Using Seawater Reverse Osmosis (SWRO) desalting system for less environmental impacts in Qatar

Desalination 309 (2013) 113–124

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Using Seawater Reverse Osmosis (SWRO) desalting system for less environmental impacts in Qatar Mohamed Darwish, Abdel Hakim Hassabou, Basem Shomar ⁎ Qatar Environment and Energy Research Institute (QEERI), Qatar Foundation, P.O. Box 5825, Doha, Qatar

H I G H L I G H T S ► ► ► ► ►

Manuscript is important for the local, regional and international scientific communities. Qatar is an arid with very harsh environment, absence of fresh water and studies. It depends 100% on desalinated water for the present and future in all levels. Paper introduces for the first time in Qatar basic information on desalination technologies. It opens new horizons for further research on water desalination and human health.

a r t i c l e

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Article history: Received 19 July 2012 Received in revised form 19 September 2012 Accepted 20 September 2012 Available online 24 October 2012 Keywords: Desalination technologies Environmental impacts Productivity and efficiency

a b s t r a c t In Qatar, the multi stage flash (MSF) system which is predominantly used for desalting seawater has negative impacts on the environment due to burning fossil fuels that emit greenhouse gases. Impingement and entrainment of marine organisms at the intake, higher temperatures and salinity as well as chemical pollution at the outfall are the main footprints on the marine environment. The objective of this paper is to show the main benefits on the environment of using the seawater reverse osmosis (SWRO) process over the MSF process for the present desalination capacity of 1.2 Mm 3/d in 2012. The study showed that the seawater intake would be reduced about 3 times from 8.4 Mm 3/d to 3.6 Mm 3/d, which decreases the impingement and entrainment of marine organisms at the intake. Hence the corresponding discharge of brine and cooling water could be reduced from 7.2 Mm 3/d to 2.4 Mm 3/d for MSF and SWRO, respectively. Moreover, the thermal pollution of the rejected effluents from MSF plants could be eliminated and the chlorine residual would be reduced dramatically when using SWRO. Finally, the use of SWRO saves up to 75% of energy use, and thus the CO2 emissions would be reduced from 3.564 to 0.891 million (M) tons per year. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Qatar depends on desalted seawater (DW) as the main (≅99%) potable (drinking) water supply. The DW production in 2010 reached 373.6 Mm3/year, or 1.0 Mm 3/day [1]. From 2006 to 2010, the production of DW more than doubled with a significant annual increase of 14%. If only 8% annual increase is assumed since 2010, the produced DW in 2012 would be 436 Mm3/year (1.2 Mm3/day or 13.82 m 3/s). The 2010 population in Qatar was 1.7 M [1], and therefore the consumed DW was about 600 liters/capita/day (L/ca/d) which is among the highest municipal water consumption in the world. The Qatar National Strategy (QNS) report for 2010–2016 stated that the average water consumption for Qatari citizens in 2009 was 1200 L/ca/d, while expatriates consumed 150 L/ca/d [2]. Qatar has very low water tariff (free for Qatari households and low-cost for non-Qatari

⁎ Corresponding author. Tel.: +974 4454 2872; fax: +974 4454 1528. E-mail address: [email protected] (B. Shomar). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.09.026

households) which may recover less than a third of the costs of water production by desalination. Desalination is a mature technology that has been providing a reliable supply of DW throughout the Gulf Co-operation Countries (GCC) for five decades. It provides water which meets or exceeds the drinking water standards. Desalination is an energy intensive process where energy is used to separate fresh water from seawater and the concentrated brine balance is rejected back to sea. It is well known that desalination plants cause a number of environmental negative impacts [3–8], briefly: 1. Entrainment and impingement of marine organisms at the seawater intake. 2. Negative impacts of the brine discharge with its chemical contaminants to sensitive marine habitat. 3. High energy use to produce desalted water [9,10]. 4. Impacts on biological resources and habitats. 5. Cumulative impacts from multiple desalination plants or other projects in the area [7,8].

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The large capacity per unit and reliability of the Multi Stage Flash (MSF) desalting system as well as the notion of low fuel cost are behind the continuous use of MSF systems since its inception in 1960 in Kuwait. While the MSF is currently the most reliable method, it is a very energy intensive process [9,10]. The use of MSF raises many concerns about DW cost, consumed energy, and its effects on the environment. The GCC governments' heavy subsidization of water and energy induces high consumption of both water and energy and distorts the choice towards desalting in favor of energy inefficient methods. Compared to the Seawater Reverse Osmosis (SWRO), the MSF and Multi Effect Distillation (MED) systems withdraw higher volumes of seawater per cubic meter of product water for cooling and partial use as feed; and discharge larger volumes of concentrated brine at temperatures higher than that of seawater. The chemical contaminants and high temperature of the rejected brine negatively affect the marine environment. General comparison between MSF and SWRO is given in Table 1. Although Qatar depends on desalinated water for domestic and major industrial uses, the available literature on the field is very limited and scientific studies are almost absent. This paper introduces the basic information about desalination technologies and potential impacts on the arid and fragile environment. The objectives of this paper are (1) to assess the negative impacts on both air and marine

Table 1 General comparison between MSF and SWRO for 1.2 Mm3/d desalted water. Item

MSF

SWRO

Environmental Impact

Specific energy demand

291 kJ/kg thermal energy

No thermal energy

4 kWh/m3 pumping energy

4 kWh/m3 pumping energy

indirect CO2 emission by MSF four times that of SWRO

9.46

3

11.352 Mm3/d 10.152 Mm3/d

Mc = F = 3.6 Mm3/d F – D = 2.4 Mm3/d

101.52 GWh/d

3.2 GWh/d

9 1022.9 kg/m3, less than ambient 50,300 1/3

1.2 1045.5 kg/m3, higher than ambient 67,500 1/3

Seawater intake to product ratio Mc/D Mc, Mm3/d Reject stream, (Mc − D), Mm3/d Thermal energy discharge, GWh/d ΔT, °C Discharge density Salinity, ppm Product/make up water, D/F Product/Intake Chlorine at intake Chlorine at outlet

D/Mc = 0.11 1–2 ppm in the intake 0.2–0.45 ppm, both the brine and the cooling water contain residual chlorine

