- Email: [email protected]
Case studies on environmental impact of seawater desalination
Desalination 185 (2005) 1–8
Case studies on environmental impact of seawater desalination J. Jaime Sadhwania*, Jose M. Vezaa, Carmelo Santanab a
University of Las Palmas de Gran Canaria, Department of Process Engineering, Campus de Tafira Baja, E-35017, Las Palmas de Gran Canaria, Spain Fax +34 928 458975; email: [email protected] b Consejo Insular de Aguas de Gran Canaria c Juan XXIII, no 7, E-35004, Las palmas de Gran Canaria, Spain Received 25 January 2005; accepted 21 February 2005
Abstract Water desalination processes have contributed to a better standard of living in a number of countries during the second half of the 20th century, following an increase in water demand for drinking purposes as well as industrial and agricultural uses. However, the technologies used in water desalination are also accompanied by adverse environmental effects. There are several effects to be considered in desalination plants, such as the use of the land, the groundwater, the marine environment, noise pollution and the use of energy, amongst others. To protect and preserve the environment, most countries turned to assess the environment impacts produced by desalination plants. Seawater desalination plants are located by the shoreline, to supply desalted water to the population of the main cities and other uses. The construction of both the desalination plants and all the required infrastructure in coastal areas affects the local environment. The impact on groundwater is due to the seawater pipes leaks which could contaminate the aquifers. The high salt concentration in the brine and several chemical products used in the desalination process are returned to the sea. Most impacts on the marine environment arise as a consequence of the brine discharge and their effects could be worse in the Mediterranean sea than in other areas. With respect to the noise pollution produced by the desalination plants, there is always an impact on the plant operators and also on the towns and villages nearby. One of the major indirect environmental impacts is the use of the energy required by desalination plants, particularly when electricity is produced by burning of oil, which in turn boosts the process of global warming. In this paper, we analyse the environmental problems of seawater reverse osmosis desalination plants, focusing on some case studies located in Canary Islands, and describing the major impacts identified. Environmental monitoring is done by the water and environmental authorities, based on regional regulations which turn out to be more restrictive than national legislation. Keywords: Environmental; Impact; Seawater; Reverse osmosis; Desalination plants.
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.02.072
2
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
1. Introduction There has never been so much discussion on the environmental impact of desalination plants in Spain as there is nowadays. It arises as a result of the last modified Water National Plan for supplying desalted water for human consumption, tourist uses, as well as agricultural and industrial consumption. Water desalination comprise a consolidated set of technologies in Spain, from the first desalination plant by MSF evaporation process installed in 1964 in Lanzarote (Canary Islands) to over 700 desalination plants existing nowadays, featuring different technologies, and with a total installed capacity production close to 8,00,000 m3/day. The large experience on brackish and seawater reverse osmosis desalination plants in Canary Islands is an important starting point to transfer knowledge about adverse environmental effects. The society at large in Canary Islands is concerned to preserve natural resources and sustainability, including the environmental impacts from seawater reverse osmosis desalination plants. 2. Environmental regulations Environmental impact assessment is a general and common technique used by industrialized countries, to preserve environment natural resources and to protect them. In Spain, environmental impact assessment was established 1986 as a basic regulation in environmental matters with Royal Decree 1302/1986 of 28th June, in agreement with European Directive 85/337, concerning the evaluation of the incidences of certain private and public projects. Later on further regulations were issued, such as including the Environmental Impact Assessment Decree (EIA), which includes a list of those activities where an environmental
impact assessment is mandatory. One of these activities is water desalination whenever new or additional capacities become larger than 3,000 m3/day. Nevertheless this EIA Decree is only the basic regulation for Spanish, since the different regions (Autonomous Communities) are entitled to produce their own procedures, including the capability to issue their own screening and scoping procedures (establishing the activities subject to environment impact assessment and the terms of reference for the assessments themselves). There are only three autonomous communities with specific regulations for EIA on desalination activity. Canary Island is one of them, with the regional Act 11/1990 (13th July) on Ecological Impact Assessment. Desalination plants over than 5,000 m3/day of water production are listed within the projects subject to carrying out an Ecological Impact Detailed Assessment. 3. Environmental impact of water desalination The water sources appropriate for desalination can have two basic origins basically: seawater and groundwater. Seawater or brackish water desalination in reverse osmosis plants may have several negatives aspects directly or indirectly on the environment: 3.1. The indirect impact on the environment due to the need to increase production electricity for desalination plants Energy consumption on desalination seawater reverse osmosis plants has improved with new technological advances in the process such as the energy recovery systems used. These desalination plants require an external supply electrical energy produced by thermal plants. To produce electricity it is necessary to burn fuels in a thermal plant,
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
