DESALINATION OF WATER
SK ANSARI, SEEMA ANSARI
June 14 - 20, 2010
Bacteria and sediment can be removed from water satisfactorily by inexpensive treatment processes.
A plentiful supply of water is clearly one of the most important factors in the development of modern societies. Availability of water for cleansing is directly related to the control or elimination of disease.
The convenience of water available in the home improves the quality of life. Inexpensive water allows individuals and communities to beautify their surroundings and to use water as a carrier for household wastes.
The residents of most communities could, in fact, reduce water consumption by 10 to 25 percent without significantly changing the life style. Forced reduction in water use due to droughts have been experienced in most areas of the world. Use bathwater by more than one person, saving bath and washer water for lawn and shrub watering, and flushing toilets only once or twice a day result in more than inconvenience.
Most of the water used by humans can be classified as fresh water because the concentration of dissolved constituent is low. Fresh water is derived from surface sources and ground water aquifers. Surface water includes lakes, rivers and those waters stored as ice or snow.
Surface water tends to be turbid a property caused by the presence of clays and other light scattering colloidal particles, and the treatment for the turbidity removal is usually necessary prior to use other than irrigation.
Ground waters usually have higher TDS concentration than surface waters because of mineral pickup from soil and rocks, and many grounds water are noted for high concentrations of particles ions or elements such as calcium and fluoride.
Ground water is a preferred source of water for individual homes and small communities. Large communities often find surface water sources in remote mountain areas attractive for their more desirable chemical characteristics and reliability.
TYPICAL PER CAPITA VALUES
FLOW, LITERS /CAPITA A DAY
. RANGE (LITERS) AVERAGE (LITERS) PERCENT BASED ON AVERAGE FLOW Domestic 150 - 480 220 36.7 Commercial & Industrial
40 - 400 260 43.3 Public Service 20 - 80 30 5 Unaccounted system losses and leakage 40 - 160 90 15 Total 250 - 160 600 100
Based on typical figure given in the table water required to meet the needs of Pakistani population is average water consumption near about 600 liter/day per person.
Nature has blessed Pakistan with adequate surface and groundwater resources. The extended droughts and non-development of additional water resources have further aggravated the water scarcity situation. The increasing gap between water supply and demand has led to severe water shortage in almost all sectors. The water shortage and increasing competition for multiple uses of water adversely affected the quality of water. Most of the reported health problems are directly or indirectly related to water.
GROWING POPULATION OF PAKISTAN
The projected population figures for 2010 and 2025 are 173 million and 221 million respectively. These estimates suggest that the country will slip below the limit of 1000 cubic meters of water per capita per year from 2010 onwards.
The principal source of drinking water for the majority in Pakistan is groundwater. Most of the rural areas and many major cities rely on it, although some cities such as Islamabad, Karachi, Hyderabad etc. get water from a number of other sources.
About 80 per cent of Punjab has fresh groundwater, with some saline water in the south and in desert areas. There are also some evidences of high fluoride or arsenic content locally in Punjab. A number of locations have also been contaminated by industrial wastewater discharges. In Sindh, less than 30 per cent of groundwater is fresh. Much of the province is underlain by highly brackish water and some instances of elevated fluoride levels. In NWFP, increasing abstraction has resulted in wells now reaching into saline layers, and much of Balochistan also has saline groundwater
According to the government figures, Punjab has the best rural water supply amongst the provinces. The vast majority of the rural population has either piped water or water from a hand pump or motor pump. It is stated that only 7 per cent of the rural population depends on a dug well or a river, canal or stream. The situation in Sindh is considerably worse: some 24 per cent of the rural population depend on these sources. The situation in rural Sindh also appears to have deteriorated. The rural water supply situation in NWFP is worse still, and is worst of all in Balochistan. In these two provinces, 46 per cent and 72 per cent of the rural population respectively depend on water from a dug well or from a river/canal/stream
Some what more than 60 per cent of the population gets their drinking water from hand or motor pumps, with the figure in rural areas being over 70 per cent. In almost all urban centers, groundwater quantity and quality has deteriorated to the extent that the availability of good quality raw water has become a serious issue. Over abstraction has also resulted in declining groundwater levels.
Uncontrolled extraction of groundwater and extended dry periods has also caused its depletion and drying up of some of the sources. Also the water table has dropped by 3 meters per year on average. The drying up of wells has important social consequences, particularly on the women and children responsible for water collection. In Islamabad, the drop has been 50 feet between 1986 and 2001 while in Lahore the drop has been about 20 feet between 1993 and 2001.
It is important to note that although there is a clear evidence that groundwater is being overexploited, yet tens of thousands of additional wells are being put into service every year.
