7. Using non-conventional resources
7.4. Desalinated water
7.4.1. General aspects and treatment processes
Seawater and brackish water desalination is progressively being used by more countries in their effort to cope with water scarcity. This is especially accentuated by the growth of urban areas, and the development of arid areas and tourism.
Desalination is a water treatment process that removes salts from saline water to produce water that is low in total dissolved solids (TDS). The use of this process in areas of water scarcity has obvious benefits, whether ocean water or brackish inland water is used.
Quite a number of different desalination processes can be applied but in practice, only a limited number of them are economically viable. The various processes considered here are distillation, electrodialysis, reverse osmosis and solar desalination. An updated review of desalination is presented by Semiat (2000).
A desalination plant can be pictured as a black box into which feed water and high-grade energy are fed and from which low grade energy, brine and desalinated water are produced. The TDS of the feed water may range from 500 to 50 000 mg/l. The feed water must be treated to the level for which the plant was designed before it is fed into the plant itself. This treatment may include physical filtration, chemical conditioning and other.
The desalinated water produced by the plant is usually in the range of 50 to 500 mg/l TDS and to be used for human consumption must at least comply with WHO limits (Table 7. 11).
The energy input into desalination plants is usually thermal, electrical or mechanical and very often is a combination of all these. Energy rejection from the plants is usually low-grade (thermal) energy. To be able to assess the performance of a desalination plant two important parameters are used - the recovery ratio (Rc) and the performance ratio (R).
Rc is defined as the ratio of the product water to the feed water while R is the reciprocal of the energy consumption.
TABLE 7.11. WHO (1995) and EEC (1999) guidelines for quality of water intended for human consumption.
EEC Max Allowed
Total Dissolved Solids* mg/l 1500 1000
* This parameter is actually total dissolved and suspended solids.
** Values for running/stagnant water sample.
The main desalination processes are:
(a) Thermal Distillation (TD). When a saline solution is boiled, the vapour that comes off is pure water and when this is cooled (condensed) the resulting water is found to contain no salt. This separation is perfect and the fact that the vapour separates very easily makes this very old method of desalination popular. The main drawback is that the energy required to evaporate a saline solution is quite high because of the high latent heat of vaporisation of water. For this to be done economically in a desalination plant, the boiling point is altered by adjusting the atmospheric pressure on the water being boiled to produce the maximum amount of water vapour under controlled conditions. The temperature required for boiling water decreases as the pressure above the water decreases. The reduction of the boiling point is important in the desalination process for two major reasons: multiple boiling and scale control. These two concepts, boiling temperature reduction and multiple boiling, have made various forms of distillation successful in locations around the world. Three types of thermal distillation units are used commercially; namely, Multistage Flash (MSF); Multiple Effect Distillation (MED); and, Vapour Compression (VC).
(b) Electrodialysis (ED). If a current is passed through a saline solution the different ions (cations and anions) in the solution will carry the current from one electrode to the other by drifting in opposite directions. If an ion selective membrane is placed in this flow, say an anion permeable membrane, only the anions will manage to pass through. If an alternating series of cation/anion selective membranes are placed in the path of the ion flow, the channels formed between the membranes will alternately become concentrated and diluted. The overall effect is that salt is being removed from every alternate channel to its neighbouring channels. The basic ED unit consists of several hundred-cell pairs bound together with electrodes on the outside and referred to as a membrane stack. Feedwater passes simultaneously through the cells to provide a continuous, parallel flow of desalted product water and brine that emerge from the stack. ED is only an economical process when used on brackish water, and tends to be most economical at TDS levels of up to 4 000 to 5 000 mg/l. ED units have a waste discharge of brackish water ranging in volume from 10% to 50% of its output of freshwater. The feedwater must be pre-treated to prevent materials from entering the membrane stack that could harm the membranes or clog the narrow channels in the cells. Post-treatment consists of stabilising the water and preparing it for distribution by removing gases such as hydrogen sulphide and adjusting the pH.
(c) Reverse Osmosis (RO). This is a membrane separation process in which the pressure of the water is raised above the osmotic pressure of the membrane. No heating or phase change is necessary for this separation, and the major energy requirement is for pressurising the feedwater. In practice, the saline feedwater is pumped into a closed vessel where it is pressurised against the membrane. As a portion of the water passes through the membrane, the salt content of the remaining feedwater increases. A portion of this saltier feedwater is discharged without passing through the membrane.
