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Sustainable RO desalination – Energy demand and environmental impact
Desalination 424 (2017) 10–16
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Engineering advance
Sustainable RO desalination – Energy demand and environmental impact Hilla Shemer, Raphael Semiat
MARK
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Rabin Desalination Laboratory, Wolfson Faculty of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords: Reverse osmosis Brine discharge, energy consumption, cost Marine environment Ecosystem
The expedient solution to water scarcity worldwide is desalinization. Nevertheless, common misconceptions of high cost, energy intensiveness and negative ecological footprint hinder global implementation. The objective of this paper is to refute some unsubstantiated claims regarding the energy demand and environmental impacts of reverse osmosis desalination. Energy consumption of RO desalination constitutes only a small fraction of a national cumulative energy demand. Meanwhile significant cost reductions of desalinated water are not expected in the near future. To date, worldwide chemical and biological monitoring programs show that brine discharge from desalination plants have localized minimal impacts on the marine environment. Properly sited, designed and operated RO desalination plants contribute to reduced energy demand and environmental footprint.
1. Introduction Water is the cheapest natural resource on earth yet, its price varies significantly worldwide. Table 1 summarizes the Rickards Real Cost Water Index™. This index is calculated using an algorithm that can be expressed in the following simplified form: (energy costs + operating expenses + capital expense + interest expense) / volume of water supplied [1]. In some places in the world tap water is free of charge due to historical reasons, religious believes or just because it is of abundance. On the other hand, the cost of 1.5 L bottled drinking water is very high, ranging from 0.3 to 3.3 US$ [2]. The costs of agricultural water (i.e., irrigation water) vary substantially with geographic location, water sources, and institutional arrangements. Water scarcity is among the main problems encountered by many societies. Two thirds of the world's population currently live in areas that experience water scarcity for at least one month a year. About 500 million people live in areas where water consumption exceeds the locally renewable water resources by a factor of two. Water shortage results from climate changes (causing spatial and temporal variations of water cycle dynamics), accelerated urbanization, increase in population and life quality, and increased demand by industry and energy production [4]. Additionally, water conflicts occurred throughout history and are still occurring now days. Water quality worldwide deteriorate due to discharge of untreated domestic and industrial wastewater, agricultural runoff and release of greenhouse gases, by polluting surface and ground water with
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nutrients, pesticides, synthetic organics, NOx and SO2 as well dissolution of naturally occurring environmental pollutants. The need to maintain clean water resources to supply all essentials is crucial. In order to do so new modern high quality water supply, which are able to accommodate the growing demand, should be prioritized. Industrially made water consume energy, require special equipment, financial expenses and trained worker. It is important on one hand, to find the best low cost and sustained solutions and on the other hand, to educate and regulate saving, smart use and minimize pollution. Water may be generated from non-conventional resources including: (i) recovery of urban wastewater for irrigation or industry use, (ii) indirect potable reuse i.e., treated effluent is discharged into groundwater or surface water, after treatment it is supplied as drinking water; (iii) desalination techniques at which water is extracted from seawater (SW) or brackish water. Desalination techniques consist of membrane separation processes such as reverse osmosis (RO) and electrodialysis (ED) or thermal processes such as multi stage flash (MSF) and multi-effect distillation (MED). At present, RO and MSF are the prevailing techniques for sea and brackish water desalination, as shown in. Fig. 1 [5]. Cost breakdown for a typical seawater RO desalination plants, including capital expenses (CAPEX) and operational and maintenance (O & M) costs, can be found in Cohen et al., 2017 [6]. Overall, the cost vary depending on plant location, plant size, feed water quality, and local electrical energy cost [6]. Public awareness as well as the scientific community raise concerns over the potential adverse effects of desalination. The objective of this
Corresponding author. E-mail address: [email protected] (R. Semiat).
http://dx.doi.org/10.1016/j.desal.2017.09.021 Received 10 August 2017; Received in revised form 19 September 2017; Accepted 19 September 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.
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consumption in the range of 3–7 kWh/m3 [12]. There are several approaches aimed to reduce the energy consumption of RO processes. These include: (i) improvements of membrane technology by developing highly permeable membranes and/or low fouling composites and by increasing the lifespan of the membrane [13–16]. (ii) Energy recovery devices (reducing the total power consumed by high pressure pumps) [17,18]. (iii) Efficient high pressure pumps (to reduce electrical power consumption) [19–21]. (iv) Optimization of the RO process (operating conditions and configuration) [13]. (v) Intermediate chemical de-mineralization to obtain higher water recovery (brackish and wastewater) [22], and (vi) use of renewable energy resources. With regard to the latter, coupling renewable energy sources (solar, wind, waves and geothermal) and desalination processes is a mean to reduce the carbon footprint of the water production process. Combining renewable technologies with desalination processes face technical challenges such as energy storage and availability of low-cost renewable energy sources. Additionally, desalination process requires a constant energy supply. Therefore, most often the produced renewable power is added to the electricity grid to overcome the intermittence of the renewable energy and to allow straitening the electricity daily sine wave consumption. Otherwise, the cost of the water produced will be much higher. More information may be found in [9,23,24]. It should be realized, that none of the above approaches could lead to energy savings of 50–80%, as claimed by forward osmosis and humidification-dehumidification experts. As example for an energy-efficient large-scale SWRO desalination plant is Tuaspring plant, Singapore. It is equipped with a self-sufficient on-site power plant, which enable significant capital and operating cost advantages by only using one intake and one outfall and associated pumps for both plants. An open seawater intake, with two separate inlet channels, is used to pump the feed water from the straits of Johor. The feed seawater salinity range between 28.5 and 32.0 g/L [25]. The lower than average salinity feed water are firstly used as cooling water in power plant. As a result, the temperature of the water, which is then fed directly into the desalination plant, rises resulting in less energy consumption by the RO pumps [26]. The intake is located about 50 m from the plant and the brine discharge about 150 m downstream as shown in Fig. 2. This configuration also contributes to the lower than average pumping energy. All of the above bring this plant to energy consumption of below 2 kWh/m3. To get prospective, today, ~ 2000 kW/year is needed to desalinate seawater to supply water for one household. This is less than that used by a household's refrigerator [27]. Pumping a cubic meter of fresh water for > 200 km requires more energy than desalinating the same amount of seawater.
Table 1 Global water cost indices [3]. Location
Water cost index (US$)
Global London Manila, Philippines Sao Paulo Singapore Uganda
1.39 2.16 0.35 1.10 1.66 1.49
Fig. 1. Total worldwide installed capacity (85.6 Mm3/d) by feed water type [5].
paper is to present misconceptions about the energy demand and environmental impacts of reverse osmosis desalination. 2. Energy consumption 2.1. Minimum work or heat energy demand The absolute minimum energy necessary for removal of cubic meter of fresh water from very large quantity of seawater at 20 °C, as calculated based on the second law of thermodynamics, is 0.79 kWh/m3 regardless of the separation technique used. For most brackish water, the minimum energy requirement is much less. For a process of 50% recovery ratio, at an initial 3.5% NaCl solution, the minimum energy increase to about 1.1 kWh/m3 [7]. Evaluation of separation processes such as desalination, based on the second law of thermodynamics, is called exergy analysis. It provides an assessment of the maximum work that can be extracted from a certain system relative to the surrounding environment i.e., it identify sources of the inefficiency. Therefore, it may be used to improve/optimize the process [8]. The simplest desalination technique of a singlestage evaporator requires approximately 650 kWh of thermal heat per one cubic meter of seawater, depending slightly on the evaporation temperature [9]. Additional energy is needed to condense the vapor by pumping cooling water. Modern evaporation techniques require much less energy, not much higher than RO, as discussed in details previously [9].