1/3 1–2 ppm in the intake can be neglected due to de-chlorination, oxidants removed to prevent membrane damage, using sodium bisulfite Chlorine load at 3.05 ton per day can be neglected due MSF effluent very outlet to de-chlorination harmful, very toxic for many organisms in mixing zone carcinogenic Trihalomethane may formed during halogenated effects, possible chlorination, but at (chlorinated and (THMs)at chronic effect, brominated) very low level due outlet more persistent to de- chlorination Present in brine, but usually below Anti scalant, not in cooling water toxic level 1–2 ppm at pretreatment

environments which result from the use of MSF and MED systems to desalt seawater, (2) to show the benefits, especially on the environment, of replacing the MSF and MED systems with the more energy efficient desalting system of SWRO. 2. Desalination pretreatment Chemical additions during seawater pretreatment (biocides, coagulants, flocculants, anti-scalants, etc.) and the disposal of residuals formed during feed water pre-treatment have effects on the marine environment and on public health due to their potential propagation into the final fresh water [4]. There is the possibility that a number of volatile organic contaminants, including those present in raw water and those resulting from disinfection, could carry over to the product water in thermal distillation processes. Membranes provide a barrier to most chemical compounds, although not always a complete barrier. The propensity of boron (as borate or boric acid) and also arsenite to pass through membranes raises the question as to what other anions and small neutral organic molecules will pass through membranes. There is potential passage of viruses through some RO membranes, which may require adequate virus inactivation downstream of RO. Also, the potential loss of integrity of membranes, which could lead to the passage of pathogens into the process water, is of concern [11]. Potential public health effects associated with pre-treatment are typically associated with the by-products formed during the chemical conditioning process and their potential propagation into the finished fresh water. Desalinated water provides some additional issues to be considered in respect to both potential chemical and microbial components. The objective of pretreatment for microbials using oxidants and biocides is to prevent fouling of the RO membranes and does not specifically address disinfection goals as oxidant doses applied are not sufficient to reach residual concentrations required for efficient disinfection. Other pretreatments include use of membranes to prepare the water for the subsequent desalination process. These pretreatment membranes include micro- and nano-filtrations which have a substantial capacity to physically remove a large proportion of particulate-associated microorganisms as well as some dissolved solids. Desalination processes significantly reduce all ions present in drinking water so that people who typically consume desalinated water may be consistently receiving smaller amounts of some nutrients relative to people who consume water from more traditional sources and thus are disadvantaged if their diets are not sufficient. Since desalinated water can be stabilized by different methods, such as the addition of lime or blending, some of these ions will be automatically replenished in that process [12,13]. Sodium can be present in desalinated water depending upon the efficiency of salt removal and the post treatment blending which could involve non-desalinated seawater. Typical daily dietary intake of sodium may range from 2000 to 10,000 mg or more and is a function of personal taste and cultural factors. Water is usually not a significant contributor to total daily sodium intake, except for persons under a physician's care who are required to be on highly restricted diets of less than 400 mg sodium per day. Pretreatment is necessary to decrease the turbidity (suspended solids) and the quantity of organic and inorganic foulants below the acceptable range for the desalination process equipment. In membrane processes, the pretreatment is extensive. It involves filtration to remove the suspended solids (particles, silt, organics, algae, etc.) and oil and grease contained in the source water. In thermal processes, the pretreatment protects downstream piping and equipment from corrosion and from formation of excessive scale of hard deposits on piping surfaces (known as scaling). Bio-fouling is often mitigated using an oxidant such as chlorine. Screening of the

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intake water is the first step of the treatment process. A typical case is the Barka MSF plant in Oman [8]. To prevent fouling, hypochlorite (ClO −) is dosed at a constant level of 3 mg/L. Besides continuous chlorination, shock chlorination is effective in regular intervals. Phosphate based anti-scalant is used at a concentration of 1.5 mg/L. Antifoaming agents are used but the dosages could not be obtained. Regulations determine that the brine at the point of discharge cannot exceed 10 °C above the ambient temperature. The brine salt concentration must be restricted to 20 g/L above ambient. In case of accidental overdosing of chemicals, additional cooling water can be mixed to the brine and an emergency mixing system is available. The outfall system with multiport diffusers can be considered as superior to the prevailing open sea outfalls from an ecological point of view. Fig. 1. Development of Desalted seawater Production in Qatar. Adapted from [1].

2.1. Pretreatment for thermal desalination plants Thermal desalination systems are more tolerant to seawater quality compared to SWRO and typically do not include any physical filtration other than screening provided at the intake. The conventional pretreatment processes in most distillation plants include feed water de-aeration (the de-aerator is often integrated in the evaporator), and treatment with polymer blends plus sodium bisulfate (NaHSO4) used for scavenging oxygen (after de-aeration) and residual chlorine. The aims of commonly used pretreatments in thermal plants are: a. Control of bio-fouling, usually by chlorination. Bio-fouling and aquatic organism growth in the intake and desalination equipment is controlled by continuous dosing of 0.5–2 mg/L active Cl2, and with site specific intermittent shock dosing. , Typical intermittent dosing figure is 3.7 mg/L for 30–120 min every 1–5 days (≈ 1.0 mg/L) [4]. Thermal processes usually use open sea water intakes. b. Control of scaling by anti-scalants. Feed water is continuously treated with scale inhibitors which are mostly polymer blends. The inhibitors are usually phosphonates, polyphosphate, polymaleic, polycarboxylic acids, or a blend of several of these. These inhibitors are crystal modifiers that avoid precipitation and development of deposits, primarily calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2). The dose is in the range of 1–8 mg/L of make-up water. An acid (usually sulfuric acid-H2SO4) is an alternative scale inhibitor that lowers the pH to avoid CaCO3 or Mg(OH)2 formation; it should be noted that acid treatment is rarely used. The dose rate of anti-scalant is ≈100 mg/L of feed water. c. Foaming reduction by antifoams. The antifoams are Poly-Othelyne Ethylene Oxide or a similar surfactant [1]. These surfactants control foaming that may cause high product total dissolved solids (TDS) carryover. The average dose of antifoam is 0.1 mg/L of make-up water and it is used intermittently in all thermal processes but primarily in MSF. d. De-aeration or use of oxygen scavengers to inhibit corrosion. Corrosion of the plant components is caused primarily by dissolved gases, suspended solids, oil, aquatic organisms and heavy metals.

