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Boron removal from seawater using FILMTECTM high rejection SWRO membranes
DESALINATION Desalination 156 (2003) 229-238
ELSEVIER
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Boron removal from seawater using FILMTECTM high rejection SWRO membranes Jorge Redondo *, Markus Busch, Jean-Pierre De Witte Deutschland GmbH & Co. OHG., Inhtstriestt.asse I. 77836 Rheinmiinstet: Germany L)ow 721
49 (-1727) 91-3774:
Fax - 49 (7227)
91-370:
emails: jredondo~do~l:com,
nlbltsch~doll’.cottt
jpdell,itre’_,do~l,.cottt
Received 20 February 2003; accepted 24 February 2003
Abstract This paper gives a quick introduction to the boron problem and its relevance in seawater desalination. It then covers the chemistry of boron in water and provides data for the pH-dependant rejection of boron with various membrane types. Field data on boron rejection from a high recovery seawater desalination plant using FILMTEC seawater reverse osmosis (SWRO) membranes is presented thereafter. Four characteristic design concepts that have
been used to achieve low boron limits are discussed from an economic and technical point of view. Ke~~u~oru’s: Boron chemistry; selective
Boron rejection;
FILMTEC
membranes; Field performance;
1. Introduction to boron problem and chemistry 1.2. Boron
problem
Due to increasing demand for water, both potable and for irrigation, coupled with a decrease in suitable water sources suppliers have to turn to alternatives. Seawater desalination or treatment of high saline, eventually contaminated surface *Corresponding author. I’re.sc~t~/c~tl (I/ //7e Etlropean l%ropeutt
Uesniintrtiot7
00 I l-91 64/03!
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Design concepts; Boron-
ion exchange resin
Cotlferetlce
Sociely
on Desalinnlion
Ir~ternationnl
waters have become standard [4]. By using those alternative sources more trace contam inants start to appear in the final product. Among those is boron. Boron in drinking water from brackish surface waters or ground water can be traced back to either residuals from waste water treatment plants (mainly borate from detergent formulations) or to leachables from subsurface strata. In the case of a seawater source the average boron concentration in the raw water is 4.5 mg/L [l].
and ihe l:nvironment:
Fresh
Waterfor All, Malta. J--X May 2003
Il’nter Associnlion.
See front matter 0 2003 Elsevier Science B.V. All rights reserved
1-9164(03)00345-X
J Redondo et al. ,’Desalination 156 (2003) 229-238
230
There are two predominant reasons for limiting boron in water: For humans boron can represent reproductive dangers and has suspected teratogenetic properties. The WHO has set a limit of 0.5 mg/L for drinking water [2]. A major limiting factor is the possible damage to plants and crops. Although boron is vital as a trace element for plant growth and is supplied in fertiliser it can be detrimental at higher concentrations. Amongst the more sensitive crops are citrus trees, which show massive leaf damage at boron levels ofmore than 0.3 mg/L in the irrigation water [3]. Excess boron also reduces fruit yield and induces premature ripening on other species such as kiwi.
??
??
As a consequence we have seen the appearance of boron limits in the tender documents for medium and large membrane desalination plants with values between 0.3 and 0.5 mg/L. I .2. Chemistry of boron Boron is usually present in water as boric acid, a weak acid which dissociates according to: H,BO, t) H’ + H2B0;
pKa 9.14
(1)
H,BO;
pKa 12.74
(2)
pKB 13.8
(3)
tiH++BO;-
HBOf- ti H’ + BO;-
Its concentration is usually expressed as “total boron” (tB), which includes all species and is expressed in terms of the molecular weight of the boron atom. tB H
At lower pH the major species is boric acid in molecular form. Due to the absence of ionic charges, the hydration of the molecule camlot enhance those charges and is less strong. This results in a smaller size and less rejection of the molecule by a membrane. The dissociated form on the other hand will be fully hydrated, resulting in a large] radius and an enhancement ofthe negative charge of the ion. This in turn results in higher rejection, both by exclusion and repulsion by the negatively charged membrane. 2. Boron rejection of FILMTEC
As seen in Fig. 1, at the natural pH of 7 to 8 of most waters used in desalination the predominant species is boric acid in molecular form. At these pH values, the percentage of the non-dissociated species H,BO, is between 99.3 (pH 7) and 93.2% (pH 8) of total boron. The rejection of that species is in the range of 82-92% for most seawater RO membrane products in the market, and the rejection for brackish water products ranges between 30 and 80%. Typical boron rejections for the FILMTEC products SW3OHR-380, SW3OHR-320, SW30380, BW30-400 and BW30LE-440 under standard test conditions are shown in Table 1, In the standard test condition at pH 8, 93.2% of molecular boron is present as neutral H,BO, and 6.8% as H,BO;. Therefore this condition reflects mainly the rejection of the neutral species. There are considerable differences between the individual products: the difference between seawater (SW) and brackish water (BW) membranes
[H,BO,]+ [H,BO;]+ [HBO;-]
+ [BO;-]
as mg/L B
70
(4)
the usual pH operating range of reverse osmosis elements, Eq. (1) is the one with the highest importance. We thus have a presence of both dissociated and non-dissociated boric acid
g
60
f
50
= -z 0
40
III
species iii the water.
membranes
30 20 10
I.
75
8
85
9
9 5
in
105
PH
Fig.
I. Distribution of H,BO,/H,BO,~
in function of PH.
!I
J. Redondo
et al. I Desalination
156 (2003)
231
229-238
Table 1 Typical boron rejection with FILMTEC seawater (SW) and brackish water (BW) membranes at standard test conditions
FILMTEC FILMTEC FILMTEC FILMTEC FILMTEC _____
Boron rejection at natural pH
Specification
Product SW30HR-380 SW30HR-320 SW30-380 BW30-400 BW30LE-440
6000 gpd (0.95 m’/h) flow, 5000 gpd (0.79 m’/h) flow, 9000 gpd (I .43 m’/h) flow, 10500 gpd (1.67 m’ih) flow, 11500 gpd (1.81 m3/h) flow,
90% 90% 88% 65% 54%
99.70% salt rejection 99.70% salt rejection 99.40% salt rejection 99.50% salt rejection 99.0% salt rejection
(88-92) (88-92) (85-90) (55-75) (43-63)
FilmTec standard test conditions: . TDS 32,000 mg/L, p =55 bar, 7’= 25 “C, pH 8, Y = 8% for SW30HR and Y==10% for SW30 products TDS 2000 mg/L, p = 16 bar, T= 25”C, pH 8, I’= 15% for BW30-400 TDS 2000 mg/L, p = 10.7 bar, T = 25 “C, pH 8, J’= 15% for BW30LE-440
??