2.3. RO as part of the national energy consumption-Israel as a case study The main issue that should be considered is the existent of water need and the national energy consumption. In Israel, for example, the water scarcity was solved by construction of five SWRO desalination plants with total production capacity of 600 Mm3/year, accounting for about 80% of domestic water consumption and approximately 40% of the total water consumption. Multiplying this number by the typical energy consumption of SWRO desalination (3.5 kWh/m3) and divide it by the cumulative national energy consumption, it can be seen that the energy used for desalination is < 1.3% of the Israeli national energy consumption. In fact, in Israel, about 60% of the desalinated water is produced during the night (off-peak time) using electricity that would been otherwise wasted. Prior to the desalination era, about 3% of the cumulative national energy consumption was used to pump water from the north to the south of Israel. Hence, it can be concluded the long distance transportation of water is more energy intensive than desalination. Taking in account all points raised above, the national energy consumption is in fact slightly negative in similar cases and add no
2.2. Energy consumption of RO desalination Common energy consumption of SWRO desalination plant is of the order of 3.5 kWh/m3 [10,11]. The range of energy demand for the RO process itself, depending on the feed water salinity, the recovery ratio, the efficiency of the pumps and the efficiency of the energy recovery system, range between 1.7 and 2.5 kWh/m3. Additional costs such as pumping, pre-treatment, brine discharge, and electric power used within the plant total to 0.3–1.5 kWh/m3. Therefore, the overall energy consumption is 2.0–4.0 kWh/m3. Smaller installations, remote locations, inexperience in design and/or operation may increase the energy 11
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Fig. 2. Feed intake point (A) and brine discharge location (B), Tuaspring, desalination plant, Singapore. (Images from the roof of the power plant by R. Semiat.)
extra environmental load. The pollutants released to the atmosphere from regular power stations are not changed if the total national energy is not increased. It is important to note that water production is inexpensive technique for energy storage. For example, pumping water (potable or seawater) up to a mountain during the off-peak time and generate energy as it flows down, providing low evaporation rate. Additional product water can be stored in aquifers and recovered when demand is higher, thereby increasing overall system efficiency and reducing cost [30].
Table 2 Desalinated water costs for medium and large size plants [27]. Cost of
Year 2016
Expected cost in 5 years
Water (US$/m3) Construction (US$/MLD) Energy consumption (kWh/m3)
0.5–1.2 1.2–2.2 2.5–4.0
0.5–1.0 1.0–1.8 2.8–3.2
Environmental concerns were a key issue for the community during planning and construction of desalination plants in Australia and California, USA, resulting in. extra costs. These costs were passed on to water consumers [28]. For example, higher price of desalinated water in Australia are attributed to urban desalination plants operating with offsetting green energy from renewable sources (i.e., wind); and an expensive tunneled intakes and outfalls design mandatory in all but the first major desalination plant [28,29]. In Carlsbad, California desalination plant, an expensive intake structure, consisting of a pump station and a wet well tied-in to the power plant discharge channel was constructed to avoid impingement and entrainment effects.
3. Costing 3.1. Seawater desalination The cost of desalination is not only depends on the technology, it is also site specific. Energy contributes much of the total cost of seawater desalination other constitutes are CAPEX and O & M (Fig. 3). Specifically, production cost of desalinated water is optimized based on the cost of land, electric energy for pumping, cost of pre- and post-treatments, equipment, brine disposal, labor, maintenance (including membranes replacement) and financial charges [6]. Currently, the cheapest cost of SWRO water is 0.5 US$/m3 while for the same design and similar equipment, the cost may be as high as by a factor of 2.5 (Table 2). The RO technique will probably prevailing the next few years, especially due to the current low cost of energy and the technological changes of the last years. However, no significant cost reductions are expected in the near future, as seen in Table 2.
3.2. Brackish water desalination Compared to seawater desalination, brackish water desalination requires less energy in the range of 0.5–2.5 kWh/m3 [30]. This is due to the lower salinity, which enable to apply lower pressure and to obtain much higher water recovery. Brackish water desalination is consolidate alternative to seawater desalination in semiarid inland areas. Yet, brine management is considered one major challenges of inland desalination, as disposal to the sea is not possible. Conventional methods for inland brine management include: (i) brine disposal such as surface water discharge, domestic sewer disposal, deep well injections, and evaporation ponds; (ii) irrigation; and (iii) brine treatment, zero-liquid-discharge (ZLD) techniques i.e., thermally driven evaporative and crystallization systems. Near ZLD technologies are available now, yet have not taken their role in brackish water desalination plants. They are capable of reducing RO brine discharge. Cost and energy consumption are the main barriers to ZLD implementation in inland desalination applications [31]. An economic study comparing between brine disposal to evaporating pond and brine concentrator combined with crystallizer or evaporation pond revealed that the land cost (1.4 km2) associated with disposal through evaporation ponds is very high at 2.98 US$/m3. Brine concentrator coupled with a crystallizer or evaporation ponds was evaluated at 1.10 and 0.82 US$/m3 respectively [32]. 3.3. Produced water desalination
Fig. 3. Cost breakdown of seawater desalination [27].
Produced water (PW) is the largest waste steam generated by oil and 12
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Table 3 Produced water range of management costs [34]. Constituent
Cost (US$/bbl)
Transportation Water sourcing Disposal Treatment
0.5–8.0 0.25–1.75 0.07–1.60 0.20–8.50
gas production. It is characterized by high salts concentration, hydrocarbons, solids, oils, gases and scaling ions. Reuse of produced water faces similar challenges to other types of wastewater. These challenges include high treatment cost, potential chronic toxicity, and public acceptance for the different applications. Additionally, the amount and properties of the produced water change over time, making customized treatment solutions a necessity [33]. Management costs in the oil and gas industry (i.e., transportation, water sourcing, treatment, and disposal) are highly variable as displayed Table 3 [34]. Application of RO membranes for treating PW is hindered by membrane scaling limiting the water recovery to 30–60% [35], fouling by hydrocarbons adsorption on the membrane surface, and damage of the polysulfone support layer of the membrane by aromatic hydrocarbons [36]. The costs of RO desalination of PW range from only 2 US$/m3 to as high as 25 US$/m3. This wide range is attributed to sharp increase in operation and maintenance costs with augmentation of the feed water TDS as well as the economies of scale formation [37]. 3.4. Developing countries Lack of safe and unreliable drinking water is a globally recognized problem in developing countries. Desalination is considered a vital option yet cost is a major obstacle in its implementation. While nongovernmental organizations can provide seed funding they are less capable of covering the running costs and the population is unable to pay the production costs. Real questions one should ask are what is the cost of: (i) Fetching water in rural areas; millions of women and girls spend hours every day walking to water sources, waiting in line and carrying heavy loads – often several times a day. The average distance women walk to fetch water in Asia and Africa is 6 km/d. UNICEF estimated that women spent 16 million hours collecting water each day in 25 countries in subSaharan Africa. This responsibility represent lost opportunities for women's employment and education [38]; (ii) Tankering drinking water, these operations are expensive and relatively time-consuming to administer; (iii) Losing water through system infrastructure leaks. Generally, leaks are thought to make up 70% of the overall losses in water distribution systems [39]; (iv) value of human life lost due to scarcity of water or at water wars.
