Comparative study of brine management technologies for desalination plants

Depósito de Investigación de la Universidad de Sevilla https://idus.us.es/ This is an Accepted Manuscript of an article published by Elsevier in Desalination, Vol. 336, on March 2014, available at: https://doi.org/10.1016/j.desal.2013.12.038 © 2014 Elsevier. En idUS Licencia Creative Commons CC BY-NC-ND Comparative study of brine management technologies for desalination plants José Morillo1, José Usero1, Daniel Rosado1, Hicham El Bakouri2*, Abel Riaza2, Francisco- Javier Bernaola2 1. Department of Chemical and Environmental Engineering, University of Seville, 41092 Seville, Spain 2. Abengoa Water S.L.U, Research Development Center, Prolongación c/ Don Remondo s/n, Barriada Fuente del Rey, 41703 Dos Hermanas, Spain. HIGHLIGHTS:  Brine management systems for desalination plants  Technologies for reducing the volume of the generated brines  Technologies for salts recovery form brines  Brine conditioning for other processes CORRESPONDING AUTHOR: * R&D Center of Abengoa Water, Prolongación c/ Don Remondo s/n, Barriada Fuente del Rey, 41703 Dos Hermanas, Spain. Tel.: 0034 955404963; E-mail address: hicham.elbakour[email protected]bengoa.com (H. El Bakouri). Comparative study of brine management technologies for desalination plants 1 Abstract In recent years, reverse osmosis (RO) has grown as an alternative to traditional potable water sources. A major disadvantage of the RO process is the huge amount of brine and its negative impact as a result of its high salinity. This brine is usually discharged to inland water bodies or to the sea and constitutes a threat to ecosystems and species, such as Posidonia oceanica in the Mediterranean Sea; thus, further research is needed for introducing environmentally friendly and economically viable management options for RO brines. This paper gives an overview of recent research as well as different technologies available at several scales to overcome the environmental problems and evaluate profitability related to discharge of RO concentrates. The treatment options have been classified into four different groups according to their final purpose: 1) technologies for reducing and eliminating brine disposal, 2) technologies for commercial salt recovery, 3) brine adaptation for industrial uses and 4) metal recovery. Solar evaporation, two-stage reverse osmosis, electrodialysis, integrated processes and brine adaptation for the chlor-alkali industry are some of the topics that this paper deals with. In the conclusion section, all of the technologies are compared emphasizing all their advantages and drawbacks, feasibility and development stage in order to provide a decision tool to select the best technology for each situation. Keywords Seawater desalination, Reverse osmosis, Brine treatment, Water recovery, Salt recovery. Comparative study of brine management technologies for desalination plants 2 1 Introduction Potable water production has become a worldwide concern; for many communities, projected population growth and associated demand exceed conventional available water resources. Over 1 billion people have no access to clean drinking water and approximately 2.3 billion people (41% of the world population) live in regions with water shortages [1]. The shortage of water supplies for drinking and irrigation purposes is already a very serious problem for the North African countries, the Middle East and several countries in Southeast Asia and Latin America. If nothing is done, acute water shortages will also occur in many countries of the European Union and the northern Mediterranean by 2020, such as Greece, Italy (southern regions and islands), Portugal (Alentejo and Algarve regions and islands such as Porto Santo, Corvo, etc.) and Spain (southern and eastern regions). For the entire Mediterranean region, conservative estimates indicate a water shortage of about 10 million m3/day by the year 2020 [2]. Desalination has become an important source of drinking water production, with thermal desalination processes developing over the past 60 years and membrane processes developing over the past 40 years [3]. Today, reverse osmosis (RO) is the leading technology for new desalination installations, with a 44% share in world desalting production capacity and an 80% share in the over 15,000 desalination plants installed worldwide [4]. The Middle East has forged ahead as the leader in large-scale seawater desalination. With only 2.9% of the world’s population, it holds approximately 50% of the world’s production capacity. In 2005, Israel opened the world’s largest seawater RO desalination plant, with a production capacity of 330,000 m3/day, or 100 million m3/year [4]. The use of membrane desalination has increased as materials have improved and costs have been reduced [3]. But the main reason why RO desalination has succeeded is because it requires less energy than thermal desalination — only 1.5-2.5 kWh/m3 for RO versus 15-25 kWh/m3 for evaporation [1]. Furthermore, improvements in membranes and energy recovery have significantly lowered the