2.1.1. Withdrawn and discharge streams from MSF System The annual DW production in Qatar is shown in Fig. 1 [1]. For the current year 2012, the average DW is predicted to be in the range of 1.2 Mm 3/d (13.8 m 3/s). Assuming that the DW is produced by the MSF desalting units, Fig. 2a and b show typical streams of the MSF plants (see also Table 2). Fig. 2a shows the major components of the MSF unit which are the heat input section (HIS), heat recovery section (HRS), and the heat rejection section (HJS). Additionally, Fig. 2b shows the concentrations of chemicals in streams that may affect the marine environment.

The streams to all MSF units producing DW of 1.2 Mm 3/d (13.89 m 3/s) include incoming cooling seawater (Mc) which is typically in the range of 7–9.5 times the amount distillate (D). For the minimum Mc = 7D = 8.4 Mm 3/d, the temperature is 35 °C in summer where it is introduced to the HJS having (j) stages (usually j = 3) of MSF units having (n) stages. Fig. 2b shows that Mc is chlorinated before entering the HJS with a chlorine concentration of 2 mg/L. The Mc exits the HJS at 46 °C. Part of the Mc leaving the HJS becomes the feed water (F The feed, F = 3D= 3.6 Mm3/d which is pre-treated and the balance (Mc − F) is rejected cooling water. This (Mc − F)= 4D= 4.8 Mm3/d is rejected back to sea at temperature of 11 °C above that of the seawater (SW). The feed, F = 3.6 Mm 3/d, is pretreated by being de-aerated and dosed with additives of anti-scalant, anti- foaming, and oxygen scavengers. The feed water is then introduced to the ended flashing brine in the MSF last stage (n) as shown in Fig. 2a. The recirculation stream (R) = 9.5 D = 11.4 Mm 3/d, is pumped from the last stage number n (# n) into the condenser of the heat recovery section (HRS), last stage (# n-3). The stream R is heated in the upper condensers of last stage of the heat recovery section, # (n − j) to stage # 1, and leaves the first stage to the brine heater (BH). The stream R is heated in the BH to the top brine temperature (TBT), usually of 110 °C. Then the stream R enters as flashing brine in the lower section of the first stage, n = 1, where it is partially evaporated as it moves from stage #1 to stage # n. The flash vapor is condensed in the condensers located at the upper part of the flashing chambers. The total flash evaporated vapor from all stages is equal to the produced distillate (D), and thus the flashing brine flow rate becomes R − D = 11.4 − 1.2 = 10.2 Mm 3/d in the last n stage. In the last stage, (R − D) is mixed with the inlet feed (F). The mixed stream [(R − D) + F] is divided to (R), which is pumped to the HRS, while (F − D) = 2 D = 2.4 Mm 3/d is brine rejected to sea. Brine rejected to sea (F− D) has typical salinity of Xr= 69,000 mg/L and is 11 °C above seawater temperature. The brine, or concentrate, (F− D) and the rejected cooling water (Mc − F) have high flow rates (from 6 D as in the given case, and up to 9 D in some MSF units). The brine is characterized by its high salt concentration and higher temperature than that of SW (8–11 °C). The rejected cooling SW and brine both contain additives and corrosion products. Additives are chemicals used for bio-fouling control (e.g. chlorine), scale control (anti-scalants), foam reduction, and corrosion inhibition that are added during the desalination process and discharged into sea as contaminants. The streams are given in Table 2 and Fig. 2c. 2.1.2. Withdrawn and discharge streams from MED with thermal vapor compression (TVC) There are misconceptions about the Multi effect distillation (MED) method used in the GCC. Some think it is the conventional

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Fig. 2. a: Multi stage flash desalting system showing the heat rejection, heat recovery and heat input sections, and conventional pretreatment. b: MSF process scheme with input and output concentrations of additives and brine characteristics. Adapted from [22]. c: MSF process scheme with input and output. Adapted from [22].

M. Darwish et al. / Desalination 309 (2013) 113–124 Table 2 Mass and brine balance of MSF desalination capacity in Qatar. Stream

Flow rate [m3/s] / [Mm3/d]

Temperature [°C]

Salinity [ppm]

Distilled water (D) Cooling water (Mc) Rejected cooling (SW) Feed (F) Brine (B) Recirculation

13.89/1.2 97.22/8.4 55.56/4.8 41.67/3.6 27.78/2.4 131.96/11.4

35 35 46 46 46 46–110

25 45,000 45,000 45,000 69,000 61,000–69,000

(or simple) MED similar to that shown in Fig. 3a; however, this is not the case. The simple MED has a top brine temperature (TBT) of approximately 65 °C, and a steam supply saturation temperature near 70 °C (0.3 bar). Mc is supplied to the end condenser at Mc ≅ 7 D. Mc leaving the end condenser is divided to the feed F at F ≅ 3 D, and (Mc − F) is rejected to sea. In simple MED, steam (as a heating medium) is supplied to the first effect (in the left of Fig. 3a) to heat the feed (F) entering this effect at t1 to the saturation temperature in this effect, T1, and evaporate part of that feed, D1. The condensed steam returns to the source. The generated vapor in the first effect D1 is directed to the second effect as heating vapor. While D1 is condensed in the second effect, it generates almost the same amount by boiling, Db2. Meanwhile, the brine leaving the first effect is directed to the second effect where its temperature is decreased from T1 to T2, by flashing part of it, Df2. T2 is the saturation temperature in the second effect, where T2 b T1. The summation of Db2 and Df2 is the total vapor generated in the second effect, D2. The vapor D2 is directed to the third effect as heating vapor and the process is repeated to the last effect, n. The vapor generated in the last effect, Dn, is directed to an end condenser, where it condenses and joins the distillate obtained from the previous effects. Another MED type uses the Thermal Vapor Compression system (TVC), and is called MED-TVC, as shown in Fig. 3b. The TVC-MED system is similar to the simple MED, but the vapor generated in the last effect, Dn, is divided into two streams, Dr and Dc. The first stream (Dr) enters a steam ejector (thermal compressor) driven by motive steam (Sm) at relatively high pressure. The steam mixture of Dr and Sm leaving the ejector at moderate pressure is directed to the first effect as heating vapor to raise the feed water to its saturation temperature and generate D1, as in the simple MED system. The process is repeated as in the simple MED to the last effect, where the vapor generated in Dn is divided again into Dr and Dc. The condensing steam in the first effect (Dr+ ms) becomes part of the distillate. This system uses motive steam at a much higher pressure (approximately 20 bar) and saturated temperature, as it compresses part of the vapor leaving the last effect, Dr, to the first effect, and does not act as a direct heating steam. The main limitation of this system is the low volumetric flow rate of the thermal vapor compressor (steam ejector). The main large capacity MED desalting system used in the GCC is a combination of both simple MED and MED-TVC systems. The combined system has two parallel frames of ME-TVC with an added simple MED unit as shown in Fig. 3c. The combined system was used to overcome the volumetric flow rate limitation of the steam ejectors. The system uses high pressure steam and has a higher Gain Output Ratio (GOR). The GOR is the distillate (D) to motive steam (S) ratio. The combined MED and MEDTVC has higher GOR compared to the simple MED for the same number of effects. Data of a similar plant [14] are given by: Capacity/unit 3.77 MIGD, (198.5 kg/s), number of evaporators = (3 × 2) + 3 = 9, top brine temperature (TBT) = 63 °C, temperature range, 19 °C, steam supply at temperature (Ts) = 126 °C, pressure (Ps) = 1.33 bar, and low rate S = 24.67 kg/s, gain ratio (GOR) = D/S 8.05, cooling water mass flow rate Mc = 6861 kg/s, Mc/D = 9.46, Feed (F) = 653.30 kg/s, and F/D = 3.29.