Fig. 4. SWRO desalination plants along the Israeli Mediterranean seashore (Haaretz 3-22017 in Hebrew).
monitoring related to the brine discharge. This enables to conduct before and after comparisons. Additionally, along a narrow strip of 80 km of the Mediterranean seashore (Fig. 4) five desalination plant (Table 4) are in full operation. Monitoring programs by the Israel Oceanographic and Limnological Research institute (IOLR) and the Israel Electric Corporation (IEC) were implemented shortly after the commissioning stage of each of these plants. The extensive monitoring of the water and sediment quality, benthic organisms and biological diversity, enables to assess the environmental impacts and their spatial extent. Sampling are conducted during the years, in the spring and fall. Monitoring of the water column (at the sea surface and near the seafloor) include the following parameters: temperature, salinity, total suspended solids, turbidity, pH, dissolved oxygen, biochemical oxygen demand, total organic carbon (TOC), nutrients, chlorophyll-a, heavy metals, and microalgae. Sediments analysis include: granulometry, TOC, heavy metals, infauna, and epifauna This is done in several locations around in the vicinity of the desalination plants [45]. Interesting is the case of Palmachim and Sorek RO desalination plants, located in south Israel. These plants production capacity is 90 and 150 Mm3/year of freshwater water, with brine discharge through two separate outfalls of 104 and 187 Mm3/year respectively. The distances between the intakes and outfalls of the two plants are short as displayed in Table 5. The outfalls are located at 1.98 and 1.85 km from the shore, for Palmachim and Sorek respectively, at water depth of 20 m. The outfalls specifications are listed in. (See Table 6.) Throughout the years of these two plants operation, no significant effects on the marine and sediments environment were monitored as compared to background measurements. Herein is a detailed description of the main findings reported by the IOLR [46]. Salinity variation was found to be within the range of the natural annual changes in coastal waters. The area with 5% higher salinity than the background was < 0.1 km2 and confined to the Sorek outfall in the spring and the
4. Environmental impacts Most of the published literature discuss the potential impacts of desalination plants on the marine and terrestrial environment [24,40–44]. However, limited field research is available, particularly on the long-term effects on the marine environment. In brief, the potential impacts include: (i) Construction stage, which share its terrestrial effects with any other land development projects; (ii) Impact on the marine environment-high salinity brine discharge, chemical disposal, entrainment and impingement of marine organisms from the intake of seawater; (iii) Air pollution and greenhouse gases emission. A comprehensive detailed analysis of the impacts of desalination on the marine environment is brought herein. Israel was chosen as a case study since regulatory requires monitoring of the marine environment baseline (before the SWRO plants operation) along with operational 13
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Table 4 Desalination plants located on the Israeli Mediterranean seashore. Plant
Water production date
Yearly production (Mm3/year)
Brine discharge
Ashkelon Palmachim Hadera Sorek Ashdod
Aug. 2005 May 2007 Dec. 2009 Nov. 2013 Oct. 2015
115 90 127 150 100
With power station cooling Outfall 1.98 km from shore With power station cooling Outfall 1.95 km from shore Outfall 2.20 km from shore
waters (20 m depth) waters (20 m depth) (22 m depth)
Table 5 Distances (in m) between intakes and outfalls of Palmachim and Sorek desalination plants.
Intake Outlet Intake Intake
Palmachim Palmachim 1 Sorek 2 Sorek
Outfall Palmachim
Intake 1 Sorek
Intake 2 Sorek
Outlet Sorek
679
685 1190
649 1177 50
712 829 584 608
Table 6 Palmachim and Sorek outfall diffusers specifications. Specifications
Palmachim
Sorek
No. pf ports Port inner diameter (m) Port spacing (m) Port elevation above the seabed (m) Discharge angle relative to horizontal (°) Outflow rate (m/s)
3 0.8 6 6 45 1.4
4 0.8 2.5 4 45 4.0 Fig. 6. Cooling water and brine discharge in Hadera. Distance between surface outfalls 80 m [47].
Palmachim outfall in the fall (Fig. 5). Higher temperature, by only up to 0.3 °C, compared to the background was measured near the bottom. Natural (below water quality criteria for the protection of marine life) of pH, turbidity, suspended particulate material, and nutrients (nitrate + nitrite, ammonium, total nitrogen, phosphate and silicic acid) were measured in both outfalls. The monitoring findings confirm that the brine discharges from the two desalination plants did not affect the chlorophyll and TOC concentrations. Total metal concentrations in the seawater (arsenic, cadmium, chromium, copper, iron, manganese, nickel, lead, selenium, vanadium, zinc, and mercury) were below or at the detection limit of the methods in all samples and lower than the seawater guidelines. Similarly, no deviations in dissolved oxygen was found from the Environmental Quality Standards.
Metal and organic carbon concentrations in the sediments were low and natural for the area and lower than the ERL criterion (i.e., low metal concentration causing biological effect in 10% of the cases). The characteristics of the faunal assemblage (number of individuals, number of taxa, species evenness and species diversity index, except perhaps the diversity index in the fall) as well as multivariate analysis of the assemblage, indicated no impact of the brine in the entire monitoring areas. Similar conclusions were reported with regard to the other three plants, which discharge brine with the cooling water of the power stations (Ashkelon, Ashdod and Hadera). Fig. 6 displays the surface
Fig. 5. Salinity distribution next to the seafloor measured in May (left) and September (right) 2015 [46].
14
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of Hormuz [51]. Time trend revealed an increased salinity in the Arabian Gulf, attributed to desalination, and a drop of > 0.2 pH predominantly attributed to CO2 sequestration in the Gulf's waters. Interestingly, the acidifying waters of the Gulf have utilized the elevated salinity to buffer the pH effect [52]. Studies, conducted in the Arabian Gulf, indicated that the levels of heavy metals measured in sediments were within the natural background levels [53]. Others have reported localized elevated concentrations of heavy metals (such as Cu, As, Cr, Mn, and Zr) in proximity to desalination plants along the coastline of the Arabian Gulf [54,55]. It should be mentioned that coastal and marine environments in the Arabian Gulf are under permanent threat from oil related pollution [50]. Field measurements, conducted by Naser (2013) [56], revealed localized severe impacts on macrobenthos at the proximity of brine discharge outlet of a desalination plants in Bahrain. Israel is located on the eastern boundary of the Mediterranean Sea (a mid-latitude semi-enclosed sea with an area of 2.5 × 106 km2). In this region, the background current circulates counterclockwise and parallel to the coastline (south to north along Israel's coast) with a mean velocity of about 15 to 25 cm/s. Its activity is observed mainly way offshore, in the region where the water depth is 20 m or more. Most of the time (90%), the speed of the near-surface current does not exceed 15–35 cm/s. The largest near-surface and near-bottom speeds recorded occur during winter [57]. It is evident, that the currents regimes of the Arabian Gulf and the eastern Mediterranean Sea are different suggesting better mixing conditions in the latter, which might assists with dispersal of the desalination plants brine. Yet, in both these geographic areas, the marine environment effect is localized in nature.