cost of RO desalination. As a result of increased interest in RO desalination, the concern about potential environmental problems has grown. RO desalination plants extract large volumes of water and discharge a dense brine concentrate back into the environment [5]. It is widely suggested that desalination plant brines have a strong potential to detrimentally impact both physicochemical and ecological attributes of receiving environments [6]. There has been worry in Mediterranean countries about Posidonia oceanica for the last few years. P. oceanica is the most abundant sea Comparative study of brine management technologies for desalination plants 3 grass species in the Mediterranean, where it covers about 40,000 km2 of the sea floor [7] and forms large meadows from the surface to 40 m depths. In addition, it is considered a very important ecosystem and is recognized by the European Habitats Directive [8] as a habitat of priority interest. Nevertheless, meadows of P. oceanica have undergone regression in several coastal areas [9] and under field conditions, P. oceanica is very sensitive to brine discharges from desalination plants [10]. Many solutions have been developed to protect this plant, mainly based on diluting brine before disposal. Brine disposal costs are high today, between 5 and 33% of total desalination cost [11], complicating implementation. This cost depends on the quality of the concentrate, treatment level before disposal, disposal method and the volume or quantity of concentrate [12]. Disposal costs for inland desalination plants are even higher than those for plants discharging brine into the sea [12]. Some of the options for brine disposal from inland desalination plants are deep well injection, evaporation ponds, discharge into surface water bodies, disposal to municipal sewers, concentration into solid salts and irrigation of plants tolerant to high salinity [12,13]. Due to the environmental problems that brine disposal can cause and high disposal cost, many technologies have been developed for recovery. Examples are renewable energy generation [14] and use in evaporation ponds to produce salt or chemicals for industry. Nevertheless, more investigation is needed to reduce brine quantity and to allow recovery and reuse of brine. In this review, current and emerging technologies are analyzed according to their origin, the maturity of the technologies and their final goal. Comparative study of brine management technologies for desalination plants 4 2 Technologies for reducing and eliminating brine disposal 2.1 Solar evaporation Solar evaporation consists of leaving brine in shallow evaporation ponds, where water evaporates naturally thanks to the sun’s energy. Salt is left in the evaporation ponds or is taken out for disposal [15]. Evaporation ponds are relatively easy to construct, while requiring low maintenance and little operator attention compared to mechanical systems. In addition, no mechanical equipment is required, except for the pump that conveys the wastewater to the pond [16]. Nevertheless, evaporation ponds for disposal of concentrate from desalination plants need to be constructed as per the design and maintained and operated properly so as not to create any environmental problem, especially with regards to groundwater pollution [16]. Solar evaporation is a suitable technology to be used in arid regions where land is available [17]. Land is crucial because shallow ponds (ranging from 25-45 cm) are optimal for maximizing the rate of evaporation [16]. However, due to the quantity of terrain needed, evaporation ponds have limited use, especially in wet areas. For instance, only 6% of the installations in the US used this method of concentrate disposal up to 1993 and only 2% after 1993, always for small plants [18]. Wind aided intensified evaporation technology (WAIV) was patented as an alternative to natural evaporation. This method reduces brine quantity thanks to natural dryness. Brine recirculates and falls, creating a thin liquid layer on vertical surfaces parallel to wind direction. Gilron et al. [17] carried out experiments in a pilot plant and demonstrated that the evaporation ratio (L/(m2∙d)) can be improved between 50% and 90% compared to evaporations ponds. Katzir et al. [15] estimated that using WAIV technology increases the evaporation rate 10-fold over natural evaporation, which allows evaporation ponds to be 10 times smaller. They also studied WAIV technology possibilities for recovery of salts and their use as raw materials. For this purpose, RO and electrodialysis concentrates from brackish groundwater were used as feedwater. Although WAIV technology has advantages compared to evaporation ponds, experiments at industrial scale are needed in order to prove feasibility for large flows of brine. 