which produces polluting flue gases (e.g. carbon dioxide) dispersed to atmosphere. The result of this process is global warming. In agreement with the Kyoto Protocol about the climate change, most of the industrialized countries must reduce gas discharges that can contribute to the global warming, 5% below 1990 level, for next period 2008– 2012. In Spain, thermal power stations produce an average 0.402 kg CO2/kWh, reference value established by the authorities. Most desalination plants in the next years need new electric infrastructure and, as a result, new thermal power stations increase the amount of pollution by gas emissions excepted if there was a replacement of conventional energies for renewable energies for the desalination industry. 3.2. Impact on the marine environment as a result of returning the concentrated brine to the sea In reverse osmosis desalination plants, the discharge brine flow rate is 30–70% of the feed water flow, which means 1.3–1.7 times the seawater concentration. In order to insure the proper dispersion of brine discharge and minimize their adverse effects on the marine environment, it is necessary to select an appropriate technology for this purpose. The main impact is due to the discharge of the concentrated brine to the sea, and its magnitude depends on environmental and hydro-geological factors which are characteristics of the sea: bathymetry, waves, currents, depth of the water column, etc. These factors would determine the extent of the mixing of the brines and therefore the geographical range of the impact [1]. Einav and Lockiev indicate that the extent of vulnerability of the marine environment to salinity differs from place to place. It is measured by the nature of the marine habitat:
3
coral reef, rocky beach or sandy surfaces and by the origin of the surrounding organism. Mathematics models are used for evaluating the range of marine environmental impact [8]. The CORMIX methodology emphasizes the role of boundary interaction on mixing brine and seawater and provide adverse effects from discharges environmental impacts [9]. Ho¨pner and Windelberg [2] divide the global marine habitats into 15 categories according to their sensitivities to the effects of desalination plants, shown in Table 1. In the Mediterranean coasts of Spain there are five kinds of angiosperms marine prairies, and the most abundant is the Posidonia oceanica endemicity. P. oceanica has experienced a remarkable regression in the last decades [10]; reason why at the moment is an species protected by law in the autonomous communities of Balearic Islands, Catalonia and Valencia, and is classified as a high-priority habitat by the European Union Directive [11]. In the Canary Islands, we have the presence of sea grass prairies on the sandy sea
Table 1 Sensitivities of marine habitats to desalination plants 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
High-energy oceanic coasts, rocky or sandy, with coast-parallel current Exposed rocky coast Mature shoreline (sediment mobility) Coastal upwelling High-energy soft tidal coast Estuaries and estuary-similar Low energy sand-, mud-, and beach rocks-flats Coastals sabkhas Fjords Shallow low-energy bay and semi-enclosed lagoon Algal (cyanobacterial) mats Seaweed bay and shallows Coral reefs Salt marsh Mangal (mangrove flats)
4
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
beds (generally associations of two species: Cymodocea nodosa and Caulerpa prolifera) of great biological importance. In Gran Canary island there are located in different areas, as in (Sardina del Norte) and Maspalomas at the south of the island. The grass prairies contribute to fix and to stabilize sandy deposits; also they can permit algae association communities development, Finally they are an important marine habitat to different fishes and invertebrate communities. 3.3. Impact on the marine environment as a result of different products used in chemical cleaning of membranes and pretreatment cleaning In addition to the high concentration of salts, brine discharges contain various chemicals used in the pretreatment stage of the desalination plant, including anti fouling materials. The chemicals used in pretreatment of seawater are mainly: Sodium hypochlorite (NaOCl) or free chlorine, used for chlorination, preventing biological growth Ferric chloride (FeCl3) used for the flocculation and removal of suspended matter from the water Sulphuric or hydrochloric acid used for pH adjustment Sodium hexametaphosphate and similar chemicals, to prevent scale formation on the pipes and on the membranes Sodium bisulphite (NaHSO3), used in order to neutralize any remains of chlorine in the feed water Cleaning pretreatment is conducted by seawater on sand filters or water product on cartridge filters. Organic matter is also accompanied in the discharge to the sea. Cleaning membranes takes place 3 or 4 times a year, and the chemicals products used are mainly weak acids and detergents
(citric acids, sodium polyphosphate and EDTA) and caustic alkali. Water cleaning chemical solution in membranes must be neutralized before being discharged to the sea. The chemical doses effects are not very important to for marine environment impact. 3.4. Impact of noise Acoustic contamination on seawater reverse osmosis desalination plants is important. High pressure pumps and energy recovery systems, such as turbines or similar, produce significant level of noise over 90 dB(A). Therefore, they should be located far away from populated areas and equipped with appropriate acoustic technology to reduce noise level. 3.5. Adverse effect on land use Desalination plants are located next to coastal areas, away from tourist developments and population. The area required for a seawater reverse osmosis desalination plants is about 10,000 m2 for a 5,000 or 10,000 m3/day product water. There is an important view adverse effect on the design of the architectural typology of buildings used to this kind of construction. Additionally, it is necessary improve infrastructure like electric energy transport to the plant, feed and product water pipes to transport, even brine concentrated discharge outfalls, etc. 3.6. Impact on the aquifer Seawater and brine pipes laid over the aquifer pose a danger to it as these pipes may leak and salt water may penetrate into the aquifer. One solution could be to use proper sealing techniques to minimize his impact on the aquifer.