??
x
100.0%
from 90% rejection for the SW30HR-380 to 65% for the tighter brackish water product BW30-400. A further difference can be seen with the lower energy membrane, which uses a more permeable membrane, and has a standard boron rejection of 54%. Dow in-house tests have confirmed a very low standard deviation for the boron rejection of standard production: e.g. 96% of all delivered FILMTEC SW30HR-380 product is between 88 and 92%. A wider analysis of various membrane products available in the market has also shown that higher variation in total boron rejection is possible. This may be due to the different membrane production techniques used and has been seen especially after membrane cleaning steps. We therefore recommend performing benchmark boron rejection tests before deciding on a specific membrane brand and element to use. At higher pH values, rejections are significantly improved. As explained earlier, this is due to the shift to the better rejected H,BO,- species, for which the rejection ranges between 99.0 and 99.8%. However, even at very high pH values, a small portion of uncharged H,BO, is present which has a significant impact on the passage of total boron (tB). The rejection of total boron of the FILMTEC products SW3OHR-380, BW30400 and BW30LE-440 is displayed in Fig. 2. varies
95.0%
.
90.0% ??
x
i
.
.
85.0% .-: 2 2
80.0% m
75.0% 70.0%
A
65.0% m
m
50.0% A
A
6
7
60 0% 55 0%
. sw3oHR-380
m
;x BW30-400 A Bw3OLm40 ‘1
A 8
pti “L
10
11
12
Fig. 2. Rejection of total boron with FILMTEC membrane elements.
Fig. 2 confirms what can be expected from the distribution of the boron species: in the region around the pH value for the dissociation of boric acid, there is substantial improvement with every unit of pH shift. A shift to pH 10 brings the total boron rejection from 93 to 99% depending on the membrane chemistry. At a pH of 11, the total boron rejection is 99.0-99.5%. It should be noted that operation in this pH region with FILMTEC membrane elements is safe, provided the appropriate anti-sealants are used. The necessity of scaling prevention will depend upon the composition of the feed water and in many situations the use of anti-sealants is not required. Based on above observations, it is obvious that high pH operation is advantageous for boron rejection.
J Redondo et al. ’Llesalination 156 12003) 229-238
232
3. Field performance of FILMTEC rejection SWRO membranes
high
A case history on an installation using FILMTEC seawater reverse osmosis (SWRO) membranes with boron rejection data is presented here. It includes not only data on boron rejection in that system but also on pressure requirements (hence normalised flow) and rejection of the total dissolved solids (TDS) fraction. This is done for the following reasons: Consideration of boron rejection needs to take system operating pressures into account. A “boron-selective” product is not cost-efficient when the nominal flow is too low and the energy requirement too high. Rejection of boron and TDS are not directly coupled. It is true that higher TDS rejection chemistries tend to also have higher boron rejection. It can not be implied though, that two products with the same TDS rejection will have the same boron rejection and vice versa. Boron rejection is principally controlled by the membrane chemistry (diffusion) and to a lesser degree by convective transport (mechanical leakage), whereas TDS rejection is controlled by both. In contrast, for high rejection products the convective transport becomes even the more dominant factor in determining TDS rejection. Therefore TDS rejection needs to be considered in addition to boron rejection, as there may be certain differences. ??
??
The plant under consideration is a seawater plant with two stages, which enables operation at high recovery. It has 4 trains and uses FILMTEC SW3OHR-380 membrane elements. Each train contains 52 pressure vessels in the 1st and 38 pressure vessels in the 2nd stage and has an interstage booster pump. Each vessel contains 7 elements. The feed TDS is 37,830 mg/L, feed boron concentration 4.98 mg/L, and temperature around 22°C. The plant produces 290 m3/h at a recovery of 55% with a designed average permeate flux of 15 L(m2h). Table 2 and Fig. 3 show the plant performance at start-up and in the following 4 months. The operating conditions are depicted to the left whereas the normalised values are shown to the right of the table. These values were normalised using the FilmTec program ROSA (Reverse Osmosis System Analysis). The performance is normalised to the SW3OHR-380 element, which was used in this case. Nominally this element has a performance of 0.95 m’/h (6000 gpd) of flow and 99.70% rejection. At start-up, feed pressure was 7.1 bar lower than predicted in the projection. This is partly explained by the lower than designed flow rate, but is also due to the outstanding average flow performance ofthe 2500 FILMTEC SW30HR-380 elements in the plant. Anormalisation ofthe startup conditions indicates that the elements had an average flow performance of 1.06 m’/h (6730 gpd), which is 12% higher than the specification. The
Table 2
Operating data for 290 m’/h plant Time (d) Projection I 3 28 64 91 133 ___
Pressure
I__Temp.
(bar)
(“C)
66.2 59.1 65.0 65.4 66.4 65. I 65.3 ___.~_
22.3 22.4 22.3 22.1 22.2 22.2 22
~_.__~ Feed Permeate flow flow (m’/h) (m3/h) 531 525 539 535 537 533 529
292 273 296 294 297 292 290
Recovery (W
55.0 52.1 55.3 54.7 55.3 54.8 54.8 __.-__
Permeate TDS (ma)
Normalised element flow (gpd)
Normalised Normalised element flow TDS reject. (m3h) W)
243 227 187 166 175 152 I63
6000 6730 6490 6310 6200 6230 6200
0.95 1.06 1.02 1.oo 0.98 0.98 0.98
99.70 99.72 99.77 99.79 99.78 99.8 1 99.80
J. Redondo
Fig. 3. Normalised 290 m’ih plant.
et al. /Desalination
flow and rejection performance
of
permeate TDS was projected to be 243 mg/L with the 99.70% nominal rejection element. For this case the permeate TDS was 227 mg/L at start-up, which corresponds to a normalised rejection of 99.72%. Within the first hours a flow decrease occurred, which is a normal phenomenon with membranes that come out of the preservation solution. After 24 h the initial flow decrease had stopped and the membranes were stabilised. A further much slower flow decrease was seen between day 3 and the end of the observation period. This slower flow decrease of 300 gpd is due to fouling. The membranes in the plant have not been cleaned since start-up and current operation suggests cleaning is still not necessary. The stable flow performance is probably due to the short leaf length of FTLMTEC elements which results in more uniform flux distribution over the element. The homogenous flux distribution avoids areas with high flux and therefore minimises fouling.
156 (2003)
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A rejection increase as function of time can be seen. In the initial stabilisation period the rejection increases substantially from 99.72% to 99.77%. This is a normal observation with FILMTEC membrane elements, which improve in rejection during operation. In the period following the initial stabilisation, a further slight increase in rejection to 99.80% can be seen. Table 3 displays the projected and observed boron concentrations in stage 1 and stage 2 of the high recovery plant. The projection was done with a simulation that has been specifically developed for projecting boron in membrane systems. This simulation uses the data and models for boron rejection presented in sections 1 and 2 and is currently being integrated to the FilmTec ROSA (Reverse Osmosis System Analysis) program. The system results confirm the model in principle. The normalised rejection in the plant is actually somewhat higher than predicted, which can be attributed to the FILMTEC membrane elements operating in the plant. The permeate of both stages in the 55% recovery plant ranges from 0.79 to 0.86 mg/L, which is below the limit of 1.O mg/L used in some desalination contracts, but above the 0.5 mg/L limit by the World Health Organisation (WHO). In fact, the 1st low recovery stage of the 2-pass plant almost meets the boron specification. Minimal post-treatment, e.g. by using a small partial 2nd pass at high pH, or a boron-selective ion exchange resin, would reduce the boron outlet to CO.5 mg/L.