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As in MSF, the discharge of brine has strong impacts on the environment due to its changed physical properties and to the residues of chemical additives or corrosion products. In the MED plants, common chemical additives are biocides, anti-scalants, antifoaming agents, and corrosion inhibitors at some plants. Fig. 3d shows where the chemicals are added and the concentrations as well as the characteristics of the brine and its concentration for the MED desalting system. The above data indicates that Mc is around 9.46 D for the MSF units and 7D for the large MED-TVC units. Both MSF and MED-TVC processes consume almost the same heat. The heat and chemical rejected to the marine environment are similar for both MSF and MED-TVC used in the GCC. 2.1.3. Thermal and chemical load discharged by the MSF and MED plants in Qatar 2.1.3.1. Thermal impact. The cooling seawater and brine discharge from thermal desalination plants cause thermal pollution. For a daily average of 1.2 Mm 3/d (13.89 kg/s), Mc = 9.46 D supplied by all MSF units, and large MED-TVC units, the cooling seawater and brine are discharged at the rate of 8.46 D, at 11 °C temperature higher than the ambient seawater. The thermal pollution can be calculated by two methods: (a) For seawater average specific heat of 4 kJ/kgC, the rejected heat is: Q ðrejectÞ ¼ 13:9 kg=s  8:46  4  9 ¼ 4:23 GW; or 101:52 GWh=d; and 37; 055 GWh=y:

(b) Another calculation method assumes that the thermal energy consumed by the desalination method, for a performance ratio of 8, the thermal energy input is 2330/8 = 291.25 kJ/kg where 2330 kJ/kg is the average latent heat of the steam at the average temperature. The pumping mechanical energy of 4 kWh/m 3 (or 14.4 kJ/kg) is transferred to heat gained by the process. So, the total rejected thermal energy per kg of distillate is 305.65 kJ/kg by both thermal and pumping energy inputs. For 1.2 Mm 3, (13.889 kg/s), the thermal pollution is 4.25 GW, which is similar to the previous method. If we assume that the heat content of one barrel (bbl) of oil is 6.1 GJ/bbl, the calculated thermal pollution of 37,055 GWh/y is equivalent to burning 21.87 million (M) barrels of oil per year. 2.1.3.2. Chemical impact. (a) Disposed salts: By assuming that the salinity of the seawater is 45,000 ppm (45 g/kg); the amount of daily minerals (salt) rejected to the sea is 54,000 t/d, or 19.71 M t/year. (b) Chlorine: The chlorine concentration in the outlet from MSFplants ranges between 0.2 ppm to 0.45 ppm, [15]. By assuming a chlorine discharge of 0.3 ppm and a product/effluent ratio of 8.46 (given before), for every 1000 m 3/d desalted water, the chlorine rejected with brine is 2.538 kg/d. Then, for 1.2 Mm 3/d desalted water per day; the discharged chlorine rate is 3.05 t/day. 2.2. Pretreatment for Seawater Reverse Osmosis (SWRO) Desalination Plants Fig. 4 is a schematic diagram of seawater reverse osmosis (SWRO) conventional pretreatment, chemical dosage, and the different waste and side streams (Table 3). The conventional pretreatment in SWRO plants operating on surface water include:

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2.2.1. Control of bio-fouling, usually by chlorination, and de-chlorination with sodium bisulfite Bio-fouling and aquatic organism growth in the intake and pretreatment facilities are controlled by chlorine in both MSF and MED systems. The dose is site specific. A typical value is 3.7 mg/L of feed water for 30–120 minutes every 1–5 days [4]. This is used for large surface and seawater intakes. Small systems using beach wells, especially those in which source water is anaerobic, may not require oxidation. The feed to all membrane processes using polyamide RO membranes should be de-chlorinated by using a reducing agent such as bisulfite after being chlorinated at the intake. The less commonly used cellulose acetate RO membranes have greater tolerance to oxidants. The bisulfite dose depends on the chlorine dosage. It is generally 2 to 4 times higher than the oxidizing agent dose.