5
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
4. Case studies We will now analyse the environmental impacts on some new and older seawater desalination reverse osmosis plants in Gran Canaria. Five cases are reviewed, providing brief descriptions of the Ecological Impact Detailed Assessment written for each plant. 4.1. Bocabarranco desalination plant This installation is located in the northwest (NW) of the island, at Bocabarranco beach, in Galdar. Technical specifications are shown in Table 2. Our installation is integrated into a larger desalination system on a 10,000 m2 plot, and all the infrastructure services are also use by the unit. There is also another seawater reverse osmosis desalination plant for agriculture purposes in a different building, and a waste water treatment plant nearby. Brine discharge is realized though two pipelines to the coastline, first one is double for the brine reject of potable water plant with 300 mm diameter FGRP and the second one is 400 mm diameter FGRP for agriculture use. Both pipelines discharge close to the beach. The solution taken consists of joining all discharges, even waste water, by a long outfall discharging into the sea [6]. Seawater and brine reject are shown in Table 3. The cleaning frequency of the sand filter in the pretreatment stage is about once a week and it is discharged into the brine pipe. Table 2 Technical specification in Bocabarranco SWRO desalination plant Capacity of production Recovery TDS water product Application
7,000 m3/day 45% 400 ppm Domestic consumption
Table 3 Chemical composition of seawater and brine reject in Bocabarranco Plant mg/l
Seawater
Brine
Calcium Magnesium Sodium Potassium Bicarbonate Chloride Sulphate Silicon TDS
450 1,520 11,415 450 250 20,800 3,110 5 38,000
814 2,751 20,657 814 452 37,639 5,628 9 68,764
Table 4 Chemical Doses used in Bocabarranco Plant Chemical doses
kg/m3
ppm
NaOCl H2SO4 NaHSO3 Antiscalant FeCl3 Calcium hypochloride
0.053 0.068 0.027 0.009 0.055 0.005
3.0 16.4 2.0 4.1 1.7 2.8
Membranes are cleaned 3–4 times a year and also discharged to the brine pipe. Finally chemical pretreatment used is shown in Table 4, based on water product. 4.2. Agricultural plant at Arucas This unit is located in the north of the island, by the shoreline, in a spot called Punta de Camello, at an altitude 33.27 m over sea, in Arucas. Technical specifications are shown in Table 5. This unit is close to another with a similar design, although the use of product water is different. New brine discharge is realized jointly with another existing discharge dated 1994, with a 400 mm diameter FGRP and is done directly to the coastline. There was one catalogued and endemic marine species, a red algae (Rissoella
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J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
Table 5 Technical specification in Arucas SWRO Desalination Plant 5,000 m3/day 45% 400 ppm Agriculture use
Capacity of production Recovery TDS water product Application
Table 7 Technical specification in Roque Prieto Desalination SW RO Plant Capacity of production Recovery TDS water product Application
5,000 m3/day 45% 400 ppm Potable water
Table 6 Chemical Doses used in Arucas Agriculture Plant Chemical doses
g/m3
kg/day
NaOCl H2SO4 NaHSO3 HSFM FeCl3 Calcium hydroxide
11.10 68.02 6.80 6.67 11.10 20
3.0 230 34 33.3 55.5 103
Verruculosa), discovered in this place and nowadays destroyed by previous impacts [7]. Analytical composition of seawater and brine reject are similar as those shown for the latter case, but chemical doses used for pretreatment and postreatment are as shown in Table 6. 4.3. Roque Prieto Plant This unit is again located in the NW of the island, in a spot called Roque Prieto, Guı´ a, not far from Bocabarranco Plant. Technical specifications are shown in Table 7. This installation is adjacent to another 1,500 m3/day product water evaporation plant. Both plants have similar type of building construction. The feed water intake is done through beach wells and the brine discharge from the RO unit is done jointly with the vapour compression plant reject. The discharge of the brine is directly at the coastline, as you can show in the next photo (Figure 1).
Fig. 1. Brine discharge point at Roque Prieto Plant.
Brine discharge reverse osmosis is diluted with water reject from evaporation plant to reach about 41,000 ppm TDS.[8] With respect to the analytical composition of seawater and brine reject are similar than the rest of studied cases. 4.4. La Aldea Plant This installation is located in the west of the island, La Aldea, in San Nicola´s de Tolentino, with 5,500 m2 plot area. There is also a waste water treatment nearby. The main use of water desalted in this area is agriculture. The total capacity of the plant is 10,000 m3/day with a 45% recovery. It was initially designed for 5,000 m3/day produced with TDS lower than 400 mg/L.