Table 3 Boron rejection of 295 m’lh plant Time (d) Projection 3 64 133
c(B), feed 1st c(B), feed2nd c(B), permeate stage stage 1st stage (mg/L) @g/L) (mg/L) 5.00 7.50 0.70 4.98 7.48 0.60 5.21 7.79 0.63 5.07 7.65 0.61
c(B) permeate 2nd stage (mg/L)
c(B), total permeate (mg/L)
I .20 I .32 1.22 1.15
0.86 0.84 0.82 0.79
- Normalized boron rejection (“/) 90.0 90.3 90.5 90.6
234
.J Redondo et al. / Desalination 156 f2003) 229- 238
4. Design concepts for boron removal The boron problem has most frequently been encountered in seawater desalination. Seawater membranes have the highest boron rejection which is however still insufficient to comply with the most stringent requirements (e.g. boron concentration of 0.4 mg/L in the final product). Several design concepts have been developed by original equipment manufacturers (OEMs), engineering companies, process consultants, end users (plant operators), etc. to achieve an efficient and safe boron removal at competitive costs. Among the proposed concepts are the following [5-10,12,14]: 1 SWRO pass with natural seawater feed pH as well as lower or higher feed pH 2 passes with increased pH, especially in the 2nd pass. Above include options with high and low recovery in the 2nd pass. 2 passes with boron-selective ion exchange resins (IER), with options treating a part of the I st pass permeate, which does not feed the 2nd pass. 3 passes with low and high pH stages in the 2nd pass. Most of the above concepts have been studied and some pilot-tested in various options, including partial stream softening, e.g. to allow the extension of the process pH range with a minimum or no use of anti-sealants. Some of the concepts including the use of boron-selective ion exchange resin propose their use straight after the SWRO, for a partial stream ofthe whole 1st pass permeate, while other propose their use only for the part of the permeate originated in the rear elements of I st pass permeate vessels. 1. I. One puss SWRO
This concept has been preposed for projects requiring 0.8 to I .O mg/L boron concentration in the RO permeate, where boron in the seawater feed is in the order of 4.0-5.0 mg/L and the seawater temperature range is 1S-26°C. The typical
water production cost for this concept is in the range of US$0.38-0.52/m’ of product [7,1 O]. A typical example for operating results with this type of design has been presented in section 3 of this paper. In that case the feed concentration was between 5.0 and 5.2 mg/L, temperature between 22.0 and 22.4”C and the boron outlet of the I st low recovery seawater stage between 0.60 and 0.70 mg/L, and the concentration in the product mix of both stages at a total recovery of 55% in the range between 0.79 and 0.86 mg/L. 4.2. Twlopasses with increased pN This concept has been widely proposed for systems where the product boron requirement is between 0.4 and 0.5 mg/L and the boron concentration of the seawater feed ranges between 4.0 and 6.3. It has also been used for water temperatures up to 34°C. The typical unit cost for this option is in the range ofUS$0.45-0.55/m3 product water [7,8, IO]. As can be seen from the pH considerations in section 2, the boron rejection of this type of system can be very high. An example for this type ofsystem: with a feed boron concentration of 6.3 mg/L, 34°C and recoveries of 45% in the 1st and 85% in the 2nd pass, the boron concentration of the permeate would be expected in the range of 0.2 mg/L providing the pH of the feed is adjusted. Depending on the pH value, the recovery and Ca and Mg rejection of the membranes used, care needs to be taken for CaCO, and Mg(OH)> scaling. Anti-sealant use should be considered, or the combination of pH and recovery needs to be adjusted to minimise scaling potential in the brine [ 131. FILMTEC membranes show stable long-term flow and rejection performance at high pH conditions. 3.3. Two reverse osmosis passes combined boron selective ion exchange resins
with
This concept has been proposed for seawaters containing 4.5-6.0 mg/L boron and with a request
J. Redondo et al. /Desalination
235
156 (2003) 229-238
Bypass 20 %, C, = 0.5 - 0.6 ppm
b
25%, C, = 0.1 ppm Boron selective IER
100 % (Blend) C,<0.4ppm
Fig. 4. Typical block diagram
for a hybrid RO/IER boron removal concept.
for boron concentration of less than 0.4 mg/L in the final blend of streams from 1st, 2nd pass RO systems and the boron selective ion exchange resin. A typical block diagram (not including pumps) of this hybrid RO/IER process is shown in Fig. 4. The diagram is generic, but the most common options include a seawater high-pressure pump and a low-pressure booster for the 2nd pass BWRO. The IER section can use (or not) a booster as well. A boron-selective ion exchange resin is available from Dow, with the denomination DOW XUS 43594 [ 111. Historically this type of products has been developed for the removal of boron and other weak acids in high purity applications, such as the semiconductor industry. Boron-selective resins will typically remove boron to levels of ~0.1 mg/L B, far below the required limits. DOW XUS 43594 has received NSF certification under standard 61 for potable water applications in October 2002. The capability of Dow to provide both the membrane element and the boronselective ion exchange resin is perceived as clear strategic advantage for systems with boron removal. Most of the options studied [7, IO, 12,141 have a variable stream distribution for the various temperatures considered. The partial 2nd pass
contributes bet-ween 50 and 60% to the final blend with a boron concentration between 0.2 and 0.35 mg/L, the IER stream can be lo-25%, the boron concentration is about 0.1 mg/L and the balance stream for the final blend is 1st pass permeate with 0.5-0.6 mg/L ofboron. The concentrations of boron depend upon feed pH and temperatures. The blend can be adjusted to have always less than 0.4 mg/L of boron and the typical chlorides concentration in the blend is in the range of 100-130 mg/L. The boron removal by IER process is in about the same range of cost as the 2nd pass treatment, i.e. it adds in average 7-9 US cents per m3 product water to the part streams undergoing the respective processes. This combination has been projected and offered for various medium and large plants in the last two years with typical unit costs in the range of US$O.50-0.55 per m3 product water, i.e. close to the figures shown in the next described process.