2.2.2. Removal of suspended material by coagulation and media filtration It is necessary to remove suspended materials from the RO feed stream. Suspended solids can cause irreversible damage to the membranes. Suspensions of concern are: clay and silt (≤ 63 μm), plankton, bacteria (≤ 3 μm) and small colloids of less than 1 nm particle size. The membranes are not tolerant to direct operation with open seawater without pretreatment. Conventional pretreatment technology relies on a combination of chemical treatment and media filtration to achieve the required feed water quality for the

membranes. Alternative pretreatment is membrane filtration such as microfiltration, ultra-filtration, and nano-filtration. For granular media filtration coagulant dosing is required. Coagulants are metal salts which form dense suspended flocks as they react to hydroxides in aqueous solutions. Typically ferric chloride (FeCl3) and ferric sulfate (Fe2 (SO4)3) salts are used for coagulation. The following equation shows the coagulation process using ferric chloride. −

FeCl3 þ 3HCO3 →FeðOHÞ3 þ 3Cl þ 3CO2 The coagulants neutralize the negative surface charge of the suspended particles and adsorb and enmesh colloid particles within the flocks. This process aggregates the particles into larger, heavier and more filterable solids. The dosage of coagulants and coagulant aids is normally correlated to the amount of suspended material in the intake water. It can range between b1 and 30 mg/L for coagulants and between 0.2 and 4 mg/L for polyelectrolytes. Ferric sulfate is added at a dose rate of 5–15 mg/L for open intake SWRO and RO treated surface water systems. Also, flocculant aid (usually a cationic polymer) is used at a dose rate of 1–5 mg/L feed water, in open intake SWRO and surface water RO. It can be used only intermittently when feed Silt density Index (SDI) is unusually high. Dosing of sulfuric acid (H2SO4) to establish slightly acidic pH values and addition of coagulant aids, such as polyelectrolytes, can enhance the coagulation process. Polyelectrolytes are organic substances with

Fig. 3. a: Conventional MED desalting system. b: Thermal vapor compression MED desalting system. c: Combined MED-TVC and conventional MED desalting system. d: MED process scheme with input and output concentrations of additives and brine characteristics [7].

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Fig. 3 (continued).

high molecular masses (like polyacrylamide) that help to bridge particles together. The particulate material is retained when the seawater passes through the filter beds. The filters are backwashed on a periodic basis using filtered seawater or permeate water, in order to clean the filters from the particulate material (which contains the natural suspended material and the coagulant chemicals). The backwash

water can either be discharged into the sea, or may be treated and the sludge disposed of in a landfill. 2.2.3. Anti-scaling Control of scaling could be achieved by acid addition (lowering the pH of the incoming seawater) and/or dosing of special anti-scalant chemicals. The acid is usually sulfuric acid which reduces the pH for

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Fig. 4. a: SWRO Flow‐scheme showing the conventional pretreatment, chemical dosage and the different waste and side streams [7]. b: SWRO process scheme with input and output concentrations of additives and brine characteristics. Adapted from [7].

inhibition of scaling and for improving the coagulation process. The dose is usually in the range of 40–50 mg/L or as required to reduce pH to 6–7. This is primarily in SWRO applications. It is not to be used in all applications. 2.2.4. Cartridge filters as a final protection barrier against suspended particles and microorganisms before the RO units The feed water is usually passed through 5 μm-size cartridge filters as a final barrier, before it enters the RO units. The cartridge filters are used in combination with both conventional and membrane pretreatment systems. The retained particles on the cartridge filters will be removed on a periodic basis, e.g. every six to eight weeks, and typically disposed of in a sanitary landfill. Fig. 4b shows where chemicals are added and their concentration for SWRO desalting system. 2.2.5. Withdrawn and discharge streams from SWRO The SWRO water recovery ratio (feed F to permeate P ratio) in the GCC is approximately 1/3. Meaning the feed is almost 3 times that of

the permeate (produced water). Compared to thermal processes, the SWRO requires significantly less intake water (about 1/3 the cooling water to MSF or 0.4 the cooling water to MED) for the same amount of product water. Consequently the loss of organisms through impingement and entrainment is lower. So, if the DW of 1.2 Mm 3/d was produced by SWRO, and not by the MSF units, the water intake would be reduced from 8.4 Mm 3/d to 3.6 Mm 3/d, and the rejected water would be reduced from 7.2 Mm 3/d to 2.4 Mm 3/d. 3. Impacts of brine discharge on the marine environment Brine discharge from desalting plants to the sea contains total dissolved solids (TDS) up to 70,000 mg/L. Salinity and chemical contaminants of the rejected stream are the most negative parameters affecting marine environment. This brine, besides its high salt concentrations, contains chemicals used in the pretreatment processes of the feed seawater. Chemical constituents of plant discharges include: 3.1. Bio-fouling agents

Table 3 Virtual mass and brine balance of SWRO assuming the same MSF desalination capacity in Qatar. Stream

Flow rate [m3/s] / [Mm3/d]

Temperature [°C]

Salinity [ppm]

Permeate (P) Water intake (F = feed) Brine (B)

13.89/1.2 41.67/3.6 27.78/2.4

35 35 36

500 45,000 69,000

As mentioned before, chlorination is the most widely used technique to control bio-fouling resulting from incoming seawater. Chlorine (Cl2) and sodium hypochlorite (NaOCl) are well known bio-fouling agents. Seawater is usually chlorinated with typical doses of 0.5–1.5 mg/L. In distillation plants, average residual chlorine concentrations of 200 and 500 μg/L have been reported in the reject streams [7]. While this level ensures that the entire plant from intake to outfall

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is protected from bio-fouling, it also means that residual chlorine is discharged to surface waters, where it may harm aquatic life. In RO plants using polyamide membranes, de-chlorination of the feed water is carried out in order to protect the membranes. However, minor residual chlorine levels can still be present in the brine and the problem of the toxic halogenated organic compounds remains [7]. Sodium bisulfite (NaHSO3), which is commonly used for dechlorination, reacts to harmless products but may cause critical oxygen depletion if overdosed. Nevertheless, the impacts of chlorine are more significant for MSF plants since usually no de-chlorination is achieved. Besides, MSF plants require larger feed water volumes which increase the loads of chlorine and its by-products. One can assume that 10–25% of chlorine concentration in the feed water (equal to 200–500 μg/L) can approximately be measured in MSF effluents. Concentrations in the mixing zone of MSF plants were reported to be around 100–300 μg/L [16,17]. The mixing zone is the area around the discharge location in which the brine and its constituents are diluted to ambient or given threshold values. At an assumed effluent concentration of 250 μg/L, the daily chlorine input of major MSF plants into the Arabian Gulf is calculated to be 21,900 kg/d, by 2009 [18]. In RO plants, the residual chlorine is neutralized before the water enters the RO units to avoid membrane damage. The RO membranes are typically made of polyamide materials which are sensitive to the oxidizing chemicals such as chlorine. Sodium bisulfite (NaHSO3) is predominantly used for de-chlorination: −

