Mg2+ ions into the water produced in Ashkelon, as a large proportion of this water is used for agricultural irrigation. Practical alternative processes that have been proposed for the addition of Mg2+ ions to desalinated water include direct chemical dosage and dissolution of dolomite minerals. Direct chemical dosage is a very expensive alternative that also results in a high concentration of unwanted counter anions (typically chloride ions). Several problems are encountered when attempting to dissolve dolomite rocks (MgCa(CO3)2) for this purpose: the most noticeable drawback is related to the dissolution kinetics of dolomite, which is much slower than that of calcite (Liu et al., 2005). As a result, much larger reactor volumes are required in order to dissolve an adequate amount of Mg2+ ions. Alternatively, higher dissolution kinetics can be achieved by maintaining a low pH value throughout the dissolution process. The effluent of such an approach would have an adequate concentration of Mg2+

3991

ions, and also a low pH and hence the alkalinity value would be much lower than that required. In such a case, a high NaOH concentration would be required to elevate alkalinity and CCPP values, and often the limit for pH (i.e. pH 8.5) would be exceeded. Moreover, NaOH is the most expensive chemical used in the stabilization process, and a high requirement of this chemical is unfeasible. Another problem associated with dolomite is that the Ca2+ to Mg2+ ratio released during its dissolution tends not to be constant with time because excavated dolomite minerals are rarely pure, and the CaCO3 component within the dolomite structure tends to dissolve more rapidly than the MgCO3 component.

1.1. Presentation of a new post-treatment process aimed at a cost-effective supply of Mg2+, Ca2+, alkalinity and SO2 4 in desalinated water A unique new post-treatment process is introduced with the aim of producing water that complies with the new criteria and also results in a substantial Mg2+ concentration. The process is based on calcite dissolution using H2SO4 for alkalinity and Ca2+ supply, but at the same time is designed to attain the required ratio between Ca2+ and alkalinity concentrations (i.e. not higher than 1.5–1) and also to result in a significant concentration of dissolved Mg2+ (and SO2 4 ) in the water. In this process, sulfate is a by-product supplied with strong acid addition (H2SO4). However, it is worthwhile to mention that SO2 4 is also a very important component in water used for agricultural irrigation, and if not supplied with the water, it has to be added as a fertilizer. The fact that sulfate is added in the proposed process (as a by-product of the use of H2SO4 as the acidic substance) is a significant advantage over, for example, the CO2(g)-based calcite dissolution process, which does not result in sulfate in the water. The focus of the new process is to replace excess Ca2+ ions generated in the H2SO4-based calcite dissolution process by

Fig. 2 – Schematic of the RO process in Ashkelon, and characterization of the streams used for the load and wash steps, all concentrations in mg/L. Original schematic was adopted from Redondo et al. (2003).

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Mg2+ ions originating from the sea (more particularly, from the first brine generated in the RO process). The Mg2+ concentration in seawater is over five times higher than that of Ca2+ (approximately 105 and 20 meq/L, respectively, in the Mediterranean Sea). Approximately the same ratio of Mg2+ to Ca2+ is maintained in the 1st stage brine of the RO process used in the Ashkelon desalination plant (approximately 240 and 44 meq/L, respectively). Fig. 2 depicts schematically the four stages used in the RO plant in Ashkelon, and where the new process is designed to connect with it. A detailed description of the four-stage RO process applied in the Ashkelon plant can be obtained elsewhere (Redondo et al., 2003). The proposed post-treatment process will utilize the 1st stage brine (which is basically seawater concentrated by a factor of about 2.2, see Fig. 2) for loading the resin with Mg2+ ions, and the 4th stage brine, which is characterized by a relatively low total dissolved solids (TDS) concentration (around 1250 mg/L) and a high boron concentration (around 80 mg/L), for washing purposes, as shown in Fig. 2. A quantification of the effect of using the 4th stage brine, as a washing solution, on the final water quality appears in Section 4. Fig. 3 describes the proposed process schematically. In the first step of the proposed method (termed the ‘‘load step’’), Mg2+ ions are preferentially separated from the 1st stage brine by means of an ion exchange resin that has a high affinity toward divalent cations (Mg2+ and Ca2+) but an extremely low affinity toward monovalent cations (namely Na+ and K+). Following the load step, the resin is washed (using the 4th brine) to prevent the enrichment of product water with sea salts. Finally, the Mg2+-loaded resin is contacted with the effluent of the calcite reactor (the ‘‘exchange step’’). In this step, Mg2+ and Ca2+ are exchanged. As a result, the Ca2+ concentration in the desalinated water decreases while the Mg2+ concentration increases to comply with the required quality criteria. All the water streams used in the ion exchange process are internal streams and form part of the desalination plant sequence irrespective of the additional ion exchange process. The 1st stage brine, used to load the resin with Mg2+ ions, is subsequently discharged back to the sea. The same is applied

to the 4th stage brine, used to wash the resin after the load step. The main advantages of using the brines from the RO process are that, first, no excess brine is produced in the ion exchange process and, second, that the brines are available at a very low cost.