2.2 Evaporation and crystallization systems Many authors have studied evaporation and crystallization systems in order to evaluate technical and economic feasibility. Zarzo et al. [19] published an article about the research Comparative study of brine management technologies for desalination plants 5 done by the companies of Sadyt and Scrinser (both in the Spanish Sacyr-Vallehermoso group) and Ecoagua. That research aimed at the removal of salts from desalination plant brines to reduce the impact of the discharges and to obtain salts or by-products. This project studied Zero Liquid Discharge (ZLD) systems based on evaporation-crystallization technologies. Two pilot plants were installed: one at laboratory scale at the Complutense University of Madrid and the other at the Cuevas de Almanzora desalination plant (25,000 m3/d of brackish water treated with RO). The Complutense University plant had an evaporation capacity of 7 L/h. It worked in continuous mode under vacuum conditions. The energy consumption was very high, about 0.9 kWh/kg, with an estimated final cost of €0.095/kg of brine evaporated. The pilot plant at the Cuevas de Almanzora desalination plant was an evaporation-crystallization plant with a flash evaporator operating under vacuum conditions and continuous feed. The plant capacity was 100 L/h, although experiments worked with a flow rate of approximately 70 L/h. The Cuevas de Almanzora desalination plant was more energy efficient than conventional evaporationcrystallization because it utilized the vapor generated in evaporation to heat the feedwater entering the evaporator. The authors concluded that evaporation-crystallization technology appears to be economically viable only if it is associated with a system to recover residual heat or steam as the process has high energy consumption. Mickley et al. [20] did research on high recovery and zero liquid discharge technologies. They suggested many alternatives for different feedwater compositions. Those alternatives are based on combinations of RO, lime softening (LS), thermal brine concentrator (BC), thermal crystallizer (CRYST), spray dryer (SD), evaporation ponds (EP) and landfill (LF) to treat brackish water with recoveries over 96%. The study concluded that the yield of the process depends basically on salinity and water composition. They reached the same conclusion for capital and operating costs, specifying that evaporation ponds and landfills are the biggest costs. In addition, the paper emphasizes large systems requiring multiple equipment modules, thus minimizing the economy of scale. 2.3 Membrane distillation systems Membrane Distillation (MD) is a non-isothermal evaporative technology that uses a hydrophobic microporous membrane being the driving force the vapor pressure difference between both membrane sides. Extended information on MD may be found in Khayet and Matsuura [21]. MD can be applied for the treatment of saline solutions with high Comparative study of brine management technologies for desalination plants 6 concentrations. Vacuum membrane distillation (VMD) is a variant of MD, in which low pressure or vacuum is applied on the permeate side of the membrane module, for example by means of vacuum pump(s). The applied permeate pressure must be lower than the saturation pressure of volatile molecules to be separated from the feed solution and condensation takes place outside the membrane module at temperatures lower than the ambient temperature. Mericq et al. [22] applied VMD configuration for the treatment of RO brines (Figure1). Simulations were performed to optimize the VMD operating conditions and then they were completed by bench-scale experiments using synthetic RO brines containing only the mineral part of seawater with total salt concentrations up to 300 g/L. High permeate fluxes were obtained even for the highest salt concentrations. However, the permeate flux was limited at high salt concentrations by scaling, mainly due to calcium precipitation. Despite this inconvenience, scaling had only a partial impact on the permeate flux (i.e. 24% decrease for 43 L/(h∙m2) permeate with the highest salt concentration). Calcium carbonate (CaCO3) and calcium sulfate (CaSO4) precipitated first due to their low solubility and formed mixed crystal deposits on the membrane surface. These phenomena only occurred on the membrane surface and did not totally cover the membrane pores. The crystals were easily removed simply by washing the membrane with water. Simulations were performed to study the yield of the process with 40,000 m3/day of 38.9 g/L seawater, achieving a recovery of 40% for VMD itself and up to 89% for overall recovery by coupling RO and VMD. Results also showed that concentrate quantity can be reduced by a factor of 5:5, making it possible to double overall water production. Figure 1. Schematic of seawater desalination by RO and vacuum membrane distillation (VMD) integrated process [22]. Ji et al. [23] investigated the performance of membrane distillation crystallization (MDC) bench-scale plant in terms of water recovery and NaCl crystallization kinetics. The extensive Comparative study of brine management technologies for desalination plants 7 contact area provided by hollow fiber membranes made it possible to achieve reliable permeate fluxes at moderate temperatures (40-50 oC) with energy consumption ranging from 15-20 kWh/m3, which is lower