Table 7 Salinity change near the seafloor during 2013–2015 [47]. Year
Season
Max. salinity addition (%)
Area with salinity > 5% (km2)
Area with salinity > 2.5% (km2)
2013
Spring Autumn Spring Autumn Spring Autumn
3.6 6.4 5.5 3.6 6.5 3.6
0.00 0.06 0.03 0.00 0.10 0.00
0.1 1.3a 0.3 0.1 1.9a 0.2
2014 2015
a
Four units of the power station operated.
outfall of the cooling water and brine in Hadera [47]. Generally, the field monitoring indicated that the brine of the SWRO plants did not have significant adverse impacts on the recipient marine environments [48]. For example, in Hadera, the area affected by salinity changes is restricted to 400–700 m from the outfall with only small differences measured between the affected area and the background salinities of one salinity unit (2.6% salinity change). Areas of 0.1–0.2 km2 near the seafloor exhibited salinity increased by more than one unit Table 7. Exceptions were observed once the power plant operation was reduced to four units. In spring of 2015 the total area where the salinity > 2.5% 1.9 km2 at distance of 1400 m of the cooling water outlets [47]. The elevated temperatures in the receiving marine areas are due to the temperature of the power plants cooling water (ΔCCooling water = 5–11 °C) while the SWRO brine temperature is ambient. The elevated temperatures and saline brine dispersing is depending on the velocity and direction of the wind as well as on hydrogeological factors characteristic of the site [24]. An example of the temperature changes is given for the Hadera desalination plant where, the sampling points most affected by temperature are located 120 to 1320 m from the outfalls exhibiting Δ TSurface of 3.8–5.4 °C. Next to the bottom the two most affected points are located 120 and 600 m from the outfall with Δ TBottom of 4.6–6.3 °C [47]. Phytoplankton biomass has not changed significantly in any of the areas. Changes in the benthic infaunal communities were confined to very small areas of < 0.2 km2 near the outfalls. It is highly unlikely that such changes have significant ecosystem impacts. Coagulant (i.e., ferric chloride) did not accumulate in the sediments indicating that sessile organisms are not exposed to smothering and sediment feeders are not at risk of ingesting foreign materials. Heavy metals concentration, in both the water column and the sediments, were much lower than regulatory levels and thus, do not threat marine life or benthic organisms [45]. To date, the Israeli experience, of > 12 years, revealed that by choosing proper sites and discharge methods, large SWRO plants brine discharge can be environmentally safe [48]. A similar conclusion was obtained for the Australia's Gold Coast Desalination Plant, based on environmental monitoring with particular focus on the marine intake and brine discharge. It was shown that the plant's features and operational procedures ensured minimal impact on the marine environment [49].
5. Concluding remarks Misconceptions of desalination energy demand, environmental impacts and overall cost hampers implementation of RO plants worldwide. The energy used for desalination constitutes only a small fraction of a cumulative national energy consumption, with the advantageous option of partially production during the off-peak time. The cost of desalination is site specific. Currently, the cheapest cost of SWRO water is 0.5 US$/m3 while for the same design and similar equipment, the cost may be as high as by a factor of 2.5. Monitoring programs showed that proper sites selection and brine discharge methods of SWRO plants can be environmentally safe. References [1] P. Williams, S. Rickards, Waterfund/IBM the true cost of water, International Conference on Sustainable Infrastructure (ICSI), Long Beach, California, USA, 6–8 Nov 2014. [2] https://tvaraj.com/2013/11/11/price-of-bottled-potable-water-around-the-world , Accessed date: July 2017. [3] http://worldswaterfund.com/water-cost-index-overview.html , Accessed date: July 2017. [4] The United Nations World Water Development Report, Wastewater the untapped resource, http://unesdoc.unesco.org/images/0024/002471/247153e.pdf, (2017) , Accessed date: July 2017. [5] V. Pankratz, F. Gasson TJ., IDA Desalination Yearbook 2015–2016, Oxford, UK, Global Water Intell, 2016. [6] Y. Cohen, R. Semiat, A. Rahardianto, A perspective on reverse osmosis water desalination: quest for sustainability, AIChE J 63 (2017) 1771–1784. [7] Report of the Desalination Research Conference, Convened by the National Academy of Sciences, National Research Council: Desalination Research and Water Problem, (1962) (Library of Congress Catalog number 61-64602). [8] R.S. El-Emam, I. Dincer, Thermodynamic and thermoeconomic analyses of seawater reverse osmosis desalination plant with energy recovery, Energy 64 (2014) 154–163. [9] R. Semiat, Energy demands in desalination processes, Environ. Sci. Technol. 42 (2008) 8193–8201. [10] Desalination project costs, in: R. Semiat, M. Chapman, P. Price, D. Hasson (Eds.), Proceedings of International Conference on Desalination Costing, Limassol, Cyprus, The middle East Desalination Research Center, Muscat, Oman, 2004.
4.1. Arabian Gulf and Eastern Mediterranean Sea The Arabian Gulf countries produce about 60% of the global desalinated water. This Gulf is a marginal and semi-enclosed sea (2.4 × 105 km2) characterized by extreme environmental conditions due to geographic position, relative shallowness and high evaporation rates. Despite these harsh environmental conditions, the Arabian Gulf supports a range of coastal and marine ecosystems such as mangrove swamps, seagrass beds, coral reefs, and mud and sand flats [50]. Weak circulation and extremely low freshwater input result in surface currents of typical speeds of 10–20 cm/s while the bottom flow attain typical speeds of 5–10 cm/s, but magnifies to 20–30 cm/s past the Strait 15
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[11] A. Tenne, Sea Water Desalination in Israel: Planning, Coping With Difficulties, and Economic Aspects of Long-term Risks, Israel Water Authority, Desalination Division, 2010. [12] A. Alkaisi, R. Mossad, A. Sharifian-Barforoush, A review of the water desalination systems integrated with renewable energy, Energy Procedia 110 (2017) 268–274. [13] M. Li, Reducing specific energy consumption in Reverse Osmosis (RO) water desalination: an analysis from first principles, Desalination 276 (2011) 128–135. [14] A. Zhu, P.D. Christofides, Y. Cohen, On RO membrane and energy costs and associated incentives for future enhancements of membrane permeability, J. Membr. Sci. 344 (2009) 1–5. [15] L. Song, J.Y. Hu, S.L. Ong, W.J. Ng, M. Elimelech, M. Wilf, Emergence of thermodynamic restriction and its implications for full-scale reverse osmosis processes, Desalination 155 (2003) 213–228. [16] M. Wilf, S. Alt, Application of low fouling RO membrane elements for reclamation of municipal wastewater, Desalination 132 (2000) 11–19. [17] T.H. Chong, S.L. Loo, A.G. Fane, W.B. Krantz, Energy-efficient reverse osmosis desalination: effect of retentate recycle and pump and energy recovery device efficiencies, Desalination 366 (2015) 15–31. [18] B. Penate, L. Garcia-Rodriguez, Energy optimization of existing SWRO (seawater reverse osmosis) plants with ERT (energy recovery turbines): technical and thermoeconomic assessment, Energy 36 (2011) 613–626. [19] V.G. Gude, Energy consumption and recovery in reverse osmosis, Desalin. Water Treat. 36 (2011) 239–260. [20] C. Fritzmann, J. Lowenberg, T. Wintgens, T. Melin, State-of-the-art of reverse osmosis desalination, Desalination 216 (2007) 1–76. [21] N.M. Eshoul, B. Agnew, A. Anderson, M.S. Atab, Exergetic and economic analysis of two-pass RO desalination proposed plant for domestic water and irrigation, Energy 122 (2017) 319–328. [22] C.J. Gabelich, A. Rahardianto, R. Northrup, T.I. Yun, Y. Cohen, Process evaluation of intermediate chemical demineralization for water recovery enhancement in production-scale brackish water desalting, Desalination 272 (2011) 36–45. [23] R. Semiat, J. Sapoznik, D. Hasson, Energy aspects in osmotic processes, Desalin. Water Treat. 15 (2010) 228–235. [24] S. Miller, H. Shemer, R. Semiat, Energy and environmental issues in desalination, Desalination 366 (2015) 2–8. [25] P.K.L. Ng, N. Sivasothi (Eds.), A Guide to Mangroves of Singapore 1: The Ecosystem and Plant Diversity, Singapore Science Centre, 2002. [26] C. Hurn, T. Hagedorn, Tuaspring Sea Water Desalination with CCPP in Singapore: An Example for Sustainable Power Generation, PowerGen Asia Bangkok, 3–5 Oct 2012. [27] N. Voutchkov, Desalination – past, present and future, The IDA International Conference on Water Reuse and Recycling: Turning Vision into Reality, Nice, France, 25–27 Sept 2016. [28] B. Spies, G. Dandy, Sustainable Water Management: Securing Australia's future in a Green Economy, Australian Academy of Technological Sciences and Engineering, 2012, http://www.atse.org.au/Documents/Publications/Reports/Water/ATSE %202012%20Sustainable%20Water%20Management%20REPORT.pdf. [29] P. Baudish, Design considerations for tunnelled seawater intakes, Chapter 2, in: T.M. Missimer, B. Jones, R.G. Maliva (Eds.), Intakes and Outfalls for Seawater Reverse-osmosis Desalination Facilities, Springer Internetional publishing, AG Switzerland, 2015. [30] N. Ghaffour, T.M. Missimer, G.L. Amy, Technical review and evaluation of the economics of water desalination: current and future challenges for better water supply sustainability, Desalination 309 (2013) 197–207. [31] T. Tiezheng, M. Elimelech, The global rise of zero liquid discharge for wastewater management: drivers, technologies, and future directions, Environ. Sci. Technol. 50 (2016) 6846–6855. [32] B.D. Stanford, J.F. Leising, R.G. Bond, S.A. Snyder, Chapter 11 inland desalination: current practices, environmental implications, and case studies in Las Vegas, NV, Sustain. Sci. Eng. 2 (2010) 327–350. [33] T. Whalen, The challenges of reusing produced water, J. Pet. Technol. 64 (2012), http://dx.doi.org/10.2118/1112-0018-JPT.