2.

Materials and methods

2.1.

Suitability tests for resins

Two resins were evaluated for use in the process: Amberlite IRC748 and Amberlite IRC747, both purchased from Rohm & Haas Inc. Five replicates of 2 g of each resin (on a dry weight basis) were immersed in 35 mL seawater for 4 h. After draining the seawater, the resin samples were immersed in distilled water to rinse residual seawater. Washing was repeated three times, until the washing solution showed an electrical conductivity (EC) of distilled water. The resin samples were then immersed in a 35 mL 1 N HCl solution in order to fully desorb the cations adsorbed to the resin. After the samples had been shaken for 2 h, solution samples were taken for cation analysis using inductively coupled plasma (ICP). Following this, the replicates were re-rinsed with distilled water and then put again in concentrated HCl for 2 h, and the extracting solution was analyzed for cations. This step was repeated until no cations were detected in the extracting solution.

2.2.

Continuous ion exchange experiments

A solution containing Ca2+ was prepared to simulate the exchange step by dissolving analytical grade CaCl2  2H2O in distilled water. Brine from the Ashkelon desalination plant was used to simulate the load step (1st stage brine). The wash step was simulated by water similar in composition to the Ashkelon 4th stage brine. Adsorption and desorption experiments of Ca2+ and Mg2+, simulating the load and the exchange steps, were carried out using a 25.2 mm internal diameter PVC column, filled with resin. Resin bed height was measured after a short period of operation, to ensure steady

Fig. 3 – Schematic of an H2SO4-based calcite-dissolution desalination post-treatment process operating in parallel to a set of batch ion exchange columns.

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packing of the bed. The resin bed volume (BV) was 82.4 mL. A peristaltic pump (Cole Parmer) was used to attain a constant flow rate of 25 BV/h into the column.

2.3.

Analyses

During the wash experiment EC was monitored using an EC meter (CyberScan 500). Chloride ion concentration was measured using the argenometric method, according to the standard methods (APHA, 1998). Mg2+, Ca2+, K+ and Na+ were analyzed by ICP emission spectrometry, Optima 3000 DV, Perkin-Elmer.

3.

Results

3.1.

Choice of resin

Both resins were received from Rohm & Haas Inc. and showed, as required, a high affinity toward divalent cations and an extremely low affinity toward monovalent cations. Table 1 shows the distribution of cations in the solid phase of both resins after they were brought to equilibrium with seawater, as compared with the natural distribution of the same cations in seawater (and also in the 1st stage brine). The advantage of Amberlite IRC747 over Amberlite IRC748 for the proposed process is apparent by the fact that IRC747 adsorbed almost no Na+ at all, whereas Na+ constituted 3% of the overall capacity of IRC748, following equilibrium with seawater. This is clearly unwanted since any adsorbed Na+ would be easily released to the product water in the following exchange step. Moreover, the minimal reaction retention time of Amberlite IRC747 (i.e. 1.5 min), suggested by the manufacturer, is much more favorable to the process. A retention time of 1.5 min is translated into a maximal flow rate of 40 BV/h, which is a much higher possible flow rate than the one allowed with IRC748, and thus much less resin is required for a given desalinated water flow rate. As a result, Amberlite IRC747 was chosen, and all further results in the paper are reported with relation to it.

3.2.