than that of conventional evaporative systems for NaCl crystallization having a specific energy consumption of 30 kWh/m3. Experimental tests carried out on artificial RO concentrates resulted in 21 kg/m3 production of NaCl crystals and the final water recovery factor increased up to 90%. Analogous investigations carried out on RO brines from natural seawater were affected by the presence of dissolved organic matter, showing 20% reduction in the amount of salt crystallized and 8% decrease of the permeate flux. Therefore, adequate pretreatment before the RO stage is needed to reduce the negative effect of dissolved organic matter on the MDC performance. This study confirms the ability of MDC to concentrate RO brines. In principle, the industrial scale-up of the MDC process involving large volumes of brines do not show any technical complexity. However, this technology is not available at a commercial scale and further investigations are needed on the environmental impacts and economics. Martinetti et al. [24] studied vacuum-enhanced direct contact membrane distillation (VEDCMD) to increase water recovery during desalination of brackish water (Figure 2). In their tests, two RO brine streams were used as feed of the VEDCMD system, with total dissolved solid concentrations ranging between 7,500 and 17,500 mg/L. A recovery factor up to 81% was achieved. However, recovery factors were always limited by the precipitation of inorganic salts on the membrane surface. Martinetti et al. [24] also showed also that cleaning techniques were able to remove the scaling layer from the membrane surfaced restoring the water permeate flux to almost its initial level. The authors also claimed that the addition of scale inhibitors during the process was effective in maintaining high water permeate flux during an extended VEDCMD operating time. Comparative study of brine management technologies for desalination plants 14 highly concentrated draw solution [37]. Since the draw solution is the driving force behind the FO process, the solute within the draw solution must meet several criteria in order to be suitable for the FO process. High osmotic efficiency is required, and the draw solute must be highly soluble in water and, at the same time, of low molecular weight, easily and economically separated and recycled, non-toxic and chemically compatible with the membrane being used [37]. McGinnis [38] and McCutcheon et al. [39] proposed draw solutes that met all the requirements. However, study results revealed that RO membranes are not suitable for the FO process because of relatively low permeability attributed to severe internal concentration polarization in the porous support and fabric layers of the RO membrane. McCutcheon et al. [39] presented a FO process using an ammonium bicarbonate ((NH4)HCO3) draw solution to extract water from saline feedwater across a semi-permeable polymeric membrane. The system yields high water fluxes and can result in very high feedwater recoveries. Upon moderate heating, (NH4)HCO3 decomposes into ammonia (NH3) and CO2 gases that can be separated and recycled as draw solutes, leaving the fresh product water. Experiments with a laboratory-scale FO unit using a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. Tang and Ng [40] investigated FO using a laboratory-scale unit. These experiments achieved 38.5% recovery using a feed stream similar to RO concentrates (1-2 M NaCl, 58.5-117 g/L) and 5 M fructose as draw solution (900 g/L). Assuming a RO-FO process for seawater with 45% recovery for RO [3], an overall recovery rate ranging from 66 to 76% could be achieved with the recovery rates for FO estimated by Tang & Ng [40]. Cath et al. [35] stated that salt rejection in FO is over 97%. McCutcheon et al. [41] indicated 95-99% and confirm Cath et al. [35], with greater reject at higher water flow rates. Energy requirements were less for FO than for other desalination technologies. McGinnis et al. [42] pointed out that the FO process proposed by McCutcheon et al. [39,41] can achieve energy savings of 72% compared to RO and 85% compared to multi-stage flash distillation. Water flow for a feed solution of 0.5 M NaCl (29.5 g/L NaCl), similar to seawater, ranged from 3.2 to 23 L/(m2h) at osmotic pressure differentials between 22 and 217 bar [41]. However, for FO it ranged from 4 to 7.2 L/m2 with 2 M NaCl (117 g/L NaCl) feed solution and pressure differentials from 80 to 127 bar. Another alternative to take into consideration is FO as a stage Comparative study of brine management technologies for desalination plants 15 prior to RO. Cath et al. [36,43] achieved recovery rates over 95% in research done with sewage. Cath et al. [44] did a study on combining FO and RO to obtain fresh water from polluted water by using seawater as draw solution. With this hybrid process, seawater is diluted before desalination, decreasing energy costs and reducing the quantity of polluted water. Martinetti et al. [24] investigated FO for water recovery enhancement in desalination of brackish water (Figure 9). In the current study, two RO brine streams with total dissolved solid concentrations averaging 7,500 and 17,500 mg/L were further desalinated by FO with a constant-concentration draw solution of 50 g/L NaCl. FO achieved water recoveries up to 90% from the brines, limited by salt precipitation on membranes. Figure 9. Schematic drawing of FO system proposed by Martinetti et al. [24]. 