−

NaHSO3 þ OCl →NaHSO4 þ Cl

Consequently, chlorine concentrations will be very low to non‐ detectable in the reject streams of RO plants. De‐chlorination with NaHSO3 may reduce the dissolved oxygen (DO) levels in the reject stream as a side effect if the NaHSO3 dosing is not properly adjusted. The chlorine and its by-products create carcinogenic effects of greatest environmental and public health concerns. Seawater contains about 6.5 mg/L of bromide (Br −). During chlorination bromine (Br2) is formed by the oxidation of bromide, leading to the formation of organobromine compounds [19]. As a result, trihalomethanes (THM) in chlorinated seawater mainly consist of bromoform (CHBr3) and dibromo-chloro-methane (CHBr2Cl). Bromoform has a slow and progressive formation, being the final product in the oxidation of organic substances. Accordingly, the majority of monitoring studies have focused on these two compounds. The resulting residual oxidant in the coolant water is generally in the range of 0.1–0.2 mg/L. Though the organo-chlorinated by-products represent a small fraction of the added chlorine, they are more persistent than residual chlorine, and thus pose a potential hazard to marine life due to their mutagenicity. 3.2. Metal salts and coagulants Most existing SWRO plants use either ferric chloride (FeCl3) or ferric sulfate (FeSO4) as a primary coagulant or flocculant in the pretreatment system. When added to water, a hydrolysis reaction produces an insoluble ferric hydroxide precipitate that binds non-reactive molecules and colloidal solids into larger aggregations that can then be more easily settled or filtered from the water. The resulting ferric hydroxide floc is retained in the filter until the filter is flushed during a backwash process. Their effect on the environment is not well known. 3.3. Scale inhibitors Although the dosing rate of anti-scalants is low, the overall consumption is huge. An estimated anti-scalant load of almost 62,000 kg/day was discharged into the Arabian Gulf in 2009 [18].

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Information is lacking about the degradation of anti-scalants and their effect on the marine environment. 3.4. Antifoams These are organic (acylated polyglycols, fatty acids and fatty acid esters) or silicon-based compounds (composition normally not revealed by manufacturers) used to suppress seawater foaming. These chemicals are certified to be non-toxic. However, no data are available on their fate in the marine environment or whether they have potentially harmful degradation products. 3.5. Oxygen scavengers Hydrazine (N2H4) is mostly used in boilers and may cause longterm damage to the environment. Hypochlorite ions (ClO −) are used in parts other than boilers to deplete oxygen in order to minimize corrosion. Sodium sulfite (Na2SO3) is also used, and has the potential of increasing the sulfate level in water, with no foreseen harm. 3.6. Acids and alkalis The well-known examples are sulfuric acid (H2SO4) and sodium hydroxide (NaOH). The H2SO4 is added to feed water in order to convert the less soluble calcium carbonate (CaCO3) and magnesium hydroxide (MgOH) to more soluble calcium and magnesium sulfate salts. The NaOH is added to seawater feed to adjust the pH above 9.0. Both sulfuric acid and sodium hydroxide impose an insignificant shift in the ionic composition of the brine discharge, and are not envisioned to have an environmental impact. Some RO plants use also sulfuric acid or hydrochloric acid at 20– 100 mg/L in order to avoid scaling. The acid solution should be neutralized as far as possible prior to discharge to the sea (pH ≅ 8.3) [18]. The impacts of desalination plants on the marine environment (e.g. Jubail Plants) are recognized primarily as bio-fouling of intake structures, membranes, pumps, water lines, water boxes, heat exchangers, and others. Bio-fouling also triggers corrosion of materials which could be costly. 4. Brine disposal by surface water discharge system An appropriate technology is required to ensure proper dispersion of the brine for minimization of its impacts on the marine environment. The most commonly used disposal methods are: a- Discharging the brines by a long pipe far into the sea, which is preferable; A critical factor of this method is the distance between the intake and the outlet of the water that has to be considerable enough to avoid or minimize the risk of feed water deterioration. The brine should be disposed by pipe sufficiently far out into the sea. If the brine concentrate is discharged directly into the sea, a plume of elevated density is formed that will descend to the sea floor and extend horizontally following the sea bottom bathymetry. For instance, in the Dhekelia plant in Cyprus the distance between the inlet and the outlet is more than 2 km. Mitigation measures should be taken so as to reduce the potential impacts on the marine environment [20]: b- Direct discharge of the brines at the coastline; In cogeneration power desalting plants, the brine is discharged to the outlet of the power station's cooling water to dilute salt concentration. In small capacity plants, the brine may be discharged directly to the salt production plant. Sea outfalls are classified according to their location (onshore surface discharges/offshore submerged discharges), their mixing

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features (single port / multiport) and their effluent characteristics (positively buoyant, or negatively buoyant). Onshore surface discharges have traditionally been installed due to their low costs. Examples are [18]: 1. Shoaiba (KSA), MSF Plant, Red Sea, 1.58 Mm 3/day, positively buoyant discharge. 2. Al Jubail (KSA), MSF plant, Arabian Gulf, 1.54 Mm 3/day, positively buoyant discharge. 3. Al Gubrah (Oman), MSF plant, 191,000 m 3/day, Gulf of Oman, positively buoyant discharge. 4. Taweelah MSF plant, Arabian Gulf, 1.12 Mm 3/day, positively buoyant discharge. 5. Ashkelon (Israel), RO plant with negatively buoyant brine discharge during backwash through an open channel at the coast into the Mediterranean. However, such discharges should be analyzed carefully and generally be avoided due to their limited mixing characteristics, high visibility, their need for large scale coastal constructions, and thus generally larger impacts. The high salinity of brine creates denser brine plumes than seawater. This effluent is generally 33% saltier than the water originally drawn into the plant. Its density causes this effluent to sink and, in some cases form stable pools on the seafloor that resists mixing. Decrease in dissolved oxygen and associated changes then kill marine animals and plants. Also, increased salinity affects the behavior of some marine animals and plants. In Kuwait, the MSF desalting units are located within a steam power plant (PP), and brine water is mixed with cooling seawater returning from the PP condenser to the sea. This process dilutes the brine and lowers the salt concentration to less than 1.1% the intake salinity. A key challenge for dedicated sea outfalls is to minimize the size of the zone in which the salinity is elevated, before adequate mixing with ambient waters. In some cases, this can be achieved by reliance on the mixing capacity of the tidal (surf) zone. However, this approach may lead to high salt concentrations along the shoreline. In other cases, where the discharge occurs beyond the tidal zone and