4.4. Three passes with high and low pH stages in the 2nd pass This concept has been proposed for a seawater containing between 4.8 and 6.3 mg/L and with simultaneous requirements for low boron (0.4 mg/L),
236
J. Redondo er 01
Desalination
low chlorides (e.g. 15 mg/L) and low TDS (e.g. 25 mg/L) in the final blended permeate output. This concept, known as IDE process (or Ashkelon process) has been proposed by the OTlD consortium (IDE+Vivendi)forthemegaprojectAshkelon. This project was developed as a BOOT tender to provide IO0 million rn? per year desalinated high quality water, mainly for potable uses and the project has been granted to the OTID consortium mentioned above. The description below follows a presentation made by IDE [9] and uses the following terms: 1st pass is called 1st RO stage 2nd pass - 1st stage (2 arrays) is called 2nd RO stage 2nd pass - 2nd stage (2 arrays) is called 3rd RO stage 3rd pass (2 arrays) is called 4th RO stage
l_i6 (2003) 229-238
surface. ‘The permeate of the 2nd stage has a low concentration of boron and constitutes Its brine has a high product water. concentration of boron, Ca and Mg ions and is processed in the 3rd RO stage. The 3rd RO stage uses the 2nd stage brine at weak-acid pH as feed and can be defined as low pH and high recovery. The pH of the feed water is low which avoids CaCO, and Mg(OH), salt and allows high recovery but results in poor boron rejection. The 4th stage uses feed from the 3rd stage permeate at high pH and reduces the boron concentration. It is used in the product water blend.
??
??
??
??
These stages operate as follows (please also refer to Fig. 5): The 1st stage RO stage is a conventional seawater RO desalination system. Boron in permeate is reduced to concentrations in the range 0.9-l .5 mg/L. The permeate from the RO membranes at the feed side of the RO vessel (front permeate) has a lower concentration of boron and can be used as product water by blending with the product water from the 2nd and the 4th stage. The 2nd RO stage can be defined as a stage of high pH and moderate recovery, to avoid CaCO, and Mg(OH), scaling on the membrane
??
??
The so called 2nd and 3rd stages, more commonly called 2nd pass 1st stage (2 arrays) and 2nd pass 2nd stage (also 2 arrays) are in reality a once-through process with a very high recovery. The borderline between the 2nd and the 3rd desalination stages can be moved depending on feed water temperature, membrane age and efficiency of anti-sealant. Frequency converters in 2nd and 3rd stage pumps allow for system flexibility. The four stage RO desalination and boron removal system offers the following: Low specific power consumption, as pressured 2nd stage brine enters third stage feed with low power losses. Power in pressurised brine of 3rd and 4th stages can be recovered in a Pelton turbine and used in 3rd stage booster. ??
Fmal product
Brine .I____ 1st stage /
///
rear
Fig. 5. Block diagram of Ashkelon process.
,’
J. Redondo
??
??
et al. / Desalination
Low chemical consumption, as pH transition between stages takes place in conditions of low bicarbonate buffer. This system offers high operational safety
This four stage boron removal process is patent pended by IDE Technologies Ltd. The typical unit cost for this option has been projected in the range of US$O.47-0.52/m’ product water.
5. Summary and conclusions There have been significant and proven improvements in the basic flow and salt rejection properties of SWRO membranes and elements. Flow rates in the range of 6000-6700 gpd (0.951.06n-G/h) and TDS rejections of 99.70-99.85% have been documented in operational SWRO systems. These developments have also yielded improvements in the rejection of boron at neutral pH values. In combination with innovative process solutions, improved membranes have enabled substantial progress in the economic reduction of boron in SWRO permeates to the stringent levels currently required. It is now feasible to consider SWR.0 designs with recovery ratios in the range of 4O-60% whilst operating on feed waters containing up to 48,000 mg/L and boron concentrations in the range of 3.5-6.8 mg/L. Typical water production costs for such processes appear to be in the order of US$O.380.50/m of treated water with product boron contents of 0.6-I .O mg/L and US$O.47-0.60/m’ with product boron contents of 0.3-0.5 mg/L for drinking and agricultural applications. The reported figures are based upon large and very large plants (1 O,OOO-300,000 m’/d) at convenient locations and infrastructures. Pumps and energy recovery devices with efficiencies in the range of 85% and up to 95% including isobaric chambers, as well as energy costs in the order of US cent 47/kWh and convenient interest rates of 3.5-6.5% per year contribute to the overall low cost of production. The combination of improved membrane
156 (2003)
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237
products with varied process possibilities for boron removal, the progress in all the ancillary equipment and positive financial climate make possible the use of this technology in very large water purification applications previously not considered viable and holds the potential for further improvements in the near future. Acknowledgements We want to first thank our customers for the good co-operation in designing plants with boron removal. We have learned a lot from this work. Thanks are also due to Steve Jons, Duane Jacobson, David Wintergrass and their respective teams for the boron data in the membrane section. Thanks to Steve Wrigley and Sara Fessler for the ion exchange resin part and Andreas Gorenflo for his support of graphic material.
References [II PI
PI
Ambient Water Quality Guidelines for Boron. British Columbia, Water Protection Branch. Guidelines for Drinking-Water Quality, 2nd ed., Addendum to vol. I, Recommendations. Geneva, World Health Organisation, 1998, pp. 4-6. Environmental Protection Agency (EPA). 1975.
Preliminary investigation of effects on the environment ofboron, indium,nickel,selenium,tin, vanadium and their compounds, vol. 1, Boron. US Environmental Protection Agency Rep. 56/2-75-005A. [41 J. Redondo, Brackish, sea and waste water desalination as sustainable water sources. Technical-economical and ecological answers based on membrane technology. Desalination, 138 (2001) 29-40. [51 Personal communication with Dr. Manuel Farinas Iglesias. [61 Extract from Ashkelon Tender Documents 2001, t71 J.L. Loidi and .L.Gonzalez, ACS-Tedaguaprojections for the two pass SWRO and IER boron rejection process, July 24 and Nov. 4, 2002. PI M. Farinas, Infraestructuras hydraulicas municipales, Expert seminar, Andalucia International University, Jan. 2003. [91 B. Liberman and I. Liberman, Method of Boron Removal in Ashkelon Desalination Plant, 5th IWA conference on Membranes in Drinking and Industrial
238
[IO] [ 1I] [ 121
[ 131 [ 141
J Redondo et al. /Desalination Water Production, Miilheim an der Ruhr, Germany, Sept. 22-26, 2002. M.A. Sanz, Meeting on Desalination Technology and Costs, Paris, Nov. 2002. Dow document, DOWEX IER, Product Information for XUS 43594.00, Jan. 2003. Y. Mansdorf, Flow sheet draft for 2 pass SWRO and IER boron rejection process, Tambour Ecology Graph, May 7,2002. FILMTEC Membranes Technical Manual, April 1995. P. Glueckstem and M. Priel , Potential cost reduction of seawater desalination, 5th IWA conference on Membranes in Drinking and Industrial Water Production, Mtilheim an der Ruhr, Germany, Sept. 222 26, 2002.