2.6 Electrodialysis Electrodialysis (ED), under the influence of an electric field, enables the separation of dissolved ions in water through selective ion exchange membranes. Korngold et al. [45] applied ED to concentrated brine solutions similar to effluents from the desalination of brackish and industrial water. Results showed that ED can be used to increase the concentration of a brine solution from 0.2-2% to 12-20% with energy consumption in the range of 1.5-7.1 kWh/m3, in contrast to approximately 25 kWh/m3 by thermal evaporation. However, electrical efficiency decreases when the brine concentration is significantly increased and CaSO4 precipitates on the membranes. Nevertheless, precipitation on membranes can be avoided by pretreatment. Korngold et al. [46] built an ED pilot plant fed with brackish water in which brine circulating through the ED cells passed through a separate CaSO4 precipitator containing gypsum seeds. Comparative study of brine management technologies for desalination plants 16 ED proved to be usable for increasing the concentration of RO brine solution from 1.5% to 10% at an energy requirement of 7.0-8.0 kWh/m3. Oren et al. [47] tested a hybrid process combining RO and ED, which was shown to be effective in recovering 97-98% of brackish water as product water with chloride levels of 200 mg/L or less. Potential for scaling on the brine side of the ED unit was prevented by acidification, operating the ED in reversal mode, called electrodialysis reversal (EDR), and a side loop crystallizer which prevented buildup of scaling components. This process was demonstrated in a series of more than eighty batch experiments of 1.5 to 1.8 m3 of RO concentrate of raw brackish groundwater from the Negev Highland in Israel. The feedwater was concentrated from 0.3% to over 10% TDS superconcentrate while producing water that could be recycled to the RO permeate. This superconcentrate from the EDR unit was further concentrated in a wind-powered WAIV unit that brought final brine TDS to over 30%. Initial economic estimates showed that this hybrid process is competitive with conventional RO and other enhanced recovery processes for inland desalination requiring use of evaporation ponds. Comparative study of brine management technologies for desalination plants 17 3 Technologies for commercial salt recovery 3.1 SAL-PROC process SAL-PROC is an integrated process for sequential extraction of dissolved elements from inorganic saline waters in the form of valuable chemical products in crystalline, slurry and liquid forms. An analysis indicated that various types of salts, including gypsum, NaCl, Mg(OH)2, calcium chloride (CaCl2), CaCO3 and sodium sulfate (Na2SO4), can be produced from the reject brine of desalination plants. This process is particularly recommended for inland brines with high concentrations of sulfate, potassium and magnesium. Figure 10 shows a simplified diagram of the process. From an economic point of view, it was estimated that by processing 405,000 m3 of reject brine per year, commercial salts worth $895,000 could be produced [48]. Although the economic benefit will probably be lower, the commercialization potential of salts is an option to improve profitability of desalination processes. Figure 10. Simplified diagram of SAL-PROC process [48]. Arakel et al. [49] used SAL-PROC to process brackish water from Tutchewop Lake (Victoria, Australia). This lake receives a discharge of approximately 64,000 t/year of salt. The process made it possible to recover high quality Mg(OH)2, NaCl, a mixture of gypsum and Mg(OH)2 and a highly concentrated solution of CaCl2. The SAL-PROC route used in this case is described in detail in Figure 11. Comparative study of brine management technologies for desalination plants 18 Figure 11. SAL-PROC process used in the treatment of brackish water from Tutchewop Lake [49]. Arakel et al. [49] combined RO and SAL-PROC in a process known as ROSP. This process was used for treating RO brine with a high bicarbonate concentration in the effluent coming from coal-based methane extraction (CBM extraction) in Queensland, Australia (Figure 12). The ROSP process produces CaCO3, Na2SO4 and NaCl. Figure 12. ROSP process for treatment of water coming from coal-based methane extraction (CBM) gas field [49]. Comparative study of brine management technologies for desalination plants 19 3.2 Zero Discharge Desalination Patent of University of South Carolina The University of South Carolina [50] developed a technology called zero discharge desalination (ZDD) for the treatment of seawater RO brines (PCT patent PCT/US03/24250). The process focuses on producing fresh water and valuable salts: NaCl, Mg(OH)2 and bromine (Br2). The process has different configurations, all based on ED. In