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
The plant design is similar to the other studied cases, with beach-well intake, chemical and physical pretreatment, formed by sand filter and pressure cartridge filter. Finally there are not significant differences on the analytical composition of seawater and brine from rest studied cases. Brine discharges into the sea with 500 mm diameter FGRP pipe diluted by waste water reject treatment [9]. 4.5. Maspalomas II plant This case has provided us with information on how to identify the mixing processes in brine discharge, obtained from work done by the plant operators [3]. It is located in the south of the island, on the left side of the Barranco del Toro, next to its outlet, Playa del Toro beach. Seawater feed flow rate is 42,000 m3/day and the production of drinking water is 25,000 m3/day. The TDS brine is about 90,000 mg/l, because they use a second stage at 90 bar pressure, raising the recovery to 60%. The study was realized on different water samples collection along the coastline, in the open sea and at the bottom, intermediate and surface brine discharge to obtain measures of salinity and turbidity. The dilution of the brine discharge from 75 psu (Practical Salinity Unit) to 38 psu at some 20 m away from discharge outfalls outlet. The flora and fauna is affected by the discharge of treated wastewater and some areas of ‘cebadales’ too as in the south of the discharge.
7
In the Canary Islands there are many reverse osmosis plants for domestical or agriculture water consumption. There is an specific regulation to prevent environmental impact on desalination plants, called an Ecological Impact Detailed Assessment. In the different cases studied in Gran Canaria, most of them are located by the coastline in areas designated to tourist population. External electric energy is supplied by thermal plants to produce electricity and normally it is required to build some electric infrastructure to transport. There is scarcely any possibility of an alternative supply for plants of large capacity. For small capacity plants, renewable energies are being used. Brine reject is always the main environmental problem, and his discharge is usually done jointly with the discharge of waste water treatment, thus diluting it. There are some marine species affected by the salinity of the brine discharged into the sea, as grass prairies called Cymodocea nodosa and Caulerpa prolifera or red algae (Rissoella Verruculosa). Other rejects products as chemical additives and chemical cleaning solutions or pretreatment cleaning is always discharged in the same brine reject pipe. Acknowledgements This paper has been made possible with the cooperation from the Consejo Insular de Aguas de Gran Canaria, which is gratefully acknowledged.
5. Conclusions
References
Seawater Desalination Reverse Osmosis Plants is a solution to grow demand for fresh water, but the technical processes used could damage the environment, with impacts such as the global warming by the increases use of energy, noise pollution, negative effects on land use and adverse effects on the marine environment.
[1] R. Einav, K. Harussi and D Perry, The footprint of the desalination processes on the environment. Desalination, 152 (2002) 141–154. [2] T. Ho¨pner and Windelberg, Elements of environmental impact studies on the coastal desalination plants. Desalination, 108 (1996) 11–18. [3] J. L. Pe´rez Talavera and J. J. Quesada Ruiz, Identification of the mixing processes in brine
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[4]
[5]
[6]
[7]
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8 discharges carried out in Precipice of the Toro Beach, south of Great Canary (Canary Islands), Desalination, 139 (2001) 277–286. AT Hidrotecnia SL. Estudio Detallado de Impacto Ecolo´gico de la Desaladora de Bocabarranco, 1999. Report Submitted to Consejo Insular Aguas Gran Canaria, (in Spanish). GESNAT. Estudio Detallado de Impacto Ecolo´gico del Proyecto ‘Incremento de recusros hidra´ulicos. Desaladora Agua Mar En El Noroeste Arucas (Gran Canaria), 2000. Report submitted to Consejo Insular de Aguas de Gran Canaria (in Spanish). AT Hidrotecnia S.L. Estudio Detallado de Impacto Ecologico de la Desaladora de Bocabarranco, 1999. Report submitted to consejo Insular de Agvas de Gran Canaria (in Spanish). GESNAT, Estudio Detallado de Impacto Ecologico del projects "Incremento de recursos hidravlicos. Desalxdora de agna de mar en el noroeste de Arvcas (Gran Canaria) 2000. Report submitted to Consejo Insular de Aguas de Gran Canaria (in Spanish)
[8] A.T. Hidrotecnia, S.L. Estudio Detallado de Impacto Ecologico de la Desaladora de Rogne Prieto, 1999. Report submitted to consejo Insular de Aguas de Gran Canaria (in Spanish). [9] Hydra consultores S.L. Estudio Detallado de Impacto Ecologico del projecto de constesccion de la Potabilizadora de san Nicola´s de Tolentino (La Alden), 2000. Report submitted to consejo Insular de agnas de Gran Canaria. (in Spanish). [10] C.F. Boudoresqne, A. Meinese. Decouverte de l0 hervier de Posidonie. Cahier Paro Nation, PortCross 4 (1982) 1–79. [11] N. Marba´, C.M. Duarte, J. Cebrian´, S. Enriqnez, M.E. Gallegos, B. Olesen, K. Sand Jenson. Growth and population dynamics of posidonia oceanica on the Spanish Mediterranean coast: elucidating seagrass decline. Marine Ecology Progress Series 137 (1996) 203–213.
