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Environmental Aspects of a Desalination Plant in Ashkelon
Abstract
The desalination plant was set up as a solution to Israel’s growing demand for fresh water. This is the first of a series of designated desalination plants of the same approximate size, which are at present in various stages of planning and licensing. The desalination plant in Ashkelon is one of the largest in the world, and the largest in the Levant Basin. The proposed solution for treating the brines of the desalination plant is unique and apparently the first of its kind for plants of this size.
According to plans, the plant will provide over 100 million cum. water annually, and for this purpose it will draw over twice as much water from the sea, and return a similar quantity of brines. In order to limit interference in the marine environment, it was decided to dilute the water with the cooling water of the Ruthenberg Power Station. Conditions of continuous operation and proper dilution will lead to a slight rise in the salinity level within a range of up to two hundred meters from the pipeline outlet.
From the very first licensing and planning stages, serious attention has been paid to the environmental factors of the project and even before a concessionaire was chosen, a review of the effect on the environment was prepared by the Water Commission. The J. Rom Company has simulated models to forecast the dispersal of brines under various environmental conditions.
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Dr. Rachel Einav* and Fredi Lokiec**
* Blue Ecosystems, environmental consulting
** IDE Technologies Ltd.
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Introduction
Nowadays, with over six billion human beings inhabiting the planet and claiming their basic right to life and quality of life which we strive to give to each individual, it is incumbent upon us to create alternative processes for recycling water to replace the natural processes. An estimate of Israel’s water shortage in 2001 ranges between 200-500 million cum. annually. A desalination plant such as the one being constructed in Ashkelon, based on the process of Reverse Osmosis, is designated to supply approximately 100 million cum. water annually, making up almost one quarter of the shortfall. Subsequently, additional plants will be set up in Hadera, the Haifa Bay and the south of Israel.
Even though the processes of seawater desalination contribute to humanity as well as to nature preservation, they also coincide with environmental damage. However, this can be minimized through careful planning. Most of the problems are of a local nature. Damage is anticipated in five fields: damage to soil usage, damage to the marine environment, increased use of energy, damage to underground water, and noise pollution (Einav et al., 2002).
Damage to soil usage is manifested in the use of beaches for the purpose of constructing the desalination plant, thus designating them for industry rather than for tourism and recreation. The construction of desalination plants in areas adjacent to nature reserves, especially rocky beaches, should be avoided. Plants should be erected in areas earmarked for industry or far from the coast line.
Damage to the marine environment is caused by laying underwater pipelines in the area of pumping brines to the sea. Even though the concentrate water contains dissolved substances originating from the sea, their high specific weight makes it sink to the sea-bottom and prevents mixing, thus creating a “salty desert” in the vicinity of the pipeline outlet. In most cases sea pollution by brines is continuous and permanent, leaving its mark on the flora and fauna around the pipeline outlet. Furthermore, the concentrate water contains chemicals accumulated during the pre-treatment of the desalination process and there is concern for damage to marine fauna and flora in the area of the brine outlet.
The effect of the brines is mainly of a local nature and does not cause accumulated damage of the sea. The range of the effect of the brines on the marine environment depends on bathymetry and on the hydrological characteristics of the sea, i.e., the type of currents and waves. Characteristics of brines such as concentration, discharge rate, outlet pressure and planning of the pipe system also influence the extent of damage to the marine environment. Current know-how for evaluating the range of the effect is based mainly on mathematical models and a few field samples made in real time. It can be assumed that in the course of time this know-how will be expanded and deepened.
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: coral reef, rocky beach or sandy surfaces (Höpner and Windelberg, 1996) and by the origin of the surrounding organisms: the range of the geographic distribution and resistance to changes. The rarity of the natural habitat and the importance of the environment are also significant. Professional literature does not specify a salinity limit above which definite damage is caused to the Benthic population (Dowes, 1982; Levington, 1998), however the range should be minimized to the salt concentration level of the Red Sea.
Increased use of energy, another factor causing direct and indirect environmental damage, stems from the production of electricity (air pollution, coal-dust pollution, thermal pollution, etc.).
Damage to underground water, relevant mainly to cases in which seawater and concentrate water piping is laid on land as well as cases of feed drilling. In such cases there is danger of damage to the aquifer by penetration of saline water or upsetting the subsurface interflow balance.
Noise pollution, a seawater desalination plant by Reverse Osmosis contains equipment such as high-pressure pumps which produce noise, and should therefore be kept away from population centers. Various technological measures can be utilized to reduce noise levels.
Environmental awareness and preventive planning can minimize the damage of the desalination process to the environment.
Land usage by the desalination plant in Ashkelon
From an engineering and economic point of view there is a clear advantage to placing seawater desalination plants close to the shoreline. However, very often, especially in Israel, due to constraints and pressures for land use, the real-estate, environmental and social value of the shoreline is very high. Therefore there is a tendency to place coastal infrastructure plants in areas designated for engineering installations. The desalination plant will be set up in the area of the EAPC (Eilat-Ashkelon Pipeline Co.) installations located near the shoreline, about 2 kilometers south of Ashkelon. It will extend over an area of 70 dunam. This area is designated and used for infrastructures dating back before the founding of the State of Israel. The plot designated for the construction of the plant is located in the center of the site and is surrounded by many infrastructure plants, including the Ruthenberg Power Station of IEC.