the basic configuration (Figure 13) NaCl is recovered as a dry salt and the waste streams of Mg(OH)2 and Br2 are returned to the sea. In the second configuration, pure NaCl is recovered by adding a crystallizer, and the waste streams that return to sea in the first case are treated by evaporation for drying and production of road salt (Figure 14). Figure 13. Process schematic for zero discharge desalination with optional seawater discharge [50]. Experiments carried out at laboratory scale showed that about 75% of NaCl in the brine was recovered as high-purity NaCl crystals in the evaporation-crystallization stage. Because of ED concentrates NaCl up to 20%, NaCl can be crystallized with only one-third of the thermal energy that would be required if the total amount of water in the RO reject were to be evaporated. Figure 14. Process schematic for zero discharge desalination [50]. Comparative study of brine management technologies for desalination plants 20 These experiments pointed out that Mg(OH)2 production of greater than 99% purity was achieved by pretreatment with Na2CO3 to remove calcium. However, using Na2CO3 also precipitates magnesium and affects process yield. Br2 production was estimated by a mathematical model developed for this purpose. Simulations showed that about 0.38 tons of bromine ion would be recovered in the ED brine associated with 3.79 million m3 of seawater RO permeate, and essentially all of that could be recovered as Br2 by conventional techniques. ZDD scale-up could be easily achieved because all the separation processes involved are available on a commercial scale. The University of South Carolina assessed ZDD profitability. The capital costs for the analysis were based on reported costs of the individual processes. Operating costs were based on data from the process model. The estimated values for recovered salt and water products were $0.60/m3 for water, $60/t for NaCl, $673/t for Mg(OH)2 as Mg and $900/t for Br2. Results indicated that the recovered products can be sold for prices that will cover the cost of their recovery. The main benefit of recovering NaCl from RO concentrate is the energy savings because the starting salt concentration is twice that of seawater. Tanaka et al. [51] indicated that the energy consumption in a salt manufacturing process using RO brine is 80% of the energy consumption in the process using seawater. But producing salt from seawater is also profitable. Japan has used ED to recover NaCl from seawater and produce edible salt on a large scale for about 40 years. In recent years, ED plants have been installed by Japanese companies in Kuwait and South Korea to recover NaCl from seawater for use in chlor-alkali plants. 3.3 Integrated processes Turek [52] investigated ED of seawater in a two-step stand with an ED-Multi-stage Flash (MSF)-Crystallization system (Figure 15). Figure 15. Schematic of desalination in ED-MSF-crystallization system [52]. ED followed by EDR in countercurrent flow mode (Figure 16) was proposed for seawater desalination. In the first step, monovalent ions were removed, producing a permeate with a total Comparative study of brine management technologies for desalination plants 21 dissolved solid concentration around 10 g/L (0.38 g/L calcium, 1.26 g/L magnesium, 4.08 g/L chloride and 2.78 g/L sulfate). EDR eliminated most of the ions, achieving a permeate ready for human consumption. A single-pass low residence time mode of operation was applied to avoid gypsum crystallization in the EDR concentrate. ED water recovery was 73.8% while the EDR recovery was 90%, yielding 66.4% total ED(R) stage recovery. Figure 16. Schematic of seawater desalination by electrodialysis in ED-MSF- crystallization system [52]. This investigation included a cost estimate of ED(R) stage seawater desalination. The cost was calculated for an industrial ED unit with 80% effective membrane surface. The energy cost was assumed as $0.06/kWh, the efficiency of pumps as 0.85 and the membranes’ life as 10 years. The total investment costs were assumed as $320/m2 of membrane and maintenance costs as 25% of investment costs. Costs were also estimated for 1 m3 of water. It was assumed that the MSF unit cost was equal to $1.0/m3. The costs of further evaporation accompanied by salt crystallization were estimated based on the study of the construction of the plant and were assumed to be $8/t of salt obtained. The value of salt obtained was $30/t while the salt recovery was 80%. The authors pointed out that with this process the potable water cost is estimated to be only $0.44/m3 and salt production is 23.7 kg/m3 of potable water. Turek [53] also investigated two arrangements: ultrafiltration (UF)-nanofiltration (NF)-MSF-crystallization (Figure 17) and UF-NF-RO-MSF-crystallization (Figure 18) for producing salt and fresh water. Seawater was assumed to be the input and NF recovery of 70% was obtained. Furthermore, the rejection coefficient of the NF membrane was assumed to be 0.83 for calcium, 0.87 for magnesium, 0.93 for sulfate and 0.1 for NaCl. The NaCl concentration in NF permeate is 28.70 g/L. If 