1.56 (2003) 229-238
Disclaimer No freedom for any patent owned by Dow Deutschland GmbH & Co. OHGiFilmTec Corporation or others is to be inferred. Because use conditions and applicable laws may differ from one location to another and may change with time, Customer is responsible for determining whether products and the information in this document are appropriate for Customer’s use and for ensuring that Customer’s workplace and disposal practices are in compliance with applicable laws and other governmental enactments. Dow Deutschland GmbH & Co. OHG / FilmTec Corporation assume no obligation or liability for the information in this document. No warranties are given; all implied warranties of merchantability or fitness for a particular purpose are expressly excluded.
ELSEVIER
www.elsevier.com/locate/desal
Boron removal from seawater using FILMTECTM high rejection SWRO membranes Jorge Redondo *, Markus Busch, Jean-Pierre De Witte Deutschland GmbH & Co. OHG., Inhtstriestt.asse I. 77836 Rheinmiinstet: Germany L)ow 721
49 (-1727) 91-3774:
Fax - 49 (7227)
91-370:
emails: jredondo~do~l:com,
nlbltsch~doll’.cottt
jpdell,itre’_,do~l,.cottt
Received 20 February 2003; accepted 24 February 2003
Abstract This paper gives a quick introduction to the boron problem and its relevance in seawater desalination. It then covers the chemistry of boron in water and provides data for the pH-dependant rejection of boron with various membrane types. Field data on boron rejection from a high recovery seawater desalination plant using FILMTEC seawater reverse osmosis (SWRO) membranes is presented thereafter. Four characteristic design concepts that have
been used to achieve low boron limits are discussed from an economic and technical point of view. Ke~~u~oru’s: Boron chemistry; selective
Boron rejection;
FILMTEC
membranes; Field performance;
1. Introduction to boron problem and chemistry 1.2. Boron
problem
Due to increasing demand for water, both potable and for irrigation, coupled with a decrease in suitable water sources suppliers have to turn to alternatives. Seawater desalination or treatment of high saline, eventually contaminated surface *Corresponding author. I’re.sc~t~/c~tl (I/ //7e Etlropean l%ropeutt
Uesniintrtiot7
00 I l-91 64/03!
%$PII:SOOl
Design concepts; Boron-
ion exchange resin
Cotlferetlce
Sociely
on Desalinnlion
Ir~ternationnl
waters have become standard [4]. By using those alternative sources more trace contam inants start to appear in the final product. Among those is boron. Boron in drinking water from brackish surface waters or ground water can be traced back to either residuals from waste water treatment plants (mainly borate from detergent formulations) or to leachables from subsurface strata. In the case of a seawater source the average boron concentration in the raw water is 4.5 mg/L [l].
and ihe l:nvironment:
Fresh
Waterfor All, Malta. J--X May 2003
Il’nter Associnlion.
See front matter 0 2003 Elsevier Science B.V. All rights reserved
1-9164(03)00345-X
J Redondo et al. ,’Desalination 156 (2003) 229-238
230
There are two predominant reasons for limiting boron in water: For humans boron can represent reproductive dangers and has suspected teratogenetic properties. The WHO has set a limit of 0.5 mg/L for drinking water [2]. A major limiting factor is the possible damage to plants and crops. Although boron is vital as a trace element for plant growth and is supplied in fertiliser it can be detrimental at higher concentrations. Amongst the more sensitive crops are citrus trees, which show massive leaf damage at boron levels ofmore than 0.3 mg/L in the irrigation water [3]. Excess boron also reduces fruit yield and induces premature ripening on other species such as kiwi.
??
??
As a consequence we have seen the appearance of boron limits in the tender documents for medium and large membrane desalination plants with values between 0.3 and 0.5 mg/L. I .2. Chemistry of boron Boron is usually present in water as boric acid, a weak acid which dissociates according to: H,BO, t) H’ + H2B0;
pKa 9.14
(1)
H,BO;
pKa 12.74
(2)
pKB 13.8
(3)
tiH++BO;-
HBOf- ti H’ + BO;-
Its concentration is usually expressed as “total boron” (tB), which includes all species and is expressed in terms of the molecular weight of the boron atom. tB H
At lower pH the major species is boric acid in molecular form. Due to the absence of ionic charges, the hydration of the molecule camlot enhance those charges and is less strong. This results in a smaller size and less rejection of the molecule by a membrane. The dissociated form on the other hand will be fully hydrated, resulting in a large] radius and an enhancement ofthe negative charge of the ion. This in turn results in higher rejection, both by exclusion and repulsion by the negatively charged membrane. 2. Boron rejection of FILMTEC
As seen in Fig. 1, at the natural pH of 7 to 8 of most waters used in desalination the predominant species is boric acid in molecular form. At these pH values, the percentage of the non-dissociated species H,BO, is between 99.3 (pH 7) and 93.2% (pH 8) of total boron. The rejection of that species is in the range of 82-92% for most seawater RO membrane products in the market, and the rejection for brackish water products ranges between 30 and 80%. Typical boron rejections for the FILMTEC products SW3OHR-380, SW3OHR-320, SW30380, BW30-400 and BW30LE-440 under standard test conditions are shown in Table 1, In the standard test condition at pH 8, 93.2% of molecular boron is present as neutral H,BO, and 6.8% as H,BO;. Therefore this condition reflects mainly the rejection of the neutral species. There are considerable differences between the individual products: the difference between seawater (SW) and brackish water (BW) membranes
[H,BO,]+ [H,BO;]+ [HBO;-]
+ [BO;-]
as mg/L B
70
(4)
the usual pH operating range of reverse osmosis elements, Eq. (1) is the one with the highest importance. We thus have a presence of both dissociated and non-dissociated boric acid
g
60
f
50
= -z 0
40
III
species iii the water.
membranes
30 20 10
I.
75
8
85
9
9 5
in
105
PH
Fig.
I. Distribution of H,BO,/H,BO,~
in function of PH.
!I
J. Redondo
et al. I Desalination
156 (2003)
231
229-238
Table 1 Typical boron rejection with FILMTEC seawater (SW) and brackish water (BW) membranes at standard test conditions
FILMTEC FILMTEC FILMTEC FILMTEC FILMTEC _____
Boron rejection at natural pH
Specification
Product SW30HR-380 SW30HR-320 SW30-380 BW30-400 BW30LE-440
6000 gpd (0.95 m’/h) flow, 5000 gpd (0.79 m’/h) flow, 9000 gpd (I .43 m’/h) flow, 10500 gpd (1.67 m’ih) flow, 11500 gpd (1.81 m3/h) flow,
90% 90% 88% 65% 54%
99.70% salt rejection 99.70% salt rejection 99.40% salt rejection 99.50% salt rejection 99.0% salt rejection
(88-92) (88-92) (85-90) (55-75) (43-63)
FilmTec standard test conditions: . TDS 32,000 mg/L, p =55 bar, 7’= 25 “C, pH 8, Y = 8% for SW30HR and Y==10% for SW30 products TDS 2000 mg/L, p = 16 bar, T= 25”C, pH 8, I’= 15% for BW30-400 TDS 2000 mg/L, p = 10.7 bar, T = 25 “C, pH 8, J’= 15% for BW30LE-440
??