The land requirements of the desalination plant include the area of the plant, the marine pumping station, corridors for the pipe system and water pools. At other sites, the pumping installations should be located about 100 meters from the shoreline. In Ashkelon however, due to topographic constraints, it must be located near the shoreline, comprising the suction pipe system and the brine outlet. So the seawater pumping station will be located south of the breakwater of IEC’s settling pool, on land reclaimed from the sea, which has low scenic value. Most of the structure will be located below sea level.
Interaction with the sea
The desalination plant in Ashkelon is designed to supply over 100 million cum. water annually (an average of 274,000 cum/day; 11,400 cum/hour). For this purpose 315 million cum. water a year will be pumped from the sea (an average of 880,000 cum/day; 36,000 cum/hour), and brines with salt concentration of 7.35% TDS will be pumped back (1.86 times seawater concentration) with a capacity of 160 million cum/year (an average of 430,000 cum/day; 18,000 cum/hour). The outlet flow will also contain the pre-treatment water, which will be diluted by rinsing water. The principal interaction with the sea is in the suction and emission systems for seawater and brines.
Seawater suction will be carried out by a system of three 1000 meter long pipes, with a diameter of 1,6 m. each. At the suction head of each pipe a system of dissolved and suspended oil sensors are planned, to monitor the water quality in real time. Should there be a risk of penetration of pollutants, pumping the water into the plant will be stopped. Laying the three suction pipes includes digging a duct on the seabed up to six meters deep. The pipeline runs through a sandy seabed and the dug up sand will be stored on the seabed at no be less than 15 meters water depth. After laying the pipes, the original stored sand will be dug up by a marine excavator and serve as refill for the duct, to cover the pipes at a water depth of up to 10 meters. In this way the seabed will be restored to its former state.
In anticipation of the construction of the plant in Ashkelon, two alternatives for the suitable removal of brines were examined: a) pumping through the pipe to sea depth combined with the use of diffuser whose purpose is to increase dilution; b) diluting the brines in the hot water emitted from the adjacent power station (coolant out). For environmental as well as other reasons, the second alternative, which guarantees maximum dilution and minimal effect on the marine environment, was selected. So the emission pipe of the brines will be located next to the northern hot water emission of the Ruthenberg Power Station.
The operation schedule of the Ruthenberg Power Station is such that three out of four of the existing generation units are always operational. Therefore we opted for releasing the brines adjacent to the emission of IEC’s generation units 1 and 4, thus guaranteeing that at least one generation unit will be operational at all times. Therefore, in a regular situation with two generation units operational, a dilution ratio of 1:10 between the salinity from the desalination plant and the cooling water of the power station will be achieved.
In anticipation of the operation of the plant, and following demands from the Ministry of Environment, models were tested to forecast the dissemination of brines under various flow conditions. Conditions with a relatively calm sea were chosen (a current flow of 0.1 or 0.2 m/second), based on the assessment that the increased flow would increase the mixing of water. Options of pumping northwards and southwards were examined. Results of the model indicate that in a normal situation when two generation units of the power station are in operation, the salinity of the diluted mixture will not exceed 10% above background water. As far as known, there are no desalination plants similar in size to the one designated for Ashkelon, in which the brine is diluted in the emission water of the power station before being pumped back into the sea at a high rate of dilution. The desalination plant in Trinidad, is an exception: the desalination plant’s emission water is diluted by the cooling water of an ammonium production plant (Menachem Zigerson, oral information).
In addition to the salinity problem, a phenomenon of sedimentation of brines close to the seabed is also anticipated. In a situation whereby one of IEC’s units is shut down, a wake of about 200 meters will be created close to the water surface, and one of about 250 m. close to the seabed, where a rise in salinity of over 10% will be observed. In a situation whereby brine is released into the sea without dilution by hot water from IEC, the effect of the large dilution will disappear, thus increasing salinity.
The salinity of most of the world’s seas varies between 32-38‰ (given in salt weight per liter), which is the extent of resistance of most marine life. The salinity of the eastern Mediterranean is relatively higher than the western part, and it changes in the course of the year, reaching levels 39.5-41‰ (Oren, 1970). The high salt concentration can lead to an increase of water turbidity, especially since some of the additives for the desalination process (mainly those containing iron) have a dark hue, black or red. This kind of pollution is called optical. In places with abundant marine life the turbid and dark fuzz is likely to prevent the penetration of light, and disrupt the photosynthesis process. The environmental sensitivity to optical pollution varies in different habitats. Naturally, in the eastern Mediterranean it is more significant in clear waters and less so in turbid waters.