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Engineering advance
Sustainable RO desalination – Energy demand and environmental impact Hilla Shemer, Raphael Semiat
MARK
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Rabin Desalination Laboratory, Wolfson Faculty of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords: Reverse osmosis Brine discharge, energy consumption, cost Marine environment Ecosystem
The expedient solution to water scarcity worldwide is desalinization. Nevertheless, common misconceptions of high cost, energy intensiveness and negative ecological footprint hinder global implementation. The objective of this paper is to refute some unsubstantiated claims regarding the energy demand and environmental impacts of reverse osmosis desalination. Energy consumption of RO desalination constitutes only a small fraction of a national cumulative energy demand. Meanwhile significant cost reductions of desalinated water are not expected in the near future. To date, worldwide chemical and biological monitoring programs show that brine discharge from desalination plants have localized minimal impacts on the marine environment. Properly sited, designed and operated RO desalination plants contribute to reduced energy demand and environmental footprint.
1. Introduction Water is the cheapest natural resource on earth yet, its price varies significantly worldwide. Table 1 summarizes the Rickards Real Cost Water Index™. This index is calculated using an algorithm that can be expressed in the following simplified form: (energy costs + operating expenses + capital expense + interest expense) / volume of water supplied [1]. In some places in the world tap water is free of charge due to historical reasons, religious believes or just because it is of abundance. On the other hand, the cost of 1.5 L bottled drinking water is very high, ranging from 0.3 to 3.3 US$ [2]. The costs of agricultural water (i.e., irrigation water) vary substantially with geographic location, water sources, and institutional arrangements. Water scarcity is among the main problems encountered by many societies. Two thirds of the world's population currently live in areas that experience water scarcity for at least one month a year. About 500 million people live in areas where water consumption exceeds the locally renewable water resources by a factor of two. Water shortage results from climate changes (causing spatial and temporal variations of water cycle dynamics), accelerated urbanization, increase in population and life quality, and increased demand by industry and energy production [4]. Additionally, water conflicts occurred throughout history and are still occurring now days. Water quality worldwide deteriorate due to discharge of untreated domestic and industrial wastewater, agricultural runoff and release of greenhouse gases, by polluting surface and ground water with
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nutrients, pesticides, synthetic organics, NOx and SO2 as well dissolution of naturally occurring environmental pollutants. The need to maintain clean water resources to supply all essentials is crucial. In order to do so new modern high quality water supply, which are able to accommodate the growing demand, should be prioritized. Industrially made water consume energy, require special equipment, financial expenses and trained worker. It is important on one hand, to find the best low cost and sustained solutions and on the other hand, to educate and regulate saving, smart use and minimize pollution. Water may be generated from non-conventional resources including: (i) recovery of urban wastewater for irrigation or industry use, (ii) indirect potable reuse i.e., treated effluent is discharged into groundwater or surface water, after treatment it is supplied as drinking water; (iii) desalination techniques at which water is extracted from seawater (SW) or brackish water. Desalination techniques consist of membrane separation processes such as reverse osmosis (RO) and electrodialysis (ED) or thermal processes such as multi stage flash (MSF) and multi-effect distillation (MED). At present, RO and MSF are the prevailing techniques for sea and brackish water desalination, as shown in. Fig. 1 [5]. Cost breakdown for a typical seawater RO desalination plants, including capital expenses (CAPEX) and operational and maintenance (O & M) costs, can be found in Cohen et al., 2017 [6]. Overall, the cost vary depending on plant location, plant size, feed water quality, and local electrical energy cost [6]. Public awareness as well as the scientific community raise concerns over the potential adverse effects of desalination. The objective of this
Corresponding author. E-mail address: [email protected] (R. Semiat).
http://dx.doi.org/10.1016/j.desal.2017.09.021 Received 10 August 2017; Received in revised form 19 September 2017; Accepted 19 September 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.
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consumption in the range of 3–7 kWh/m3 [12]. There are several approaches aimed to reduce the energy consumption of RO processes. These include: (i) improvements of membrane technology by developing highly permeable membranes and/or low fouling composites and by increasing the lifespan of the membrane [13–16]. (ii) Energy recovery devices (reducing the total power consumed by high pressure pumps) [17,18]. (iii) Efficient high pressure pumps (to reduce electrical power consumption) [19–21]. (iv) Optimization of the RO process (operating conditions and configuration) [13]. (v) Intermediate chemical de-mineralization to obtain higher water recovery (brackish and wastewater) [22], and (vi) use of renewable energy resources. With regard to the latter, coupling renewable energy sources (solar, wind, waves and geothermal) and desalination processes is a mean to reduce the carbon footprint of the water production process. Combining renewable technologies with desalination processes face technical challenges such as energy storage and availability of low-cost renewable energy sources. Additionally, desalination process requires a constant energy supply. Therefore, most often the produced renewable power is added to the electricity grid to overcome the intermittence of the renewable energy and to allow straitening the electricity daily sine wave consumption. Otherwise, the cost of the water produced will be much higher. More information may be found in [9,23,24]. It should be realized, that none of the above approaches could lead to energy savings of 50–80%, as claimed by forward osmosis and humidification-dehumidification experts. As example for an energy-efficient large-scale SWRO desalination plant is Tuaspring plant, Singapore. It is equipped with a self-sufficient on-site power plant, which enable significant capital and operating cost advantages by only using one intake and one outfall and associated pumps for both plants. An open seawater intake, with two separate inlet channels, is used to pump the feed water from the straits of Johor. The feed seawater salinity range between 28.5 and 32.0 g/L [25]. The lower than average salinity feed water are firstly used as cooling water in power plant. As a result, the temperature of the water, which is then fed directly into the desalination plant, rises resulting in less energy consumption by the RO pumps [26]. The intake is located about 50 m from the plant and the brine discharge about 150 m downstream as shown in Fig. 2. This configuration also contributes to the lower than average pumping energy. All of the above bring this plant to energy consumption of below 2 kWh/m3. To get prospective, today, ~ 2000 kW/year is needed to desalinate seawater to supply water for one household. This is less than that used by a household's refrigerator [27]. Pumping a cubic meter of fresh water for > 200 km requires more energy than desalinating the same amount of seawater.