??
x
100.0%
from 90% rejection for the SW30HR-380 to 65% for the tighter brackish water product BW30-400. A further difference can be seen with the lower energy membrane, which uses a more permeable membrane, and has a standard boron rejection of 54%. Dow in-house tests have confirmed a very low standard deviation for the boron rejection of standard production: e.g. 96% of all delivered FILMTEC SW30HR-380 product is between 88 and 92%. A wider analysis of various membrane products available in the market has also shown that higher variation in total boron rejection is possible. This may be due to the different membrane production techniques used and has been seen especially after membrane cleaning steps. We therefore recommend performing benchmark boron rejection tests before deciding on a specific membrane brand and element to use. At higher pH values, rejections are significantly improved. As explained earlier, this is due to the shift to the better rejected H,BO,- species, for which the rejection ranges between 99.0 and 99.8%. However, even at very high pH values, a small portion of uncharged H,BO, is present which has a significant impact on the passage of total boron (tB). The rejection of total boron of the FILMTEC products SW3OHR-380, BW30400 and BW30LE-440 is displayed in Fig. 2. varies
95.0%
.
90.0% ??
x
i
.
.
85.0% .-: 2 2
80.0% m
75.0% 70.0%
A
65.0% m
m
50.0% A
A
6
7
60 0% 55 0%
. sw3oHR-380
m
;x BW30-400 A Bw3OLm40 ‘1
A 8
pti “L
10
11
12
Fig. 2. Rejection of total boron with FILMTEC membrane elements.
Fig. 2 confirms what can be expected from the distribution of the boron species: in the region around the pH value for the dissociation of boric acid, there is substantial improvement with every unit of pH shift. A shift to pH 10 brings the total boron rejection from 93 to 99% depending on the membrane chemistry. At a pH of 11, the total boron rejection is 99.0-99.5%. It should be noted that operation in this pH region with FILMTEC membrane elements is safe, provided the appropriate anti-sealants are used. The necessity of scaling prevention will depend upon the composition of the feed water and in many situations the use of anti-sealants is not required. Based on above observations, it is obvious that high pH operation is advantageous for boron rejection.
J Redondo et al. ’Llesalination 156 12003) 229-238
232
3. Field performance of FILMTEC rejection SWRO membranes
high
A case history on an installation using FILMTEC seawater reverse osmosis (SWRO) membranes with boron rejection data is presented here. It includes not only data on boron rejection in that system but also on pressure requirements (hence normalised flow) and rejection of the total dissolved solids (TDS) fraction. This is done for the following reasons: Consideration of boron rejection needs to take system operating pressures into account. A “boron-selective” product is not cost-efficient when the nominal flow is too low and the energy requirement too high. Rejection of boron and TDS are not directly coupled. It is true that higher TDS rejection chemistries tend to also have higher boron rejection. It can not be implied though, that two products with the same TDS rejection will have the same boron rejection and vice versa. Boron rejection is principally controlled by the membrane chemistry (diffusion) and to a lesser degree by convective transport (mechanical leakage), whereas TDS rejection is controlled by both. In contrast, for high rejection products the convective transport becomes even the more dominant factor in determining TDS rejection. Therefore TDS rejection needs to be considered in addition to boron rejection, as there may be certain differences. ??
??
The plant under consideration is a seawater plant with two stages, which enables operation at high recovery. It has 4 trains and uses FILMTEC SW3OHR-380 membrane elements. Each train contains 52 pressure vessels in the 1st and 38 pressure vessels in the 2nd stage and has an interstage booster pump. Each vessel contains 7 elements. The feed TDS is 37,830 mg/L, feed boron concentration 4.98 mg/L, and temperature around 22°C. The plant produces 290 m3/h at a recovery of 55% with a designed average permeate flux of 15 L(m2h). Table 2 and Fig. 3 show the plant performance at start-up and in the following 4 months. The operating conditions are depicted to the left whereas the normalised values are shown to the right of the table. These values were normalised using the FilmTec program ROSA (Reverse Osmosis System Analysis). The performance is normalised to the SW3OHR-380 element, which was used in this case. Nominally this element has a performance of 0.95 m’/h (6000 gpd) of flow and 99.70% rejection. At start-up, feed pressure was 7.1 bar lower than predicted in the projection. This is partly explained by the lower than designed flow rate, but is also due to the outstanding average flow performance ofthe 2500 FILMTEC SW30HR-380 elements in the plant. Anormalisation ofthe startup conditions indicates that the elements had an average flow performance of 1.06 m’/h (6730 gpd), which is 12% higher than the specification. The
Table 2
Operating data for 290 m’/h plant Time (d) Projection I 3 28 64 91 133 ___
Pressure
I__Temp.
(bar)
(“C)
66.2 59.1 65.0 65.4 66.4 65. I 65.3 ___.~_
22.3 22.4 22.3 22.1 22.2 22.2 22
~_.__~ Feed Permeate flow flow (m’/h) (m3/h) 531 525 539 535 537 533 529
292 273 296 294 297 292 290
Recovery (W
55.0 52.1 55.3 54.7 55.3 54.8 54.8 __.-__
Permeate TDS (ma)
Normalised element flow (gpd)
Normalised Normalised element flow TDS reject. (m3h) W)
243 227 187 166 175 152 I63
6000 6730 6490 6310 6200 6230 6200
0.95 1.06 1.02 1.oo 0.98 0.98 0.98
99.70 99.72 99.77 99.79 99.78 99.8 1 99.80
J. Redondo
Fig. 3. Normalised 290 m’ih plant.
et al. /Desalination
flow and rejection performance
of
permeate TDS was projected to be 243 mg/L with the 99.70% nominal rejection element. For this case the permeate TDS was 227 mg/L at start-up, which corresponds to a normalised rejection of 99.72%. Within the first hours a flow decrease occurred, which is a normal phenomenon with membranes that come out of the preservation solution. After 24 h the initial flow decrease had stopped and the membranes were stabilised. A further much slower flow decrease was seen between day 3 and the end of the observation period. This slower flow decrease of 300 gpd is due to fouling. The membranes in the plant have not been cleaned since start-up and current operation suggests cleaning is still not necessary. The stable flow performance is probably due to the short leaf length of FTLMTEC elements which results in more uniform flux distribution over the element. The homogenous flux distribution avoids areas with high flux and therefore minimises fouling.