RO units have a waste discharge of brackish water or brine which could range from 35% to 100% of its output of fresh water, depending on the feedwater being treated.
Two improvements have helped reduce the operating costs of RO plants during the past decade. These are the development of membranes that can operate efficiently at lower pressures and the use of energy recovery devices.
(d) Solar Desalination (SD). There are three basic ways in which solar energy is used to desalinate saltwater. These are humidification, distillation, and photovoltaic separation. Solar humidification imitates a part of the natural hydrologic cycle by using the Sun's rays to heat a saline water source to produce water vapour. This vapour, or humidity, is then condensed on a cooler surface and the condensate collected as product water. In the solar distillation process, a solar collector is used to concentrate solar energy to heat the feedwater so that it can be used in the high temperature end of a standard thermal desalination process. This is usually a multiple effect or multistage flash process. These units tend to be very capital intensive and require specialised staff to operate them over a long period of time. In addition, they require additional energy inputs to pump the water through the process. Desalination with photovoltaics use photovoltaics to provide electrical energy to operate standard desalting processes like reverse osmosis or electrodialysis. Batteries are used to store energy and inverters are needed to supply alternating current when necessary. The availability of solar energy for only part of the day requires commercial units to be oversized to produce the quantity of water required.
7.4.2. Extent of use and costs of the various desalination methods
Distillation accounts for about 65% of the worlds installed desalination capacity, with the MSF process making up the highest proportion of distillation units. The MSF and MED processes are often used as part of a dual purpose facility where the steam to run the desalination unit is taken from the low pressure end of a steam turbine that is used to generate electricity. The remaining steam and condensate is then returned to the boiler to be reheated and reused. Individual MSF or MED units generally have a capacity of 1000 to 20000 m3/day but several of these units can be grouped around an electrical generating plant. Facilities with a total water output of 200000 m3/day or more are not uncommon in the Middle East, while smaller facilities, consisting of several 5000 m3/day units, are common. VC units are also widely used but, individually, these have much smaller capacities, and, hence, a lower overall total capacity than that of the MSF and MED plants.
Electrodialysis makes up about 5% of the world’s installed desalination capacity.
Electrodialysis units are used in applications requiring smaller volumes of water and can be purchased in units with individual capacities ranging from 10 to 4000 m3/day.
Reverse osmosis makes up about 30% of the worlds installed desalination capacity.
RO units are also small relative to thermal distillation units, and can be purchased in units with individual capacities from 10 to 4000 m3/day. The largest plants are in the range of 40000 m3/day made up of a number of 2000 m3/day individual units. RO plants include a 20000 m3/day plant in Malta for seawater desalination, commissioned in 1983 and producing in 1986 about 30% of the island's total water supply; a 30 000 m3/day plant on Gran Canaria which supplements the water supply of the capital; in Cyprus, a 40000 m3/day plant was commissioned in 1997 and a new plant installed in 2001 produces 51000 m3/day desalinated from seawater, meeting about half the domestic water supply demand of the island. Other smaller plants are located on a number of islands around the world.
Solar desalination is not used extensively and remains largely experimental. There are no large-scale installations, generally because of the large solar collection area requirements, high capital cost, vulnerability to weather-related damage and complexity of
operation. More than 100 plants are listed in over 25 countries with capacities of less than 20 m3/day.
The capital cost of the MSF and MED distillation units tends to be in the range of
$1000 to $2000/m3/d of installed capacity, exclusive of the steam supply and site preparation. The capital cost for VC units tends to be around $2500 to $3000/m3/d of installed capacity. These units require less site preparation. In general, production costs tend to be in the range of $1 to $4/m3/d of water produced, depending on the size of the unit. The 1995 capital cost of electrodialysis units tended to range from $250 to $750/m3/d of installed capacity, exclusive of the site preparation, buildings, and development of the raw water supply. Production costs, including depreciation, tended to be in the range $0.25 to $1/m3/d of water produced depending on the size.