Table 1 Global water cost indices [3]. Location
Water cost index (US$)
Global London Manila, Philippines Sao Paulo Singapore Uganda
1.39 2.16 0.35 1.10 1.66 1.49
Fig. 1. Total worldwide installed capacity (85.6 Mm3/d) by feed water type [5].
paper is to present misconceptions about the energy demand and environmental impacts of reverse osmosis desalination. 2. Energy consumption 2.1. Minimum work or heat energy demand The absolute minimum energy necessary for removal of cubic meter of fresh water from very large quantity of seawater at 20 °C, as calculated based on the second law of thermodynamics, is 0.79 kWh/m3 regardless of the separation technique used. For most brackish water, the minimum energy requirement is much less. For a process of 50% recovery ratio, at an initial 3.5% NaCl solution, the minimum energy increase to about 1.1 kWh/m3 [7]. Evaluation of separation processes such as desalination, based on the second law of thermodynamics, is called exergy analysis. It provides an assessment of the maximum work that can be extracted from a certain system relative to the surrounding environment i.e., it identify sources of the inefficiency. Therefore, it may be used to improve/optimize the process [8]. The simplest desalination technique of a singlestage evaporator requires approximately 650 kWh of thermal heat per one cubic meter of seawater, depending slightly on the evaporation temperature [9]. Additional energy is needed to condense the vapor by pumping cooling water. Modern evaporation techniques require much less energy, not much higher than RO, as discussed in details previously [9].
2.3. RO as part of the national energy consumption-Israel as a case study The main issue that should be considered is the existent of water need and the national energy consumption. In Israel, for example, the water scarcity was solved by construction of five SWRO desalination plants with total production capacity of 600 Mm3/year, accounting for about 80% of domestic water consumption and approximately 40% of the total water consumption. Multiplying this number by the typical energy consumption of SWRO desalination (3.5 kWh/m3) and divide it by the cumulative national energy consumption, it can be seen that the energy used for desalination is < 1.3% of the Israeli national energy consumption. In fact, in Israel, about 60% of the desalinated water is produced during the night (off-peak time) using electricity that would been otherwise wasted. Prior to the desalination era, about 3% of the cumulative national energy consumption was used to pump water from the north to the south of Israel. Hence, it can be concluded the long distance transportation of water is more energy intensive than desalination. Taking in account all points raised above, the national energy consumption is in fact slightly negative in similar cases and add no
2.2. Energy consumption of RO desalination Common energy consumption of SWRO desalination plant is of the order of 3.5 kWh/m3 [10,11]. The range of energy demand for the RO process itself, depending on the feed water salinity, the recovery ratio, the efficiency of the pumps and the efficiency of the energy recovery system, range between 1.7 and 2.5 kWh/m3. Additional costs such as pumping, pre-treatment, brine discharge, and electric power used within the plant total to 0.3–1.5 kWh/m3. Therefore, the overall energy consumption is 2.0–4.0 kWh/m3. Smaller installations, remote locations, inexperience in design and/or operation may increase the energy 11
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Fig. 2. Feed intake point (A) and brine discharge location (B), Tuaspring, desalination plant, Singapore. (Images from the roof of the power plant by R. Semiat.)
extra environmental load. The pollutants released to the atmosphere from regular power stations are not changed if the total national energy is not increased. It is important to note that water production is inexpensive technique for energy storage. For example, pumping water (potable or seawater) up to a mountain during the off-peak time and generate energy as it flows down, providing low evaporation rate. Additional product water can be stored in aquifers and recovered when demand is higher, thereby increasing overall system efficiency and reducing cost [30].
Table 2 Desalinated water costs for medium and large size plants [27]. Cost of
Year 2016
Expected cost in 5 years
Water (US$/m3) Construction (US$/MLD) Energy consumption (kWh/m3)
0.5–1.2 1.2–2.2 2.5–4.0
0.5–1.0 1.0–1.8 2.8–3.2
Environmental concerns were a key issue for the community during planning and construction of desalination plants in Australia and California, USA, resulting in. extra costs. These costs were passed on to water consumers [28]. For example, higher price of desalinated water in Australia are attributed to urban desalination plants operating with offsetting green energy from renewable sources (i.e., wind); and an expensive tunneled intakes and outfalls design mandatory in all but the first major desalination plant [28,29]. In Carlsbad, California desalination plant, an expensive intake structure, consisting of a pump station and a wet well tied-in to the power plant discharge channel was constructed to avoid impingement and entrainment effects.
3. Costing 3.1. Seawater desalination The cost of desalination is not only depends on the technology, it is also site specific. Energy contributes much of the total cost of seawater desalination other constitutes are CAPEX and O & M (Fig. 3). Specifically, production cost of desalinated water is optimized based on the cost of land, electric energy for pumping, cost of pre- and post-treatments, equipment, brine disposal, labor, maintenance (including membranes replacement) and financial charges [6]. Currently, the cheapest cost of SWRO water is 0.5 US$/m3 while for the same design and similar equipment, the cost may be as high as by a factor of 2.5 (Table 2). The RO technique will probably prevailing the next few years, especially due to the current low cost of energy and the technological changes of the last years. However, no significant cost reductions are expected in the near future, as seen in Table 2.
3.2. Brackish water desalination Compared to seawater desalination, brackish water desalination requires less energy in the range of 0.5–2.5 kWh/m3 [30]. This is due to the lower salinity, which enable to apply lower pressure and to obtain much higher water recovery. Brackish water desalination is consolidate alternative to seawater desalination in semiarid inland areas. Yet, brine management is considered one major challenges of inland desalination, as disposal to the sea is not possible. Conventional methods for inland brine management include: (i) brine disposal such as surface water discharge, domestic sewer disposal, deep well injections, and evaporation ponds; (ii) irrigation; and (iii) brine treatment, zero-liquid-discharge (ZLD) techniques i.e., thermally driven evaporative and crystallization systems. Near ZLD technologies are available now, yet have not taken their role in brackish water desalination plants. They are capable of reducing RO brine discharge. Cost and energy consumption are the main barriers to ZLD implementation in inland desalination applications [31]. An economic study comparing between brine disposal to evaporating pond and brine concentrator combined with crystallizer or evaporation pond revealed that the land cost (1.4 km2) associated with disposal through evaporation ponds is very high at 2.98 US$/m3. Brine concentrator coupled with a crystallizer or evaporation ponds was evaluated at 1.10 and 0.82 US$/m3 respectively [32]. 3.3. Produced water desalination
Fig. 3. Cost breakdown of seawater desalination [27].