Case studies on environmental impact of seawater desalination J. Jaime Sadhwania*, Jose M. Vezaa, Carmelo Santanab a
University of Las Palmas de Gran Canaria, Department of Process Engineering, Campus de Tafira Baja, E-35017, Las Palmas de Gran Canaria, Spain Fax +34 928 458975; email: [email protected] b Consejo Insular de Aguas de Gran Canaria c Juan XXIII, no 7, E-35004, Las palmas de Gran Canaria, Spain Received 25 January 2005; accepted 21 February 2005
Abstract Water desalination processes have contributed to a better standard of living in a number of countries during the second half of the 20th century, following an increase in water demand for drinking purposes as well as industrial and agricultural uses. However, the technologies used in water desalination are also accompanied by adverse environmental effects. There are several effects to be considered in desalination plants, such as the use of the land, the groundwater, the marine environment, noise pollution and the use of energy, amongst others. To protect and preserve the environment, most countries turned to assess the environment impacts produced by desalination plants. Seawater desalination plants are located by the shoreline, to supply desalted water to the population of the main cities and other uses. The construction of both the desalination plants and all the required infrastructure in coastal areas affects the local environment. The impact on groundwater is due to the seawater pipes leaks which could contaminate the aquifers. The high salt concentration in the brine and several chemical products used in the desalination process are returned to the sea. Most impacts on the marine environment arise as a consequence of the brine discharge and their effects could be worse in the Mediterranean sea than in other areas. With respect to the noise pollution produced by the desalination plants, there is always an impact on the plant operators and also on the towns and villages nearby. One of the major indirect environmental impacts is the use of the energy required by desalination plants, particularly when electricity is produced by burning of oil, which in turn boosts the process of global warming. In this paper, we analyse the environmental problems of seawater reverse osmosis desalination plants, focusing on some case studies located in Canary Islands, and describing the major impacts identified. Environmental monitoring is done by the water and environmental authorities, based on regional regulations which turn out to be more restrictive than national legislation. Keywords: Environmental; Impact; Seawater; Reverse osmosis; Desalination plants.
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.02.072
2
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
1. Introduction There has never been so much discussion on the environmental impact of desalination plants in Spain as there is nowadays. It arises as a result of the last modified Water National Plan for supplying desalted water for human consumption, tourist uses, as well as agricultural and industrial consumption. Water desalination comprise a consolidated set of technologies in Spain, from the first desalination plant by MSF evaporation process installed in 1964 in Lanzarote (Canary Islands) to over 700 desalination plants existing nowadays, featuring different technologies, and with a total installed capacity production close to 8,00,000 m3/day. The large experience on brackish and seawater reverse osmosis desalination plants in Canary Islands is an important starting point to transfer knowledge about adverse environmental effects. The society at large in Canary Islands is concerned to preserve natural resources and sustainability, including the environmental impacts from seawater reverse osmosis desalination plants. 2. Environmental regulations Environmental impact assessment is a general and common technique used by industrialized countries, to preserve environment natural resources and to protect them. In Spain, environmental impact assessment was established 1986 as a basic regulation in environmental matters with Royal Decree 1302/1986 of 28th June, in agreement with European Directive 85/337, concerning the evaluation of the incidences of certain private and public projects. Later on further regulations were issued, such as including the Environmental Impact Assessment Decree (EIA), which includes a list of those activities where an environmental
impact assessment is mandatory. One of these activities is water desalination whenever new or additional capacities become larger than 3,000 m3/day. Nevertheless this EIA Decree is only the basic regulation for Spanish, since the different regions (Autonomous Communities) are entitled to produce their own procedures, including the capability to issue their own screening and scoping procedures (establishing the activities subject to environment impact assessment and the terms of reference for the assessments themselves). There are only three autonomous communities with specific regulations for EIA on desalination activity. Canary Island is one of them, with the regional Act 11/1990 (13th July) on Ecological Impact Assessment. Desalination plants over than 5,000 m3/day of water production are listed within the projects subject to carrying out an Ecological Impact Detailed Assessment. 3. Environmental impact of water desalination The water sources appropriate for desalination can have two basic origins basically: seawater and groundwater. Seawater or brackish water desalination in reverse osmosis plants may have several negatives aspects directly or indirectly on the environment: 3.1. The indirect impact on the environment due to the need to increase production electricity for desalination plants Energy consumption on desalination seawater reverse osmosis plants has improved with new technological advances in the process such as the energy recovery systems used. These desalination plants require an external supply electrical energy produced by thermal plants. To produce electricity it is necessary to burn fuels in a thermal plant,
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