in low energy environments, it may be necessary to install diffusers to accelerate and facilitate mixing. It is a requirement to set guidelines to be followed for new power and desalting plant projects concerning the major environmental issues associated with the project. Potential options to address those issues, including facility siting, facility operations (e.g., intake impacts, entrainment/impingement), discharge impacts, greenhouse impacts, etc. should be identified and evaluated early to insure that the guidelines are followed before approving the projects by Kuwait Environment Protection Agency. Intake and discharge characteristics such as type (e.g., beach well or open water intake), location, volumes, salinity levels and other critical water quality parameters in the vicinity of intakes and outfalls should be addressed in the reviewing and approval processes. Environmental impacts resulting from desalination plants should be avoided or minimized through (1) appropriate siting, (2) choosing an intake which is suitable (beneath coastal sediments and in open, well circulated, marine waters), and (3) treating rejected brine properly to save marine species. 5. Indirect impacts of desalination on air environment and CO2 emissions Combustion of fossil fuels used to generate the energy required for the desalination process adversely impacts the air environment. Most of the desalination plants in Qatar are combined with a gas turbine combined cycle (GTCC) power plant (Fig. 5). This plant uses back pressure steam turbine (BPST) discharging its expanded steam at 2–3 bar to the desalination MSF units. Details are shown in Fig. 6 where the flow rates of steam supply the three 45 MIGD total capacity MSF units. For this specific plant, the steam discharged to the three MSF units has 287.7 kg/s flow rate and 2782 kJ/kg enthalpy, and leaves as condensate at 496 kJ/kg enthalpy. Therefore, there is 656 MW heat delivered to the three desalination units to produce 45 MIGD (2367 kg/s) or 277 MJ/m 3. If this steam was expanded in the LP turbine to produce power instead of DW, it would produce 127.71 MW. In other words, there is 127.71 MW work loss due to supplying 656 MW heat to the MSF units. Similar work loss

Fig. 5. Arrangement of Gas turbine combined cycle (GTCC) with BPST supplying steam to 3 MSF units of 45 MIGD desalting capacity. Adapted from [23].

M. Darwish et al. / Desalination 309 (2013) 113–124

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Fig. 6. Arrangement of BPST supplying steam to 3 MSF units of 45 MIGD desalting capacity. Adapted from [23].

due to the steam is supplied to the steam ejectors of the 3 MSF units equal to 7.5 MW. Besides heat, pumping energy is consumed to move the streams of the MSF units at a rate of 4 kWh/m 3 (14.4 kJ/kg pumping energy), or total pumping energy of 34.04 MW. Therefore, 169.25 MW total equivalent mechanical energy is consumed by the MSF units to produce 2367 kg/s (or 71.5 kJ/kg work ≅ 20 kWh/m 3). If the overall efficiency of the GTCC power plant (including the equivalent work of desalting is 0.48) the fuel consumed to produce 1 m3 of DW is 150 MJ/m3. The expected 2012 production of 436 Mm3 DW means consumption of 65.4×106 GJ. If natural gas (NG) is used, 62.31 billion cubic foot (BCF), or 62.31×0.0208=1.296 million (M) tons of NG are needed to be combusted annually for DW production in 2012. The 1.296 M tons of NG combustion produce CO2 equal to 1.296×0.75×44/12=3.564 M tons of CO2. This indicates that each single m3 of desalinated water results in the emission of 8.18 kg of CO2. If SWRO of 5 kWh/m 3 consumption was used in place of MSF, the expected 2012 production of 436 Mm 3 DW means consumption of 16.35 × 10 6 GJ. If natural gas is used, 15.57 billion cubic foot (BCF), or 15.577 × 0.0208 = 0.324 million (M) tons of natural gas need to be combusted annually for DW production in 2012. The 0.324 M tons of NG combustion produce CO2 equal to 0.324 × 0.75 × 44/12 = 0.89 M tons of CO2. This indicates that each one cubic meter of desalinated water results in the emission of 2.045 kg of CO2, or 25% of that consumed by the MSF units. These calculations are summarized in Table 4. 6. Conclusions Seawater desalination is the main source of water in Qatar and it is used for all domestic, industrial, and recreational purposes, and

covers a fraction of irrigation water. Almost all of the desalination plants in operation today in Qatar (as well as the GCC countries) are based on thermal desalination technologies. It has been realized that the continuous use of thermal technologies for many decades has a significant environmental footprint compared to Seawater Reverse Osmosis systems (SWRO). Environmental impacts of desalination technologies are inherently related to energy efficiency and discharge of chemical additives and sub-products in the brine, which result from pretreatment and post treatment processes. Throughout this paper, Qatar desalination practices were analyzed with greater care, to assess the main environmental impacts caused by extensive energy consumption and brine discharges that may heavily affect air quality and the marine environment. Recommendations on using SWRO in place of MSF and MED are outlined to reduce the environmental footprint. It has been shown that the present and projected environmental loads related to desalination capacity in Qatar can be considerably reduced when adopting SWRO as the main technology. Integrating desalination processes with renewable energy production systems could also play a vital role for both resource and environmental sustainability.

Table 4 Energy and CO2 emission for MSF and SWRO. MSF Equivalent mechanical energy Fuel consumption/m3 desalted water Total consumed NG/y for 1.2 Mm3/d Emitted CO2/y CO2 emission/m3 desalted water