85% NaCl recovery is assumed, then 17.07 kg of NaCl per 1 m3 of UF permeate is obtained. The cost estimation for 1 m3 of UF permeate is presented in Table 2. A value as low as $0.18/m3 in the NF process was assumed because the pretreatment cost was considered Comparative study of brine management technologies for desalination plants 22 separately as UF cost. The costs of further evaporation accompanied by salt crystallization were estimated based on the study of the construction of the plant and were assumed to be $8/t of salt obtained. The value of salt obtained was $30/t and the cost of desalinated water was $0.71/m3. Since concentrating by RO is cheaper than by MSF in the range of relatively low salt concentrations, then pre-concentrating by RO may be assumed to decrease the cost of the desalination-salt production process. The cost estimate for this process is presented in Table 3. It was assumed that RO recovery was 65% while its cost was $0.63/m3 and the value of salt obtained was also $30/t. The cost of desalinated water was then $0.43/m3. Figure 17. Schematic of desalination in UF-NF-MSF-crystallization system [53]. Figure 18. Schematic of desalination in UF-NF-RO-MSF-crystallization system [53]. Table 2. Cost of desalination and salt production in UF-NF-MSF-crystallization system per 1 m3 of UF permeate [53]. Cost , $ Unit cost Per 1 m 3 of UF permeate UF 0 . 07 /m 3 0 . 070 /m 3 NF 0 . 18/ m 3 0 . 126/m 3 MSF 1 . 0 0 / m 3 0 . 613/m 3 Crystallization 8 . 00 / t 0 . 137/m 3 Total 0 . 946/m 3 Comparative study of brine management technologies for desalination plants 23 Table 3. Cost of desalination and salt production in UF-NF-RO-MSF- crystallization system per 1 m3 of UF permeate [53]. Cost, $ Unit cost Per 1 m 3 of UF permeate UF 0 . 07 /m 3 0 . 070/m 3 NF 0 . 18/m 3 0 . 126/m 3 RO 0 . 63/m 3 0 . 287/m 3 MSF 1 . 0 0 /m 3 0 . 158/m 3 Crystallization 8 . 00 /t 0 . 137/m 3 Total 0 . 778/m 3 Nanofiltration is a pressure-driven process between RO and ultrafiltration. Its main advantages are lower working pressure and greater rejection as divalent cations than monovalents. Nanofiltration can be used combined with other technologies for seawater treatment. Drioli et al. [54] developed an integrated membrane system in order to recover CaCO3, NaCl and magnesium sulfate heptahydrate (MgSO4·7H2O) from seawater nanofiltration retentate. The diagram is shown in Figure 19. In this work, nanofiltration retentate calcium ions were precipitated as carbonates by reaction with sodium bicarbonate (NaHCO3) and Na2CO3 solutions. These solutions were previously produced by reactive transfer of CO2 into NaOH solutions in a hollow fiber membrane contactor (3.85 cm x 12.32 cm LiquiCel-Celgard) with 1.4 m2 of contact area. The alkaline solution was fed in continuous mode on the shell side in countercurrent to the gaseous CO2 stream flowing through the fibers. The amount of CO2 transferred from the gas phase to the liquid phase was calculated using the mass balance of the gas stream. The flow diagram was completed with a crystallization system based on a membrane process that allows supersaturation. In all tests, the solution was fed into the crystallizer and recirculated through the membrane fibers with a flow rate of 120 L/h. Temperatures measured at the module inlet on retentate and distillate sides were 35 and 15 oC, respectively. The pH of the crystallizing solution was adjusted to 5 by hydrochloric acid (HCl) addition in order to prevent Mg(OH)2 precipitation, which avoids the formation of magnesium sulfate crystals. Higher removals were also obtained at higher pH values due to the reaction of CO2 with hydroxide ions to give bicarbonate and carbonate. Comparative study of brine management technologies for desalination plants 30 configuration following a reverse osmosis process could increase water recovery between 19 and 29%. Furthermore, the energy cost of direct osmosis is lower than that of other desalination technologies, although it is closely related to the carryover of solutes (osmotic solutions) with suitable characteristics. Technologies oriented toward obtaining commercial salts show greater potential than those whose main purpose is to eliminate effluents. It is undeniable that the potential for commercializing salts is a key option to improve the cost-effectiveness ratio of desalination processes. These technologies combine different types of processes depending on the objective sought. The aim of the SAL-PROC patent is to recover products contained in the brine: a mixture of gypsum and magnesium hydroxide, magnesium hydroxide, sodium chloride, calcium carbonate, sodium sulfate and calcium chloride. The process is particularly appropriate for brine with high levels of dissolved sulfate, potassium and magnesium salts. Theoretical studies highlight the economic feasibility of this technology; although the data should be taken with certain reservations since there are currently no seawater desalination plants with this technology installed (it has been tested only with brackish water). The University of South Carolina’s ZDD patent was