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4 1 (200 7) 398 9 – 399 7

Simulation of the exchange, load and wash steps

Continuous experiments were conducted in order to determine the number of BVs that can be passed through the resin in the load and exchange steps in order to attain the required water quality. Assuming that 25% of the raw desalinated water passes through the calcite reactor and thus also

through the ion exchange column (see Fig. 3), the concentrations of the components in the water after the exchange step should be four times higher than the required product water concentrations (because the water is diluted at a 4–1 dilution factor, prior to the NaOH addition). A split flow of 20–25% is typical in the operation of H2SO4-based calcite dissolution reactors, e.g. at the Ashkelon plant. A case study that simulates the process in Ashkelon is now presented as an example to the possible implementation of the new process. As mentioned before, the alkalinity concentration in the product water of the Ashkelon plant is planned to be raised to 65 mg/L as CaCO3 (1.3 meq/L). In such a case, the Ca2+ concentration expected at the outlet of the calcite reactor will be 600 mg/L as CaCO3 or 12 meq/L (assuming that 25% of the plant’s water will be pumped into the calcite reactor, and that a Ca2+ to alkalinity ratio of 2.3–1 is attained at the outlet of the calcite reactor, as explained in Section 1). Considering that the final product water (a mixture of the ion exchange column effluent and the raw desalinated water—see Fig. 3) should have a final dissolved calcium concentration of approximately 2 meq/L (i.e. 100 mg/L as CaCO3), the calcium concentration in the water should be reduced during the exchange step from 12 to 8 meq/L. The resulting Mg2+ concentration will thus be 4 meq/L prior to the dilution with the raw desalinated water, and 1 meq/L after the dilution. Such a magnesium concentration meets the recent recommendation presented in the WHO expert meeting (WHO, 2006). The number of BVs in the exchange step for the described conditions is shown in Fig. 4, which presents the cumulative concentrations of Ca2+ and Mg2+ in the water leaving the ion exchange, when a 1st brine-loaded resin is contacted with a 600 mg/L Ca2+–CaCO3 (12 meq/L) solution. Fig. 4 shows that after around 220 BVs, the cumulative concentrations of Ca2+ and Mg2+ are around 8 and 4 meq/L, respectively. Fig. 5 shows the number of BVs required to load the resin with the 1st stage brine following the exchange step. It is shown that the load step can be practically stopped after 15 BV. At the end of the load step, the resin has to be washed to remove residual 1st stage brine water and prevent it from mixing with the product water. During the wash step, lowdensity (light) water (4th stage brine) is pumped downwards through the column and the concentrated (heavy) 1st stage brine water is pushed out of the reactor. In other words, washing is done by passing a cheap, low-TDS brine through the resin bed, until the salt concentration of the water in the bed tends toward the salt content of the washing water.

Table 1 – Cation distribution in the solid phase of Amberlite IRC747 and Amberlite IRC747 loaded with seawater versus the cation distribution in Mediterranean seawater, and minimal contact time recommended by the manufacturer for each resin Resin Amberlite IRC747 Amberlite IRC748 Seawater (and 1st stage brine)

Mg2+ (%)

Ca2+ (%)

K+ (%)

Na+ (%)

Minimal contact time (min)

77.4 78.3 17.5

22.3 18.4 3.4

0.1 0.2 1.7

0.1 3.1 77.5

1.5 15–30

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10 Mg2+ and Ca2+ concentrations (meq/L)

Requested water quality

Ca2+

8 6 4

Mg2+ 2 0 50

0

100

150

200

250

300

350

Number of bed volumes Fig. 4 – Results of an exchange experiment–cumulative Mg2+ (squares) and Ca2+ (diamonds) concentrations at the outlet of a single ion exchange column (resin loaded with 1st stage brine; inlet water quality: [Mg2+] ¼ 0, [Ca2+] ¼ 12 meq/L). 300 Mg2+ and Ca2+ concentrations (meq/L)

250 Mg2+ 200 150 100 Ca2+

50 0 0

4

8

12

16

20

24

Number of Bed Volumes

Electrical Conductivity (ms/cm)

Fig. 5 – Mg2+ (diamonds) and Ca2+ (squares) concentrations in the effluent of the load step (experimental conditions: loading solution: 1st stage brine; original resin loaded with mostly Ca2+, as a result of a previous exchange step depicted in Fig. 4). 100 80

1st Brine

60 40

4th Brine

20 0 0

1

2

3

4

5

6

7

8

Number of Bed Volumes Fig. 6 – Electrical conductivity of the water leaving the bed during the wash step: washing solution simulative of the 4th stage brine in the Ashkelon plant.