The capital cost of brackish water reverse osmosis units ranges from $250 to
$750/m3/d of installed capacity, exclusive of the site preparation, utilities, buildings, and development of the raw water supply. The capital cost of a seawater RO unit could range from $800 to $1250/m3/d of installed capacity. Production costs for a brackish water plant, including depreciation, range from $0.25 to $1/m3/d of water produced, depending on the size. Similarly, for a seawater plant, production costs could range from $1 to $4/m3/d. Table 7.12 lists some recent prices from International Tenders/Contracts for seawater RO desalination plants for privatised water supply i.e. adopting the Build Own Operate Transfer (BOOT) project approach. The dramatic reduction of prices is obvious.
Since there is limited commercialisation of solar units, the capital cost and operating cost are not as well established as for the other processes. The economics of operating photovoltaic, solar desalting units tend to be related to the cost of producing energy with these alternative energy devices. The capital cost of an 80 m3/d solar-assisted MED facility installed at Umm Al Nar in Abu Dhabi has recently been estimated at about $2 million, or about $25 000/m3/d of installed capacity.
TABLE 7.12. Seawater R.O Desalination. Trend in Costs from International Tenders/Contracts*.
Capacity
(m3/day) Location Country Year Notes Tender price
(USD/ m3/d)
3500 Galdar Spain 1988 Common Site Mobilization 1,943
10000 Suresle Spain 1990 1,622
10000 Galdar Spain 1990 Common Site Mobilization 1,336
25000 California USA 1990 1,458
20000 Dhekeleia Cyprus 1990 First Tender, Invalidated 1,472
20000 Dhekeleia Cyprus 1995 1,220
10000 Bahamas Bahamas 1995 1,254
8000 St. Maarlen NL Antilles 1995 1,690
11000 Canary Island Spain 1995 Estimated for 1/3 Expansion 1,184
150000 Florida USA 1996 Budgetary for Tender File 1,039
45000 Las Palmas Spain 1996 Under Turnkey Contract with
O/M Cost Duty 1,027
56000 Marbella Spain 1996 Variable up to 1,100 1,000
40000 Larnaca Cyprus 2000 Tenders 0,840
20000 Limassol Cyprus 1999 Tenders 0,660
* DuPont Regional Office, Athens 1996 except for the last two entries.
7.4.3. Seawater desalination: the Cyprus experience
Cyprus is an island with a semi-arid climate. Its water resources are intensely utilised and it is suffering from structural and temporary water shortages. During the last ten years the available water from Government projects and from other sources was very limited, which led to supply cuts of up to 30% of normal demand in the domestic sector and up to 70% of the normal demand in the agricultural sector. These have serious adverse effects on the social and economic activities and a negative impact on the environment. In order to cope with the water scarcity, the Government of Cyprus has decided to construct a number of desalination plants. The target capacity of these plants will be 120,000 m3/d or 40 million cubic metres per year with the objective of increasing water availability and the level of reliability for domestic water supply systems. The first desalination plant was commissioned in April 1997, with a nominal output of 40,000 m3/day, whilst the second desalination plant with a nominal output of 51,667 m3/day started operating in May 2001.
Both plants use the RO process with a recovery of 50% and energy from the Electric Power grid. They both use an open sea intake and the water is undergoing a pre-treatment for reducing the silt density index and the pH for the protection of the membranes. The desalinated water is post-treated for achieving an acceptable quality complying with the Cyprus and European drinking water quality standards. The BOOT principle was chosen as the method of project financing on a ten-year basis. The desalinated water is presently sold to the Government, at source, at a varying unit price, about US$ 0.85/m3 for the plant commissioned in 1997 and US$ 0.68/ m3 for the plant that was put in operation in 2001.
Undoubtedly, the discharge of brine impacts the marine environment. At the Dhekelia plant in Cyprus (40 000 m3/d), the discharge of brine with a salinity of 72 0/00 through an outfall which ends at a multi-point diffuser, at a depth of about 5 mand at a distance of 250 m from the shore, resulted in an increase in salinity within a distance of 200 m from the point of discharge. The sea environment around the brine disposal point is monitored on a continuous basis and the results so far are at acceptable levels. In all cases, environmental impact assessment studies were carried out before construction and mitigation measures were imposed on the contractors for minimising adverse environmental effects.