developed explicitly for seawater. The process is oriented to the production of fresh water and valuable salts from reverse osmosis reject in seawater treatment: sodium chloride, magnesium hydroxide and bromine. Theoretical studies carried out by the University of South Carolina show the economic feasibility of the processes developed. It is therefore a technology well worth bearing in mind, although it is not currently in operation. There are other relevant technologies for the production of fresh water and salts from seawater, such as ultrafiltration and nanofiltration, although they are likewise not currently operating on an industrial scale. One of the alternatives with potential for application is the treatment of brine for use in the chlor-alkali industry. This requires processes to concentrate the brine, among which electrodialysis is one of the most attractive. It is also necessary to eliminate the divalent cations in brine because they exceed the specifications for membrane electrolysis. These treatments involve high costs, which can be offset by the products obtained from electrolysis. It is advisable to install the electrolysis plant as an annex to the desalination plant. The recovery of metals from seawater or brine is another promising alternative, considering the metals that can potentially be obtained from seawater and their economic value. Research is Comparative study of brine management technologies for desalination plants 31 needed in this field to develop selective extraction processes for the desired elements found in seawater and brine. Comparative study of brine management technologies for desalination plants 32 Table 5. Comparison of methods. Part 1. Technology Development status Technical observations Economic observations Evaporation ponds Industrial scale Large extents of land. Simple operation. Possible contamination of groundwater. Possibility of salts recovery. Low economic cost. WAIV technology Pilot plant scale Large extents of land. Possible contamination of groundwater. 50 to 90% higher evaporation rate than evaporation ponds. Possibility of salts recovery. Low economic cost. Evaporator- Crystallizer Industrial scale Technology available. More development needed to reduce energy use. High economic cost. No valuable chemicals recovery . Membrane distillation Pilot plant scale From 81 to 90% water recycled when coupled with reverse osmosis systems. Technically feasible for treating large amounts of water in seawater desalination plants. Possible problems with decreased flow caused by precipitation of salts on membranes. Very high energy use (15 to 20 kWh/m3) relative to energy use of reverse osmosis, but less than traditional evaporation and crystallization systems. Two-stage reverse osmosis Industrial scale Technology available. Can increase recycled water to 60%. The energy cost is affordable with energy recovery systems. Chemical precipitation processes Well known technology. Not tested with seawater brine. Technology available. No studies done with seawater brine. Can increase recycled water from 60 to 90%. High reagent dosage that considerably increases process cost. Forward osmosis Pilot plant scale. Extensive research experience Simple technology. Recycles 76% of water when coupled with reverse osmosis systems. Precipitation of salts on membranes diminishes flow. More development needed in membrane technology. Low energy requirements as compared to other technologies. Electrodialysis Industrial scale Problems with precipitation on the electrodialysis membrane. Increase from 1.5 to 10% achieved in brine concentration. Electric energy use is 7-8 kWh/m3 of concentrated brine from RO. Comparative study of brine management technologies for desalination plants 33 Table 6. Comparison of methods. Part 2. Technology Development status Technical observations Economic observations SAL-PROC process Patented. Not tested with seawater brine Simple technology based on chemical precipitation reactions. Recovers commercial salts. Studies show it is economically feasible for brackish inland waters. ZDD technology Patented for seawater All the processes included in the technology are currently available. Studies based on mathematical models and tests assert 76 to 100% water recovery . Studies indicate it is economically feasible. Recovers commercial salts. Integrated processes Pilot plant scale for seawater Combine various available technologies. 90% water recovery. Salt production of 23.7 kg/m3 from desalinated water. Asserts production of desalinated water with costs between 0.43 and 0.71 USD/m3. Integrated system for recovery of CaCO3, NaCl and MgSO 4 •7H 2 O Pilot plant scale for seawater Available technology based on nanofiltration combined with precipitation and crystallization. Up to 95% water recovery. 78% of dissolved NaCl recovered. Production of 8.4 kg MgSO4·7H2O per m3 of reject from the nanofiltration unit. Brine conditioning for the chlor-alkali industry Industrial scale Simple, available technology. Need to concentrate and eliminate divalent ions. 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