The number of BVs required to complete this procedure is three to four, as shown in Fig. 6. At the end of the wash step, the resin is allowed to drain so that the volume of water (4th stage brine) that remains in the bed is approximately 0.11 BV (this draining step lasts about 1.5 min; after 2.5 min an almost identical volume of 0.096 BV remains in the bed). Acknowl-

edging that the TDS and boron concentrations in the brine are 1250 and 80 mg/L, respectively, one can easily calculate the additional values added to the product water: when 220 BV of product water are used after the wash step, the TDS added to the product water (DTDS) and the boron concentration added to the product water (DBoron) will be 0.15 mg TDS/L and

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Table 2 – Estimation of operational and capital costs of the post-treatment approach proposed in the work as compared with the cost of two operational modes of the current post-treatment process implemented in the Ashkelon desalination plant Units

Target quality Alkalinity CCPP [Ca2+] [Mg2+] pH

Alternative #0 current process in Ashkelon desalination plant

Alternative #1 current process (upgraded water quality)

Alternative #2 proposed process

48

65

66

0.5 110 0 8.15

2.1 150 0 8.07

2.1 99.5 12.15 8.27

71

100

100

110 8.5

149.4 14

149.4 14.8

0.0138

0.0194

0.0194

0.0039 0.0071 – 0.00004

0.0052 0.0117 – 0.00004

0.0052 0.0123 0.0004 0.00007

0.0248

0.0363

0.0374

2500

2500

2500

–

–

1545

–

–

1320

$/m3

2500 0.0020

2500 0.0020

5365 0.0043

$/m3

0.027

0.038

0.042

mg/L as CaCO3

mg/L

Chemical dosage H2SO4

g/m3 product water

CaCO3 NaOH Operating expenses H2SO4

$/m3 product water

CaCO3 NaOH Resin amortization Energy costs Total Capital expenses Calcite reactors (including pipes, pumps, etc.) Ion exchange columns (including pipes, pumps, control etc.) Cost of resin (110 m3) Total Total per m3 product water Overall cost of post-treatment per 1 m3 of product water

$103

0.0098 mg B/L, respectively (these values were calculated for a case where 0.11 BV are left in the bed as a result of the draining stage that follows the wash step). An example clarifying the calculations is given in Eq. (1) for the additional boron concentration: " # 80 mg B=Lð4th stage brineÞ DBoron ¼

0:11ðBV of brine remaining after washÞ

retention time required for the ion exchange reaction is 1.5 min, or, in other words, a maximum flow rate of 40 BV/h (manufacturer’s data). The overall design flow rate in the Ashkelon plant is 14,000 m3/h, out of which 25% is designed to flow through the calcite reactor and the ion exchange columns. For the sake of simplicity, a flow rate of 35 BV/h was assumed to calculate the volume of resin required in the exchange step:

220ðBV during exchangeÞ  4ðsplit ratioÞ

¼ 0:0098 mg B=L:

ð1Þ

3.3. General design of the process for the case study of the Ashkelon desalination plant The volume of resin required in the exchange step is determined by Eq. (2), acknowledging that the minimal

Vresin ¼

14; 000m3 =h =4 35BV=h

¼ 100 m3 =BV:

(2)

The volume of resin required in the load and wash steps together amounts to around 9% of the amount in the exchange step (15 and 4 BV in the load and wash steps, respectively, versus 220 BV in the exchange step); thus a total volume of 109 m3 resin is required in the process.

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Thus, the general design can assume 11 ion exchange columns, each with 10 m3 of resin: at all times 10 columns are in the exchange step and the other column is in the load/ wash step. Ion exchange is, by definition, a non-steady-state reaction. A single ion exchange column will produce water at the beginning of the exchange step that is high in Mg2+ and low in Ca2+ and exactly the opposite at the end of the exchange step. However, under the current design, the 10 resin columns are operated at a time gap of 37 min from each other (220 BV at a flow rate of 35 BV/h, i.e. a full cycle of a single column lasts 6.29 h and one-tenth of it is 37 min). Under such an operational regime, the effluents of the ion exchange columns are mixed and the Mg2+ and Ca2+ concentrations in the final product water would change linearly with time during 37-min repeating cycles from 7.53 to 8.34 meq/L ([Ca2+]) and from 4.54 to 3.56 meq/L ([Mg2+]).

3.4.