which produces polluting flue gases (e.g. carbon dioxide) dispersed to atmosphere. The result of this process is global warming. In agreement with the Kyoto Protocol about the climate change, most of the industrialized countries must reduce gas discharges that can contribute to the global warming, 5% below 1990 level, for next period 2008– 2012. In Spain, thermal power stations produce an average 0.402 kg CO2/kWh, reference value established by the authorities. Most desalination plants in the next years need new electric infrastructure and, as a result, new thermal power stations increase the amount of pollution by gas emissions excepted if there was a replacement of conventional energies for renewable energies for the desalination industry. 3.2. Impact on the marine environment as a result of returning the concentrated brine to the sea In reverse osmosis desalination plants, the discharge brine flow rate is 30–70% of the feed water flow, which means 1.3–1.7 times the seawater concentration. In order to insure the proper dispersion of brine discharge and minimize their adverse effects on the marine environment, it is necessary to select an appropriate technology for this purpose. The main impact is due to the discharge of the concentrated brine to the sea, and its magnitude depends on environmental and hydro-geological factors which are characteristics of the sea: bathymetry, waves, currents, depth of the water column, etc. These factors would determine the extent of the mixing of the brines and therefore the geographical range of the impact [1]. Einav and Lockiev indicate that the extent of vulnerability of the marine environment to salinity differs from place to place. It is measured by the nature of the marine habitat:
3
coral reef, rocky beach or sandy surfaces and by the origin of the surrounding organism. Mathematics models are used for evaluating the range of marine environmental impact [8]. The CORMIX methodology emphasizes the role of boundary interaction on mixing brine and seawater and provide adverse effects from discharges environmental impacts [9]. Ho¨pner and Windelberg [2] divide the global marine habitats into 15 categories according to their sensitivities to the effects of desalination plants, shown in Table 1. In the Mediterranean coasts of Spain there are five kinds of angiosperms marine prairies, and the most abundant is the Posidonia oceanica endemicity. P. oceanica has experienced a remarkable regression in the last decades [10]; reason why at the moment is an species protected by law in the autonomous communities of Balearic Islands, Catalonia and Valencia, and is classified as a high-priority habitat by the European Union Directive [11]. In the Canary Islands, we have the presence of sea grass prairies on the sandy sea
Table 1 Sensitivities of marine habitats to desalination plants 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
High-energy oceanic coasts, rocky or sandy, with coast-parallel current Exposed rocky coast Mature shoreline (sediment mobility) Coastal upwelling High-energy soft tidal coast Estuaries and estuary-similar Low energy sand-, mud-, and beach rocks-flats Coastals sabkhas Fjords Shallow low-energy bay and semi-enclosed lagoon Algal (cyanobacterial) mats Seaweed bay and shallows Coral reefs Salt marsh Mangal (mangrove flats)
4
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
beds (generally associations of two species: Cymodocea nodosa and Caulerpa prolifera) of great biological importance. In Gran Canary island there are located in different areas, as in (Sardina del Norte) and Maspalomas at the south of the island. The grass prairies contribute to fix and to stabilize sandy deposits; also they can permit algae association communities development, Finally they are an important marine habitat to different fishes and invertebrate communities. 3.3. Impact on the marine environment as a result of different products used in chemical cleaning of membranes and pretreatment cleaning In addition to the high concentration of salts, brine discharges contain various chemicals used in the pretreatment stage of the desalination plant, including anti fouling materials. The chemicals used in pretreatment of seawater are mainly: Sodium hypochlorite (NaOCl) or free chlorine, used for chlorination, preventing biological growth Ferric chloride (FeCl3) used for the flocculation and removal of suspended matter from the water Sulphuric or hydrochloric acid used for pH adjustment Sodium hexametaphosphate and similar chemicals, to prevent scale formation on the pipes and on the membranes Sodium bisulphite (NaHSO3), used in order to neutralize any remains of chlorine in the feed water Cleaning pretreatment is conducted by seawater on sand filters or water product on cartridge filters. Organic matter is also accompanied in the discharge to the sea. Cleaning membranes takes place 3 or 4 times a year, and the chemicals products used are mainly weak acids and detergents
(citric acids, sodium polyphosphate and EDTA) and caustic alkali. Water cleaning chemical solution in membranes must be neutralized before being discharged to the sea. The chemical doses effects are not very important to for marine environment impact. 3.4. Impact of noise Acoustic contamination on seawater reverse osmosis desalination plants is important. High pressure pumps and energy recovery systems, such as turbines or similar, produce significant level of noise over 90 dB(A). Therefore, they should be located far away from populated areas and equipped with appropriate acoustic technology to reduce noise level. 3.5. Adverse effect on land use Desalination plants are located next to coastal areas, away from tourist developments and population. The area required for a seawater reverse osmosis desalination plants is about 10,000 m2 for a 5,000 or 10,000 m3/day product water. There is an important view adverse effect on the design of the architectural typology of buildings used to this kind of construction. Additionally, it is necessary improve infrastructure like electric energy transport to the plant, feed and product water pipes to transport, even brine concentrated discharge outfalls, etc. 3.6. Impact on the aquifer Seawater and brine pipes laid over the aquifer pose a danger to it as these pipes may leak and salt water may penetrate into the aquifer. One solution could be to use proper sealing techniques to minimize his impact on the aquifer.