Produced water (PW) is the largest waste steam generated by oil and 12
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Table 3 Produced water range of management costs [34]. Constituent
Cost (US$/bbl)
Transportation Water sourcing Disposal Treatment
0.5–8.0 0.25–1.75 0.07–1.60 0.20–8.50
gas production. It is characterized by high salts concentration, hydrocarbons, solids, oils, gases and scaling ions. Reuse of produced water faces similar challenges to other types of wastewater. These challenges include high treatment cost, potential chronic toxicity, and public acceptance for the different applications. Additionally, the amount and properties of the produced water change over time, making customized treatment solutions a necessity [33]. Management costs in the oil and gas industry (i.e., transportation, water sourcing, treatment, and disposal) are highly variable as displayed Table 3 [34]. Application of RO membranes for treating PW is hindered by membrane scaling limiting the water recovery to 30–60% [35], fouling by hydrocarbons adsorption on the membrane surface, and damage of the polysulfone support layer of the membrane by aromatic hydrocarbons [36]. The costs of RO desalination of PW range from only 2 US$/m3 to as high as 25 US$/m3. This wide range is attributed to sharp increase in operation and maintenance costs with augmentation of the feed water TDS as well as the economies of scale formation [37]. 3.4. Developing countries Lack of safe and unreliable drinking water is a globally recognized problem in developing countries. Desalination is considered a vital option yet cost is a major obstacle in its implementation. While nongovernmental organizations can provide seed funding they are less capable of covering the running costs and the population is unable to pay the production costs. Real questions one should ask are what is the cost of: (i) Fetching water in rural areas; millions of women and girls spend hours every day walking to water sources, waiting in line and carrying heavy loads – often several times a day. The average distance women walk to fetch water in Asia and Africa is 6 km/d. UNICEF estimated that women spent 16 million hours collecting water each day in 25 countries in subSaharan Africa. This responsibility represent lost opportunities for women's employment and education [38]; (ii) Tankering drinking water, these operations are expensive and relatively time-consuming to administer; (iii) Losing water through system infrastructure leaks. Generally, leaks are thought to make up 70% of the overall losses in water distribution systems [39]; (iv) value of human life lost due to scarcity of water or at water wars.
Fig. 4. SWRO desalination plants along the Israeli Mediterranean seashore (Haaretz 3-22017 in Hebrew).
monitoring related to the brine discharge. This enables to conduct before and after comparisons. Additionally, along a narrow strip of 80 km of the Mediterranean seashore (Fig. 4) five desalination plant (Table 4) are in full operation. Monitoring programs by the Israel Oceanographic and Limnological Research institute (IOLR) and the Israel Electric Corporation (IEC) were implemented shortly after the commissioning stage of each of these plants. The extensive monitoring of the water and sediment quality, benthic organisms and biological diversity, enables to assess the environmental impacts and their spatial extent. Sampling are conducted during the years, in the spring and fall. Monitoring of the water column (at the sea surface and near the seafloor) include the following parameters: temperature, salinity, total suspended solids, turbidity, pH, dissolved oxygen, biochemical oxygen demand, total organic carbon (TOC), nutrients, chlorophyll-a, heavy metals, and microalgae. Sediments analysis include: granulometry, TOC, heavy metals, infauna, and epifauna This is done in several locations around in the vicinity of the desalination plants [45]. Interesting is the case of Palmachim and Sorek RO desalination plants, located in south Israel. These plants production capacity is 90 and 150 Mm3/year of freshwater water, with brine discharge through two separate outfalls of 104 and 187 Mm3/year respectively. The distances between the intakes and outfalls of the two plants are short as displayed in Table 5. The outfalls are located at 1.98 and 1.85 km from the shore, for Palmachim and Sorek respectively, at water depth of 20 m. The outfalls specifications are listed in. (See Table 6.) Throughout the years of these two plants operation, no significant effects on the marine and sediments environment were monitored as compared to background measurements. Herein is a detailed description of the main findings reported by the IOLR [46]. Salinity variation was found to be within the range of the natural annual changes in coastal waters. The area with 5% higher salinity than the background was < 0.1 km2 and confined to the Sorek outfall in the spring and the
4. Environmental impacts Most of the published literature discuss the potential impacts of desalination plants on the marine and terrestrial environment [24,40–44]. However, limited field research is available, particularly on the long-term effects on the marine environment. In brief, the potential impacts include: (i) Construction stage, which share its terrestrial effects with any other land development projects; (ii) Impact on the marine environment-high salinity brine discharge, chemical disposal, entrainment and impingement of marine organisms from the intake of seawater; (iii) Air pollution and greenhouse gases emission. A comprehensive detailed analysis of the impacts of desalination on the marine environment is brought herein. Israel was chosen as a case study since regulatory requires monitoring of the marine environment baseline (before the SWRO plants operation) along with operational 13
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Table 4 Desalination plants located on the Israeli Mediterranean seashore. Plant
Water production date
Yearly production (Mm3/year)
Brine discharge
Ashkelon Palmachim Hadera Sorek Ashdod
Aug. 2005 May 2007 Dec. 2009 Nov. 2013 Oct. 2015
115 90 127 150 100
With power station cooling Outfall 1.98 km from shore With power station cooling Outfall 1.95 km from shore Outfall 2.20 km from shore
waters (20 m depth) waters (20 m depth) (22 m depth)
Table 5 Distances (in m) between intakes and outfalls of Palmachim and Sorek desalination plants.
Intake Outlet Intake Intake
Palmachim Palmachim 1 Sorek 2 Sorek
Outfall Palmachim
Intake 1 Sorek
Intake 2 Sorek
Outlet Sorek
679
685 1190
649 1177 50
712 829 584 608
Table 6 Palmachim and Sorek outfall diffusers specifications. Specifications
Palmachim
Sorek
No. pf ports Port inner diameter (m) Port spacing (m) Port elevation above the seabed (m) Discharge angle relative to horizontal (°) Outflow rate (m/s)
3 0.8 6 6 45 1.4
4 0.8 2.5 4 45 4.0 Fig. 6. Cooling water and brine discharge in Hadera. Distance between surface outfalls 80 m [47].
Palmachim outfall in the fall (Fig. 5). Higher temperature, by only up to 0.3 °C, compared to the background was measured near the bottom. Natural (below water quality criteria for the protection of marine life) of pH, turbidity, suspended particulate material, and nutrients (nitrate + nitrite, ammonium, total nitrogen, phosphate and silicic acid) were measured in both outfalls. The monitoring findings confirm that the brine discharges from the two desalination plants did not affect the chlorophyll and TOC concentrations. Total metal concentrations in the seawater (arsenic, cadmium, chromium, copper, iron, manganese, nickel, lead, selenium, vanadium, zinc, and mercury) were below or at the detection limit of the methods in all samples and lower than the seawater guidelines. Similarly, no deviations in dissolved oxygen was found from the Environmental Quality Standards.
Metal and organic carbon concentrations in the sediments were low and natural for the area and lower than the ERL criterion (i.e., low metal concentration causing biological effect in 10% of the cases). The characteristics of the faunal assemblage (number of individuals, number of taxa, species evenness and species diversity index, except perhaps the diversity index in the fall) as well as multivariate analysis of the assemblage, indicated no impact of the brine in the entire monitoring areas. Similar conclusions were reported with regard to the other three plants, which discharge brine with the cooling water of the power stations (Ashkelon, Ashdod and Hadera). Fig. 6 displays the surface
Fig. 5. Salinity distribution next to the seafloor measured in May (left) and September (right) 2015 [46].