Preliminary cost estimation

Table 2 shows the chemical dosages and energy requirements in the post-treatment process (calcite dissolution and ion exchange combined) required for attaining the final water quality defined in the studied case (for supplying 100,000,000 m3/year) and their associated costs per 1 m3 of product water ($/m3). Table 2 also lists estimations of infrastructure costs (calcite reactors, ion exchange columns, pumps, etc.), also expressed in units of $/m3 (assuming an annual interest of 5% and an average life expectancy of 20 years). It is noted that the infrastructure costs are estimations only and are not based on actual quotes. However, we believe that a considerable safety factor is already included in them. The overall cost of 1 m3 of product water in the shown case study is $0.042 as compared with $0.027/m3 estimated for the current operation in Ashkelon and $0.038/m3 estimated for the case in which the post-treatment in the Ashkelon plant is upgraded to result in a higher buffer capacity (alkalinity value of 65 mg/L, see Table 2), which conforms better to the new Israeli regulations (the plant cannot, practically, increase the alkalinity value to 480 mg/L with the existing post-treatment infrastructure).

4.

washing the resin with the 4th stage brine following the load step was found to be between three and four and the volume of water that remains in the bed at the end of the wash step was approximately 0.11 BV. The TDS and boron concentrations added to the product water as a result of the water that remained in the bed following the wash step were shown to be 0.15 mg TDS/L and 0.0098 mg B/L, respectively, representing a negligible addition of salts. Based on these data, it was calculated that an overall volume of 110 m3 resin is required in the process (100, 7 and 3 m3 resin required at all times at the exchange, load and wash steps, respectively). If 11 ion exchange columns are applied (i.e. 10 m3 of resin per column, 10 columns in the exchange step, and the 11th is first loaded and then rinsed before switching back to the exchange mode), the Ca2+ and Mg2+ concentrations in the product water will range from 94.1 to 104.3 mg/L as Ca–CaCO3, and 10.8 to 13.8 mg Mg2+/L, respectively. If a narrower concentration range is required, one can either increase the number of operative ion exchange columns or shift to a continuous ion exchange operation mode. The additional cost (per 1 m3 of product water) of adding the ion exchange component to the calcite dissolution reactor and raising the alkalinity concentration to 65 mg/L as CaCO3 is $0.015/m3, relative to the current operation and $0.004/m3 as compared to a situation where the plant is required to improve the water quality only by increasing the alkalinity value of the water to 65 mg/L as CaCO3 but without adding magnesium. This appears a more than reasonable cost for the supply of 12 mg/L of Mg2+. Note that direct addition of MgSO4 to attain a similar Mg2+ concentration amounts to approximately $0.05/m3.

5.

 The paper introduces a new two-step post-treatment



Discussion

 A case study that is based on the RO process currently implemented in the new Ashkelon desalination plant is presented as an example for the possible application of the proposed post-treatment process. Mg2+ addition is based on separating it from the 1st RO brine using an ion exchange resin that is characterized by high affinity toward divalent cations along with an extremely low affinity toward monovalent cations and a low minimal hydraulic retention time of 1.5 min. Under the described specific operational conditions, the resin was shown to operate for 220 BV in the exchange step before it was necessary to switch to the load step, in which Mg2+ was reloaded on the resin by passing through it 1st RO brine water. A total of 15 BV were required in the load step, suggesting that the time in which the resin would spend in the load step is approximately 7% of the time it would spend in the exchange step. The number of BVs required for

Conclusions





process for desalinated water that can be used to meet quality criteria for a balanced combination of alkalinity, Ca2+, Mg2+ and SO2 4 concentrations. The process is based on exchanging excess Ca2+ ions generated in the common H2SO4-based calcite dissolution post-treatment process by Mg2+ ions that originate from the sea, using a specific cation exchange resin. All streams used to load and wash the resin in the process are by-products of the RO desalination process, and can be discarded to the sea following their usage in the posttreatment process. No problematic brine is generated in the ion exchange step. It has been shown that additional TDS and boron concentrations to the product water, as a result of the new process, are negligible. From an economic standpoint, the process is based on the most cost-effective post-treatment process for the supply and Ca2+ (i.e. the H2SO4-based CaCO3 of alkalinity, SO2 4 dissolution process). The operational cost of Mg2+ addition amounts to (low) energy requirements to pump the water through the ion exchange columns (i.e. a head difference of several meters), since sea-based Mg2+ ions come at no cost. Capital expenses include the cost of the resin, ion exchange columns, control and paraphernalia (pipes,

ARTICLE IN PRESS WAT E R R E S E A R C H

4 1 (200 7) 398 9 – 399 7

pumps, etc.), out of which the cost of the resin is the most marked component. The overall cost of the proposed posttreatment process per 1 m3 of product water is $0.042/m3, only $0.004/m3 higher than the cost of supplying an almost identical water quality, without the Mg2+ addition.

R E F E R E N C E S

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