156 (2003)
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229-238
A rejection increase as function of time can be seen. In the initial stabilisation period the rejection increases substantially from 99.72% to 99.77%. This is a normal observation with FILMTEC membrane elements, which improve in rejection during operation. In the period following the initial stabilisation, a further slight increase in rejection to 99.80% can be seen. Table 3 displays the projected and observed boron concentrations in stage 1 and stage 2 of the high recovery plant. The projection was done with a simulation that has been specifically developed for projecting boron in membrane systems. This simulation uses the data and models for boron rejection presented in sections 1 and 2 and is currently being integrated to the FilmTec ROSA (Reverse Osmosis System Analysis) program. The system results confirm the model in principle. The normalised rejection in the plant is actually somewhat higher than predicted, which can be attributed to the FILMTEC membrane elements operating in the plant. The permeate of both stages in the 55% recovery plant ranges from 0.79 to 0.86 mg/L, which is below the limit of 1.O mg/L used in some desalination contracts, but above the 0.5 mg/L limit by the World Health Organisation (WHO). In fact, the 1st low recovery stage of the 2-pass plant almost meets the boron specification. Minimal post-treatment, e.g. by using a small partial 2nd pass at high pH, or a boron-selective ion exchange resin, would reduce the boron outlet to CO.5 mg/L.
Table 3 Boron rejection of 295 m’lh plant Time (d) Projection 3 64 133
c(B), feed 1st c(B), feed2nd c(B), permeate stage stage 1st stage (mg/L) @g/L) (mg/L) 5.00 7.50 0.70 4.98 7.48 0.60 5.21 7.79 0.63 5.07 7.65 0.61
c(B) permeate 2nd stage (mg/L)
c(B), total permeate (mg/L)
I .20 I .32 1.22 1.15
0.86 0.84 0.82 0.79
- Normalized boron rejection (“/) 90.0 90.3 90.5 90.6
234
.J Redondo et al. / Desalination 156 f2003) 229- 238
4. Design concepts for boron removal The boron problem has most frequently been encountered in seawater desalination. Seawater membranes have the highest boron rejection which is however still insufficient to comply with the most stringent requirements (e.g. boron concentration of 0.4 mg/L in the final product). Several design concepts have been developed by original equipment manufacturers (OEMs), engineering companies, process consultants, end users (plant operators), etc. to achieve an efficient and safe boron removal at competitive costs. Among the proposed concepts are the following [5-10,12,14]: 1 SWRO pass with natural seawater feed pH as well as lower or higher feed pH 2 passes with increased pH, especially in the 2nd pass. Above include options with high and low recovery in the 2nd pass. 2 passes with boron-selective ion exchange resins (IER), with options treating a part of the I st pass permeate, which does not feed the 2nd pass. 3 passes with low and high pH stages in the 2nd pass. Most of the above concepts have been studied and some pilot-tested in various options, including partial stream softening, e.g. to allow the extension of the process pH range with a minimum or no use of anti-sealants. Some of the concepts including the use of boron-selective ion exchange resin propose their use straight after the SWRO, for a partial stream ofthe whole 1st pass permeate, while other propose their use only for the part of the permeate originated in the rear elements of I st pass permeate vessels. 1. I. One puss SWRO
This concept has been preposed for projects requiring 0.8 to I .O mg/L boron concentration in the RO permeate, where boron in the seawater feed is in the order of 4.0-5.0 mg/L and the seawater temperature range is 1S-26°C. The typical
water production cost for this concept is in the range of US$0.38-0.52/m’ of product [7,1 O]. A typical example for operating results with this type of design has been presented in section 3 of this paper. In that case the feed concentration was between 5.0 and 5.2 mg/L, temperature between 22.0 and 22.4”C and the boron outlet of the I st low recovery seawater stage between 0.60 and 0.70 mg/L, and the concentration in the product mix of both stages at a total recovery of 55% in the range between 0.79 and 0.86 mg/L. 4.2. Twlopasses with increased pN This concept has been widely proposed for systems where the product boron requirement is between 0.4 and 0.5 mg/L and the boron concentration of the seawater feed ranges between 4.0 and 6.3. It has also been used for water temperatures up to 34°C. The typical unit cost for this option is in the range ofUS$0.45-0.55/m3 product water [7,8, IO]. As can be seen from the pH considerations in section 2, the boron rejection of this type of system can be very high. An example for this type ofsystem: with a feed boron concentration of 6.3 mg/L, 34°C and recoveries of 45% in the 1st and 85% in the 2nd pass, the boron concentration of the permeate would be expected in the range of 0.2 mg/L providing the pH of the feed is adjusted. Depending on the pH value, the recovery and Ca and Mg rejection of the membranes used, care needs to be taken for CaCO, and Mg(OH)> scaling. Anti-sealant use should be considered, or the combination of pH and recovery needs to be adjusted to minimise scaling potential in the brine [ 131. FILMTEC membranes show stable long-term flow and rejection performance at high pH conditions. 3.3. Two reverse osmosis passes combined boron selective ion exchange resins
with
This concept has been proposed for seawaters containing 4.5-6.0 mg/L boron and with a request
J. Redondo et al. /Desalination
235
156 (2003) 229-238
Bypass 20 %, C, = 0.5 - 0.6 ppm
b
25%, C, = 0.1 ppm Boron selective IER
100 % (Blend) C,<0.4ppm
Fig. 4. Typical block diagram
for a hybrid RO/IER boron removal concept.
for boron concentration of less than 0.4 mg/L in the final blend of streams from 1st, 2nd pass RO systems and the boron selective ion exchange resin. A typical block diagram (not including pumps) of this hybrid RO/IER process is shown in Fig. 4. The diagram is generic, but the most common options include a seawater high-pressure pump and a low-pressure booster for the 2nd pass BWRO. The IER section can use (or not) a booster as well. A boron-selective ion exchange resin is available from Dow, with the denomination DOW XUS 43594 [ 111. Historically this type of products has been developed for the removal of boron and other weak acids in high purity applications, such as the semiconductor industry. Boron-selective resins will typically remove boron to levels of ~0.1 mg/L B, far below the required limits. DOW XUS 43594 has received NSF certification under standard 61 for potable water applications in October 2002. The capability of Dow to provide both the membrane element and the boronselective ion exchange resin is perceived as clear strategic advantage for systems with boron removal. Most of the options studied [7, IO, 12,141 have a variable stream distribution for the various temperatures considered. The partial 2nd pass
contributes bet-ween 50 and 60% to the final blend with a boron concentration between 0.2 and 0.35 mg/L, the IER stream can be lo-25%, the boron concentration is about 0.1 mg/L and the balance stream for the final blend is 1st pass permeate with 0.5-0.6 mg/L ofboron. The concentrations of boron depend upon feed pH and temperatures. The blend can be adjusted to have always less than 0.4 mg/L of boron and the typical chlorides concentration in the blend is in the range of 100-130 mg/L. The boron removal by IER process is in about the same range of cost as the 2nd pass treatment, i.e. it adds in average 7-9 US cents per m3 product water to the part streams undergoing the respective processes. This combination has been projected and offered for various medium and large plants in the last two years with typical unit costs in the range of US$O.50-0.55 per m3 product water, i.e. close to the figures shown in the next described process.