14
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of Hormuz [51]. Time trend revealed an increased salinity in the Arabian Gulf, attributed to desalination, and a drop of > 0.2 pH predominantly attributed to CO2 sequestration in the Gulf's waters. Interestingly, the acidifying waters of the Gulf have utilized the elevated salinity to buffer the pH effect [52]. Studies, conducted in the Arabian Gulf, indicated that the levels of heavy metals measured in sediments were within the natural background levels [53]. Others have reported localized elevated concentrations of heavy metals (such as Cu, As, Cr, Mn, and Zr) in proximity to desalination plants along the coastline of the Arabian Gulf [54,55]. It should be mentioned that coastal and marine environments in the Arabian Gulf are under permanent threat from oil related pollution [50]. Field measurements, conducted by Naser (2013) [56], revealed localized severe impacts on macrobenthos at the proximity of brine discharge outlet of a desalination plants in Bahrain. Israel is located on the eastern boundary of the Mediterranean Sea (a mid-latitude semi-enclosed sea with an area of 2.5 × 106 km2). In this region, the background current circulates counterclockwise and parallel to the coastline (south to north along Israel's coast) with a mean velocity of about 15 to 25 cm/s. Its activity is observed mainly way offshore, in the region where the water depth is 20 m or more. Most of the time (90%), the speed of the near-surface current does not exceed 15–35 cm/s. The largest near-surface and near-bottom speeds recorded occur during winter [57]. It is evident, that the currents regimes of the Arabian Gulf and the eastern Mediterranean Sea are different suggesting better mixing conditions in the latter, which might assists with dispersal of the desalination plants brine. Yet, in both these geographic areas, the marine environment effect is localized in nature.
Table 7 Salinity change near the seafloor during 2013–2015 [47]. Year
Season
Max. salinity addition (%)
Area with salinity > 5% (km2)
Area with salinity > 2.5% (km2)
2013
Spring Autumn Spring Autumn Spring Autumn
3.6 6.4 5.5 3.6 6.5 3.6
0.00 0.06 0.03 0.00 0.10 0.00
0.1 1.3a 0.3 0.1 1.9a 0.2
2014 2015
a
Four units of the power station operated.
outfall of the cooling water and brine in Hadera [47]. Generally, the field monitoring indicated that the brine of the SWRO plants did not have significant adverse impacts on the recipient marine environments [48]. For example, in Hadera, the area affected by salinity changes is restricted to 400–700 m from the outfall with only small differences measured between the affected area and the background salinities of one salinity unit (2.6% salinity change). Areas of 0.1–0.2 km2 near the seafloor exhibited salinity increased by more than one unit Table 7. Exceptions were observed once the power plant operation was reduced to four units. In spring of 2015 the total area where the salinity > 2.5% 1.9 km2 at distance of 1400 m of the cooling water outlets [47]. The elevated temperatures in the receiving marine areas are due to the temperature of the power plants cooling water (ΔCCooling water = 5–11 °C) while the SWRO brine temperature is ambient. The elevated temperatures and saline brine dispersing is depending on the velocity and direction of the wind as well as on hydrogeological factors characteristic of the site [24]. An example of the temperature changes is given for the Hadera desalination plant where, the sampling points most affected by temperature are located 120 to 1320 m from the outfalls exhibiting Δ TSurface of 3.8–5.4 °C. Next to the bottom the two most affected points are located 120 and 600 m from the outfall with Δ TBottom of 4.6–6.3 °C [47]. Phytoplankton biomass has not changed significantly in any of the areas. Changes in the benthic infaunal communities were confined to very small areas of < 0.2 km2 near the outfalls. It is highly unlikely that such changes have significant ecosystem impacts. Coagulant (i.e., ferric chloride) did not accumulate in the sediments indicating that sessile organisms are not exposed to smothering and sediment feeders are not at risk of ingesting foreign materials. Heavy metals concentration, in both the water column and the sediments, were much lower than regulatory levels and thus, do not threat marine life or benthic organisms [45]. To date, the Israeli experience, of > 12 years, revealed that by choosing proper sites and discharge methods, large SWRO plants brine discharge can be environmentally safe [48]. A similar conclusion was obtained for the Australia's Gold Coast Desalination Plant, based on environmental monitoring with particular focus on the marine intake and brine discharge. It was shown that the plant's features and operational procedures ensured minimal impact on the marine environment [49].
5. Concluding remarks Misconceptions of desalination energy demand, environmental impacts and overall cost hampers implementation of RO plants worldwide. The energy used for desalination constitutes only a small fraction of a cumulative national energy consumption, with the advantageous option of partially production during the off-peak time. The cost of desalination is site specific. Currently, the cheapest cost of SWRO water is 0.5 US$/m3 while for the same design and similar equipment, the cost may be as high as by a factor of 2.5. Monitoring programs showed that proper sites selection and brine discharge methods of SWRO plants can be environmentally safe. References [1] P. Williams, S. Rickards, Waterfund/IBM the true cost of water, International Conference on Sustainable Infrastructure (ICSI), Long Beach, California, USA, 6–8 Nov 2014. [2] https://tvaraj.com/2013/11/11/price-of-bottled-potable-water-around-the-world , Accessed date: July 2017. [3] http://worldswaterfund.com/water-cost-index-overview.html , Accessed date: July 2017. [4] The United Nations World Water Development Report, Wastewater the untapped resource, http://unesdoc.unesco.org/images/0024/002471/247153e.pdf, (2017) , Accessed date: July 2017. [5] V. Pankratz, F. Gasson TJ., IDA Desalination Yearbook 2015–2016, Oxford, UK, Global Water Intell, 2016. [6] Y. Cohen, R. Semiat, A. Rahardianto, A perspective on reverse osmosis water desalination: quest for sustainability, AIChE J 63 (2017) 1771–1784. [7] Report of the Desalination Research Conference, Convened by the National Academy of Sciences, National Research Council: Desalination Research and Water Problem, (1962) (Library of Congress Catalog number 61-64602). [8] R.S. El-Emam, I. Dincer, Thermodynamic and thermoeconomic analyses of seawater reverse osmosis desalination plant with energy recovery, Energy 64 (2014) 154–163. [9] R. Semiat, Energy demands in desalination processes, Environ. Sci. Technol. 42 (2008) 8193–8201. [10] Desalination project costs, in: R. Semiat, M. Chapman, P. Price, D. Hasson (Eds.), Proceedings of International Conference on Desalination Costing, Limassol, Cyprus, The middle East Desalination Research Center, Muscat, Oman, 2004.
4.1. Arabian Gulf and Eastern Mediterranean Sea The Arabian Gulf countries produce about 60% of the global desalinated water. This Gulf is a marginal and semi-enclosed sea (2.4 × 105 km2) characterized by extreme environmental conditions due to geographic position, relative shallowness and high evaporation rates. Despite these harsh environmental conditions, the Arabian Gulf supports a range of coastal and marine ecosystems such as mangrove swamps, seagrass beds, coral reefs, and mud and sand flats [50]. Weak circulation and extremely low freshwater input result in surface currents of typical speeds of 10–20 cm/s while the bottom flow attain typical speeds of 5–10 cm/s, but magnifies to 20–30 cm/s past the Strait 15
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