The susceptibility to salinity rise varies from species to species, and apparently, no work and/or research has been done to systematically examine the resistance of the various species found in the eastern basin of the Mediterranean. The plankton population is made up of marine organisms, which by nature exist in an osmotic equilibrium with the marine environment. An increase in the salt concentration in their environment can lead to an egression of water from the cells, to a drop in Turgor pressure and to extinction (mainly of larvae and young individuals). The susceptibility of invertebrates, mainly crustaceans, varies. But in general, those with a long stomach are more sensitive to a rise in salinity than those with a short one. Crustacean and other invertebrate larvae floating in the water are also more sensitive to variations in salinity than fully developed individuals (Levinton, 1995; Dawes, 1998; Einav, 1993; oral information Prof. Baruch Kimor, Prof. Bella Galil). Some of the species, mainly the morphological ones, are resistant to high salinity levels (these species are found in coastal salt marshes, such as the Bardawil salt marsh), but most of the species will not survive. Some are capable of enduring a relatively high level of salinity after a period of acclimatization, but the nature of the outflow will not enable the consolidation of a population of halophilous species at the outlet of the pipe. The biomass on Israel’s shoreline consists of species originating in the Pacific and Atlantic Oceans. The Atlantic species are found in the eastern Mediterranean at the limit of their capacity to endure saline water, while the Pacific species are relatively capable of enduring a certain rise in salinity.
Energy use by the desalination plant in Ashkelon
Use of energy for desalination purposes bears an indirect influence on the quality of the environment. An additional energy consumer requires an increase in the production of electricity, including air pollution, coal-dust pollution, thermal pollution, etc. On the other hand, setting up a large energy consumer such as the desalination plant in Ashkelon, characterized by a steady and continuous electricity consumption system, can reduce electricity production costs and moderate the fluctuations in electricity consumption from the network. This kind of consumer can help increase efficiency of the power station, thereby minimizing environmental damage caused by the energy producer. The initiator who has won the tender in Ashkelon intends to set up an independent, gas-fired, power station, adjacent to the desalination plant. It is a well-known fact that gas-fired power stations cause far less environmental damage than coal-fired power stations. Therefore, the operation of a private power station (backed up by the national network), which is designed to operate at peak consumption hours, is expected to reduce air pollution levels.
Damage to underground water
During the initial planning stages, the option of feed drillings was examined. This is an existing technology with a reasonable level of reliability. The main advantage of this method is the supply of clean and filtered seawater, without pollution risks, and with a steady temperature. The use of drill water will also save on pre-treatment costs.
The main disadvantage of this method is the lack of certainty with regards to the system’s capacity to supply the required quantity of feed water (hydrological limitation). Moreover, it is likely that in the course of time the drillings will become clogged. Mercado (2000) checked the subsurface interflow data in order to produce, by drilling, underground water with a salinity level close to that of seawater. Due to the sensitivity and danger of damaging the aquifer, this alternative was ruled out by the initiators of the project and by the Water Commission, and therefore is not included in the project instructions.
Another possible danger to the aquifer would be when seawater and concentrate water piping is laid on the land-based segment of the site. In these cases there is the risk of damage to the aquifer as a result of leakage and penetration of salt water, or of upsetting the balance of the subsurface interflow. In Ashkelon the laying of sea and concentrate water piping on land is not anticipated, only close to the shoreline. Moreover, the general direction of the aquifer flow is towards the sea and there is a coastal drain in place.
Noise pollution
A plant for seawater desalination by Reverse Osmosis uses noise-producing equipment, including high-pressure pumps. The location of the plant in an area designated for industry and far from population centers neutralizes the effect of noise as an environmental factor.
Summary
Around the desalination plant in Ashkelon, as the forerunner of large plants on the coasts of Israel, a great deal of attention was devoted to environmental aspects, strictly observing high standards environment preservation. With the start of operations of the desalination plant in Ashkelon, the increase in desalination operations on Mediterranean coasts and gaining experience from other large plants throughout the world, attention to environmental aspects of these plants will expand, as well as the level of know-how. Larger budgets will be allocated to monitoring and research, thereby allowing appropriate deployment to reduce environmental problems, mainly those involving the pumping of concentrate back to the sea.
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Literature site
Dawes, C. J. 1998. Marine Botany. John Wiley & Sons, Inc.
Einav, R. 1993. Ecophysiological adaptation strategies of intertidal marine macroalgae Mediterranean, Israel. Dissertations Botanicae. J. Cramer, Berlin, Stuttgart, Bd 208,
Einav, R. Haroussi, K. and Perry, D. 2002. Effects of the Desalination Processes on the Marine Environment – Evidence from Various Sites Around the World. Desalination 152:141-154
Höepner, H. and Windelberg, J. 1996. Elements of environmental impact studies on coastal desalination plant. Desalination 108:11-18.
Levinton, J. S. 1982. Marine Ecology. Prentice-Hall, Inc.
Levinton, J. S. 1995. Marine Biology. Oxford University Press.
Marcado, A. 2000. (in Hebrew).
Oren, O.H. 1970, Seasonal Changes in the Physical and Chemical characteristics and the production in the low tropic level of the Mediterranean waters of Israel.
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