SWRO 3

20 kWh/m 150 MJ/m3 1.296 M t/year 3.564 M t/year 8.18 kg/m3

5 kWh/m3 37.5 MJ/m3 0.324 M t/year 0.89 M t/year 2.05 kg/m3

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Desalination capacity in Qatar has reached 1.2 Mm 3/day in 2012. For desalting the same amount of water using SWRO technology, the seawater intake would be reduced about 3 times from 8.4 Mm 3/d to 3.6 Mm 3/d, as the MSF and MED technologies require huge amounts of cooling water for condensation. This decreases the impingement and entrainment of marine species at the intake and reduces the discharge of brine and cooling water from 7.2 Mm 3/d to 2.4 Mm 3/d. The brine discharge of the MSF and MED-TVC is at a temperature 10 °C higher than that of seawater temperature, while it is the same as ambient for the SWRO case. The elevated temperature may cause tremendous impacts on seashore, habitats and all components of the ecosystem. Moreover, the brine rejected from the MSF and MEDTVC has significant residual amounts of chlorine compared to SWRO. Finally, the use of SWRO saves 75% of CO2 emissions from 3.564 to 0.891 M tons of CO2 per year due to better energy efficiency of RO systems. In addition to the direct environmental impacts, reducing water leakage in the network and controlling socially-induced factors leading to the notably higher per capita consumption of fresh water in Qatar would significantly contribute to sustainability. Acknowledging that water scarcity is an inherent characteristic, especially in the arid region of GCC countries, water saving incentives and measures should be adopted through policy interventions such as properly regulating water prices and tariff structures to change the supply–demand nexus. Even the European Declaration for a New Water Culture recognizes the different functions and values of water from an ethical point of view, according to which a distinction should be made based on the purpose served by a freshwater supply as: a. Water for Life; which represents the top priority from the human rights point of view. b. Water for General Interest Purposes; which is necessary for preserving health and social cohesion. c. Water for Economic Growth: which is recognized as a third level of priority. Hence, energy efficient SWRO desalination capacity, with less than 4 kWh/m 3, would and should primarily be designed to meet essential human needs rather than satisfying luxury. The World Bank looks at desalination as a solution of last resort, to be elected only after all appropriate water demand management measures have been undertaken [21]. Undoubtedly, saving water is much better than adding new capacities for water supply, both from the environmental and economic perspectives. References [1] Qatar General Electricity & Water Corporation KAHRAMAA, Statistical Year Book, KAHRAMA Publications, September 2010. [2] Towards Qatar National Vision (2030), Qatar National Development Strategy, 2011–2016, Qatar General Secretariat for Development Planning, 2011. First Published March.

[3] R. Danoun, The Ocean Technology Group Desalination Plants: Potential impacts of brine discharge on marine life, University of Sydney, 2007. Final Project 05/06/2007. http://ses.library.usyd.edu.au/bitstream/2123/1897/1/Desalination%20Plants.pdf. [4] Desalination for Safe Water Supply, Guidance for the Health and Environmental Aspects Applicable to Desalination, Public Health and the Environment, World Health Organization, Geneva, 2007. WHO/SDE/WSH/07/0. [5] Desalination Resource and Guidance Manual for Environmental Impact Assessments. United Nations Environment Programme, Regional Office for West Asia, Manama, and World Health Organization, Regional Office for the Eastern Mediterranean, Cairo, Principal author and editor: Sabine Lattemann Co‐authors: Khalil H. Mancy, Bradley S. Damitz, Hosny K. Khordagui, Greg Leslie, UNEP/ROWA 2008 ISBN: 978‐92‐807‐2840‐8. (2008). http://www. unep.org.bh/Newsroom/pdf/EIA-guidance-final.pdf. [6] M. Darwish, F. Al Awadhi, M. Abdul Raheem, The MSF: enough is enough, Desalin. Water Treat. 22 (2010) 193–203. [7] S. Lattemann, T. Höpner, Environmental impact and impact assessment of seawater desalination, Desalination 220 (2008) 1–15. [8] F. Münk, Ecological and economic analysis of seawater desalination plants. Diploma Thesis. Institute for Hydromechanics, University of Karlsruhe, Karlsruhe. April (2008). http://www.ifh.uni-karlsruhe.de/science/envflu/research/brinedis/muenkdiplomathesis.pdf. [9] M. Darwish, Critical comparison between energy consumption in large capacity reverse osmosis (RO) and multistage flash (MSF) seawater desalting plants, Desalination 63 (1987) 143–161. [10] M. Darwish, Desalting fuel energy cost in Kuwait in view of $75/barrel oil price, Desalination 208 (2007) 306–320. [11] Safe drinking-water from desalination, WHO/HSE/WSH/11.03, World Health Organization, WHO Press, Geneva, 2011. [12] In: Joseph Cotruvo, John Fawell, Gunther Craun (Eds.), Nutrients in Drinking Water, WHO Press, Geneva, 2005, ISBN 9241593989. WHO Press, Geneva. (2005). www.who.int/water_sanitation_health/dwq/nutrientsindw/en. [13] Health Effects of Calcium and Magnesium in Drinking Water, WHO Press, Geneva, 2006. www.who.int/water_sanitation_health. [14] M. Darwish, A. Al-Sairafi, Technical comparison between TVC/MEB and MSF, Desalination 170 (2004) 223–239. [15] T. Hoepner, S. Lattemann, Chemical impacts from seawater desalination plants — a case study of the northern Red Sea, Desalination 152 (2002) 133–140. [16] M. Dawoud, M. Al Mulla, Environmental impacts of seawater desalination: Arabian gulf case study, Int. J. Environ. Sustain. 1 (2012) 22–37. [17] M. Abdel-Jawad, M. Al-Tabtabaei, Impact of current power generation and water desalination activities on Kuwaiti marine environment, in: Proceedings of IDA World Congress on Desalination and Water Reuse, San Diego, 3, 1999, pp. 231–240. [18] T. Bleninger, G.H. Jirka, Environmental planning, prediction and management of brine discharges from desalination plants, Final report, Middle East Desalination Research Center, Muscat, Sultanate of Oman, MEDRC Series of R&D Reports, MEDRC Project: 07-AS-003, December 2010. [19] M. Taniguchi, Y. Fusaoka, T. Nishikawa, M. Kurihara, Boron Removal in Seawater RO Desalination, Toray Industries, Inc., Japan, 2011. www.toraywater.com/ knowledge/pdf/Boron_Removal_in_seawater_RO_desalination.pdf. [20] R. Einav, K. Hamssib, D. Periy, The footprint of the desalination processes on the environment, Desalination 152 (2002) 141–154. [21] M. Schiffler, Perspectives and challenges for desalination in the 21st century, Desalination 165 (2004) 1–9. [22] Ministry of Electricity and Water, Shuaiba North Cogeneration (Power/Distillation) Plant, Plant Operation Philosophy Report. Contract No.: MEW/C/3656-2007/2008, Document Number: J / PM / GNRL – DPD / 002, State of Kuwait, June 2008. [23] F. Alasfour, M. Darwish, A. Bin Amer, Thermal analysis of ME-TVC+MEE desalination systems, Desalination 174 (2005) 39–61.