4.4. Three passes with high and low pH stages in the 2nd pass This concept has been proposed for a seawater containing between 4.8 and 6.3 mg/L and with simultaneous requirements for low boron (0.4 mg/L),
236
J. Redondo er 01
Desalination
low chlorides (e.g. 15 mg/L) and low TDS (e.g. 25 mg/L) in the final blended permeate output. This concept, known as IDE process (or Ashkelon process) has been proposed by the OTlD consortium (IDE+Vivendi)forthemegaprojectAshkelon. This project was developed as a BOOT tender to provide IO0 million rn? per year desalinated high quality water, mainly for potable uses and the project has been granted to the OTID consortium mentioned above. The description below follows a presentation made by IDE [9] and uses the following terms: 1st pass is called 1st RO stage 2nd pass - 1st stage (2 arrays) is called 2nd RO stage 2nd pass - 2nd stage (2 arrays) is called 3rd RO stage 3rd pass (2 arrays) is called 4th RO stage
l_i6 (2003) 229-238
surface. ‘The permeate of the 2nd stage has a low concentration of boron and constitutes Its brine has a high product water. concentration of boron, Ca and Mg ions and is processed in the 3rd RO stage. The 3rd RO stage uses the 2nd stage brine at weak-acid pH as feed and can be defined as low pH and high recovery. The pH of the feed water is low which avoids CaCO, and Mg(OH), salt and allows high recovery but results in poor boron rejection. The 4th stage uses feed from the 3rd stage permeate at high pH and reduces the boron concentration. It is used in the product water blend.
??
??
??
??
These stages operate as follows (please also refer to Fig. 5): The 1st stage RO stage is a conventional seawater RO desalination system. Boron in permeate is reduced to concentrations in the range 0.9-l .5 mg/L. The permeate from the RO membranes at the feed side of the RO vessel (front permeate) has a lower concentration of boron and can be used as product water by blending with the product water from the 2nd and the 4th stage. The 2nd RO stage can be defined as a stage of high pH and moderate recovery, to avoid CaCO, and Mg(OH), scaling on the membrane
??
??
The so called 2nd and 3rd stages, more commonly called 2nd pass 1st stage (2 arrays) and 2nd pass 2nd stage (also 2 arrays) are in reality a once-through process with a very high recovery. The borderline between the 2nd and the 3rd desalination stages can be moved depending on feed water temperature, membrane age and efficiency of anti-sealant. Frequency converters in 2nd and 3rd stage pumps allow for system flexibility. The four stage RO desalination and boron removal system offers the following: Low specific power consumption, as pressured 2nd stage brine enters third stage feed with low power losses. Power in pressurised brine of 3rd and 4th stages can be recovered in a Pelton turbine and used in 3rd stage booster. ??
Fmal product
Brine .I____ 1st stage /
///
rear
Fig. 5. Block diagram of Ashkelon process.
,’
J. Redondo
??
??
et al. / Desalination
Low chemical consumption, as pH transition between stages takes place in conditions of low bicarbonate buffer. This system offers high operational safety
This four stage boron removal process is patent pended by IDE Technologies Ltd. The typical unit cost for this option has been projected in the range of US$O.47-0.52/m’ product water.
5. Summary and conclusions There have been significant and proven improvements in the basic flow and salt rejection properties of SWRO membranes and elements. Flow rates in the range of 6000-6700 gpd (0.951.06n-G/h) and TDS rejections of 99.70-99.85% have been documented in operational SWRO systems. These developments have also yielded improvements in the rejection of boron at neutral pH values. In combination with innovative process solutions, improved membranes have enabled substantial progress in the economic reduction of boron in SWRO permeates to the stringent levels currently required. It is now feasible to consider SWR.0 designs with recovery ratios in the range of 4O-60% whilst operating on feed waters containing up to 48,000 mg/L and boron concentrations in the range of 3.5-6.8 mg/L. Typical water production costs for such processes appear to be in the order of US$O.380.50/m of treated water with product boron contents of 0.6-I .O mg/L and US$O.47-0.60/m’ with product boron contents of 0.3-0.5 mg/L for drinking and agricultural applications. The reported figures are based upon large and very large plants (1 O,OOO-300,000 m’/d) at convenient locations and infrastructures. Pumps and energy recovery devices with efficiencies in the range of 85% and up to 95% including isobaric chambers, as well as energy costs in the order of US cent 47/kWh and convenient interest rates of 3.5-6.5% per year contribute to the overall low cost of production. The combination of improved membrane
156 (2003)
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237
products with varied process possibilities for boron removal, the progress in all the ancillary equipment and positive financial climate make possible the use of this technology in very large water purification applications previously not considered viable and holds the potential for further improvements in the near future. Acknowledgements We want to first thank our customers for the good co-operation in designing plants with boron removal. We have learned a lot from this work. Thanks are also due to Steve Jons, Duane Jacobson, David Wintergrass and their respective teams for the boron data in the membrane section. Thanks to Steve Wrigley and Sara Fessler for the ion exchange resin part and Andreas Gorenflo for his support of graphic material.
References [II PI
PI
Ambient Water Quality Guidelines for Boron. British Columbia, Water Protection Branch. Guidelines for Drinking-Water Quality, 2nd ed., Addendum to vol. I, Recommendations. Geneva, World Health Organisation, 1998, pp. 4-6. Environmental Protection Agency (EPA). 1975.
Preliminary investigation of effects on the environment ofboron, indium,nickel,selenium,tin, vanadium and their compounds, vol. 1, Boron. US Environmental Protection Agency Rep. 56/2-75-005A. [41 J. Redondo, Brackish, sea and waste water desalination as sustainable water sources. Technical-economical and ecological answers based on membrane technology. Desalination, 138 (2001) 29-40. [51 Personal communication with Dr. Manuel Farinas Iglesias. [61 Extract from Ashkelon Tender Documents 2001, t71 J.L. Loidi and .L.Gonzalez, ACS-Tedaguaprojections for the two pass SWRO and IER boron rejection process, July 24 and Nov. 4, 2002. PI M. Farinas, Infraestructuras hydraulicas municipales, Expert seminar, Andalucia International University, Jan. 2003. [91 B. Liberman and I. Liberman, Method of Boron Removal in Ashkelon Desalination Plant, 5th IWA conference on Membranes in Drinking and Industrial
238
[IO] [ 1I] [ 121
[ 131 [ 141
J Redondo et al. /Desalination Water Production, Miilheim an der Ruhr, Germany, Sept. 22-26, 2002. M.A. Sanz, Meeting on Desalination Technology and Costs, Paris, Nov. 2002. Dow document, DOWEX IER, Product Information for XUS 43594.00, Jan. 2003. Y. Mansdorf, Flow sheet draft for 2 pass SWRO and IER boron rejection process, Tambour Ecology Graph, May 7,2002. FILMTEC Membranes Technical Manual, April 1995. P. Glueckstem and M. Priel , Potential cost reduction of seawater desalination, 5th IWA conference on Membranes in Drinking and Industrial Water Production, Mtilheim an der Ruhr, Germany, Sept. 222 26, 2002.
1.56 (2003) 229-238
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