5
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8
4. Case studies We will now analyse the environmental impacts on some new and older seawater desalination reverse osmosis plants in Gran Canaria. Five cases are reviewed, providing brief descriptions of the Ecological Impact Detailed Assessment written for each plant. 4.1. Bocabarranco desalination plant This installation is located in the northwest (NW) of the island, at Bocabarranco beach, in Galdar. Technical specifications are shown in Table 2. Our installation is integrated into a larger desalination system on a 10,000 m2 plot, and all the infrastructure services are also use by the unit. There is also another seawater reverse osmosis desalination plant for agriculture purposes in a different building, and a waste water treatment plant nearby. Brine discharge is realized though two pipelines to the coastline, first one is double for the brine reject of potable water plant with 300 mm diameter FGRP and the second one is 400 mm diameter FGRP for agriculture use. Both pipelines discharge close to the beach. The solution taken consists of joining all discharges, even waste water, by a long outfall discharging into the sea [6]. Seawater and brine reject are shown in Table 3. The cleaning frequency of the sand filter in the pretreatment stage is about once a week and it is discharged into the brine pipe. Table 2 Technical specification in Bocabarranco SWRO desalination plant Capacity of production Recovery TDS water product Application
7,000 m3/day 45% 400 ppm Domestic consumption
Table 3 Chemical composition of seawater and brine reject in Bocabarranco Plant mg/l
Seawater
Brine
Calcium Magnesium Sodium Potassium Bicarbonate Chloride Sulphate Silicon TDS
450 1,520 11,415 450 250 20,800 3,110 5 38,000
814 2,751 20,657 814 452 37,639 5,628 9 68,764
Table 4 Chemical Doses used in Bocabarranco Plant Chemical doses
kg/m3
ppm
NaOCl H2SO4 NaHSO3 Antiscalant FeCl3 Calcium hypochloride
0.053 0.068 0.027 0.009 0.055 0.005
3.0 16.4 2.0 4.1 1.7 2.8
Membranes are cleaned 3–4 times a year and also discharged to the brine pipe. Finally chemical pretreatment used is shown in Table 4, based on water product. 4.2. Agricultural plant at Arucas This unit is located in the north of the island, by the shoreline, in a spot called Punta de Camello, at an altitude 33.27 m over sea, in Arucas. Technical specifications are shown in Table 5. This unit is close to another with a similar design, although the use of product water is different. New brine discharge is realized jointly with another existing discharge dated 1994, with a 400 mm diameter FGRP and is done directly to the coastline. There was one catalogued and endemic marine species, a red algae (Rissoella
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Table 5 Technical specification in Arucas SWRO Desalination Plant 5,000 m3/day 45% 400 ppm Agriculture use
Capacity of production Recovery TDS water product Application
Table 7 Technical specification in Roque Prieto Desalination SW RO Plant Capacity of production Recovery TDS water product Application
5,000 m3/day 45% 400 ppm Potable water
Table 6 Chemical Doses used in Arucas Agriculture Plant Chemical doses
g/m3
kg/day
NaOCl H2SO4 NaHSO3 HSFM FeCl3 Calcium hydroxide
11.10 68.02 6.80 6.67 11.10 20
3.0 230 34 33.3 55.5 103
Verruculosa), discovered in this place and nowadays destroyed by previous impacts [7]. Analytical composition of seawater and brine reject are similar as those shown for the latter case, but chemical doses used for pretreatment and postreatment are as shown in Table 6. 4.3. Roque Prieto Plant This unit is again located in the NW of the island, in a spot called Roque Prieto, Guı´ a, not far from Bocabarranco Plant. Technical specifications are shown in Table 7. This installation is adjacent to another 1,500 m3/day product water evaporation plant. Both plants have similar type of building construction. The feed water intake is done through beach wells and the brine discharge from the RO unit is done jointly with the vapour compression plant reject. The discharge of the brine is directly at the coastline, as you can show in the next photo (Figure 1).
Fig. 1. Brine discharge point at Roque Prieto Plant.
Brine discharge reverse osmosis is diluted with water reject from evaporation plant to reach about 41,000 ppm TDS.[8] With respect to the analytical composition of seawater and brine reject are similar than the rest of studied cases. 4.4. La Aldea Plant This installation is located in the west of the island, La Aldea, in San Nicola´s de Tolentino, with 5,500 m2 plot area. There is also a waste water treatment nearby. The main use of water desalted in this area is agriculture. The total capacity of the plant is 10,000 m3/day with a 45% recovery. It was initially designed for 5,000 m3/day produced with TDS lower than 400 mg/L.
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The plant design is similar to the other studied cases, with beach-well intake, chemical and physical pretreatment, formed by sand filter and pressure cartridge filter. Finally there are not significant differences on the analytical composition of seawater and brine from rest studied cases. Brine discharges into the sea with 500 mm diameter FGRP pipe diluted by waste water reject treatment [9]. 4.5. Maspalomas II plant This case has provided us with information on how to identify the mixing processes in brine discharge, obtained from work done by the plant operators [3]. It is located in the south of the island, on the left side of the Barranco del Toro, next to its outlet, Playa del Toro beach. Seawater feed flow rate is 42,000 m3/day and the production of drinking water is 25,000 m3/day. The TDS brine is about 90,000 mg/l, because they use a second stage at 90 bar pressure, raising the recovery to 60%. The study was realized on different water samples collection along the coastline, in the open sea and at the bottom, intermediate and surface brine discharge to obtain measures of salinity and turbidity. The dilution of the brine discharge from 75 psu (Practical Salinity Unit) to 38 psu at some 20 m away from discharge outfalls outlet. The flora and fauna is affected by the discharge of treated wastewater and some areas of ‘cebadales’ too as in the south of the discharge.
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In the Canary Islands there are many reverse osmosis plants for domestical or agriculture water consumption. There is an specific regulation to prevent environmental impact on desalination plants, called an Ecological Impact Detailed Assessment. In the different cases studied in Gran Canaria, most of them are located by the coastline in areas designated to tourist population. External electric energy is supplied by thermal plants to produce electricity and normally it is required to build some electric infrastructure to transport. There is scarcely any possibility of an alternative supply for plants of large capacity. For small capacity plants, renewable energies are being used. Brine reject is always the main environmental problem, and his discharge is usually done jointly with the discharge of waste water treatment, thus diluting it. There are some marine species affected by the salinity of the brine discharged into the sea, as grass prairies called Cymodocea nodosa and Caulerpa prolifera or red algae (Rissoella Verruculosa). Other rejects products as chemical additives and chemical cleaning solutions or pretreatment cleaning is always discharged in the same brine reject pipe. Acknowledgements This paper has been made possible with the cooperation from the Consejo Insular de Aguas de Gran Canaria, which is gratefully acknowledged.
5. Conclusions
References
Seawater Desalination Reverse Osmosis Plants is a solution to grow demand for fresh water, but the technical processes used could damage the environment, with impacts such as the global warming by the increases use of energy, noise pollution, negative effects on land use and adverse effects on the marine environment.
[1] R. Einav, K. Harussi and D Perry, The footprint of the desalination processes on the environment. Desalination, 152 (2002) 141–154. [2] T. Ho¨pner and Windelberg, Elements of environmental impact studies on the coastal desalination plants. Desalination, 108 (1996) 11–18. [3] J. L. Pe´rez Talavera and J. J. Quesada Ruiz, Identification of the mixing processes in brine
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[4]
[5]
[6]
[7]
J.J. Sadhwani et al. / Desalination 185 (2005) 1–8 discharges carried out in Precipice of the Toro Beach, south of Great Canary (Canary Islands), Desalination, 139 (2001) 277–286. AT Hidrotecnia SL. Estudio Detallado de Impacto Ecolo´gico de la Desaladora de Bocabarranco, 1999. Report Submitted to Consejo Insular Aguas Gran Canaria, (in Spanish). GESNAT. Estudio Detallado de Impacto Ecolo´gico del Proyecto ‘Incremento de recusros hidra´ulicos. Desaladora Agua Mar En El Noroeste Arucas (Gran Canaria), 2000. Report submitted to Consejo Insular de Aguas de Gran Canaria (in Spanish). AT Hidrotecnia S.L. Estudio Detallado de Impacto Ecologico de la Desaladora de Bocabarranco, 1999. Report submitted to consejo Insular de Agvas de Gran Canaria (in Spanish). GESNAT, Estudio Detallado de Impacto Ecologico del projects "Incremento de recursos hidravlicos. Desalxdora de agna de mar en el noroeste de Arvcas (Gran Canaria) 2000. Report submitted to Consejo Insular de Aguas de Gran Canaria (in Spanish)
[8] A.T. Hidrotecnia, S.L. Estudio Detallado de Impacto Ecologico de la Desaladora de Rogne Prieto, 1999. Report submitted to consejo Insular de Aguas de Gran Canaria (in Spanish). [9] Hydra consultores S.L. Estudio Detallado de Impacto Ecologico del projecto de constesccion de la Potabilizadora de san Nicola´s de Tolentino (La Alden), 2000. Report submitted to consejo Insular de agnas de Gran Canaria. (in Spanish). [10] C.F. Boudoresqne, A. Meinese. Decouverte de l0 hervier de Posidonie. Cahier Paro Nation, PortCross 4 (1982) 1–79. [11] N. Marba´, C.M. Duarte, J. Cebrian´, S. Enriqnez, M.E. Gallegos, B. Olesen, K. Sand Jenson. Growth and population dynamics of posidonia oceanica on the Spanish Mediterranean coast: elucidating seagrass decline. Marine Ecology Progress Series 137 (1996) 203–213.