PER CAPITA WATER AVAILABILITY
YEAR POPULATION (LACS) PER CAPITA AVAILABILITY (M3) 1951 340 5300 1961 460 3950 1971 650 2700 1981 840 2100 1991 1150 1600 2000 1480 1200 2013 2070 850 2025 2670 659
PER CAPITA WATER AVAILABILITY IN SELECTED COUNTRIES (M3)
COUNTRY 1955 1990 2025 China 4597 2427 1818 Mexico 11396 4226 2597 Philippines 13507 5173 3072 Iraq 184414 6029 2356 USA 14934 9913 7695 Pakistan 4490 1672 659
DESALINATION PLANT IN PAKISTAN
Pakistan's first reheat type Multi-Effect Distillation (MED) desalination plant was installed in 1995 and commissioned in 1996. It was manufactured by Sasakura-Japan and supplied by Ishikiwajima Heavy Industries (IHI), also of Japan. The MED Plant is part of HUB Power Station, which is around 60 kilometers from Karachi. Hub Power Station is situated at a coastal area where no fresh water source is available, therefore these desalination plants have very important and critical role in smooth operation of the power plant.
Each desalination unit consists of four effects where seawater vapors are condensed to generate distillate under vacuum. Salt reduction is over 99.99 per cent during normal operation. The internals of the effects have neoprene rubber coating that guarantees a longer life without operational problems such as fouling and corrosion.
Scaling becomes significant only where temperature controls are not proper or feed flow is insufficient. Sasakura has selected best combination of materials. It is mostly stainless steel.
Thermal energy required to operate the plant is extracted from a boiler. On the coastal belt of Pakistan, Hub Power Station is the only power plant, which has desalination plant to produce water for boiler and auxiliary services. For the last fourteen years, the desalination plant is in service without any major repair or maintenance work and seems to be very reliable source for water production. Since its installation, only routine maintenance and inspection have been carried out with minimum downtime and maintenance cost.
Out of three installed, one has some problem therefore it is generating 80 per cent out of designed capacity, where as other two are generating 100 per cent designed capacity. They are in service since their installation in 1996.
The quality of product water remains very good and consistent.
Salt reduction is over 99.99 per cent during normal operation. The smooth running may continue indefinitely without any major break down with minimum maintenance that is usually required.
DHA COGEN LIMITED (DCL)
The DCL project was planned to generate 105 megawatts (MW) of electricity and five million gallons of water. The desalination plant has been established by DHA Cogen Ltd, a company set up under joint venture between DHA and Sacoden (Pvt) Ltd from Singapore. The plant has been built on 10 acres of land in DHA phase-VIII alongside Arabian Sea with an approximately cost of $185 million/day.
The DCL Plant was designed to in take sea water with two-meter diameter pipeline suspended five meters above the sea and at a distance of 26 meters long toward the sea. The intake pipe is open and, to control growth of marine organisms, chlorinating is restored to in addition to which divers are sent down to clean the intake piping at interval. Although the DCL plant has an access to filter out the fishes, spores, and jelly fish etc., all of which can cause problems and eventual shutdown of the plant.
The main disadvantage of the plant is the lacking of working capital faced by the current management of the DHA Cogen Ltd and due to this lack of capital it could not readily arrange for finances needed for replacement of equipment of the plant that is not covered under the warranty given by Siemens. The DHA Cogen management needed a minimum of $1.5 million for replacement of faulty equipment of the plant not covered by the warranty.
The incoming seawater is de-aerated and preheated in the heat rejection condenser, and then divided into two streams. One stream is discharged as coolant (e.g. back to the sea), and the other becomes feed for the distillation process. The feed is pretreated with a scale inhibiting additive and introduced into the lowest temperature group of heat recovery effects. A spray nozzle system distributes it over the top rows of tubes in each effect, where it flows in thin films down each bank of tubes.
The production capacity of the MED unit is proportional to, and inherently follows, the motive steam input. Therefore, the MED unit production rate can be varied automatically by regulating the steam input rate. For instance, in dual purpose installations (power and water product), the system will produce maximum quantities of fresh water during peak demand periods up to 110 per cent of the nominal rated output.
During non-peak hours of production, the MED unit can turn down to as low as 65 per cent of its nominal capacity without operator intervention, and the surplus steam may then be used for increased electrical energy production.
The input motive steam is fed into the tubes of the hottest effect where it condenses, releasing its latent heat to the saline water flowing over the outer surface of the tubes.
MULTI-STAGE FLASH DISTILLATION (MSF)
In the MSF process, sea water is heated in a vessel called the brine heater. This is generally done by condensing steam on a bank of tubes that passes through the vessel which in turn heats the sea water. This heated sea water then flows into another vessel, called a stage, where the ambient pressure is such that the water will immediately boil. The sudden introduction of the heated water into the chamber causes it to boil rapidly, almost exploding or flashing into steam. Generally, only a small percentage of this water is converted to steam (water vapor), depending on the pressure maintained in this stage since boiling will continue only until the water cools (furnishing the heat of vaporization) to the boiling point.
The concept of distilling water with a vessel operating at a reduced pressure is not new and has been used for well over a century. In the 1950s, a unit that used a series of stages set at increasingly lower atmospheric pressures was developed. In this unit, the feed water could pass from one stage to another and be boiled repeatedly without adding more heat. Typically, an MSF plant can contain anywhere from 4 to 40 stages.
The steam generated by flashing is converted to fresh water by being condensed on tubes of heat exchangers that run through each stage. The tubes are cooled by the incoming feed water going to the brine heater. This, in turn, warms up the feed water so that the amount of thermal energy needed in the brine heater to raise the temperature of the sea water is reduced.
NEW INNOVATED SOLAR SEAWATER DESALINATION BASED ON MED TECHNOLOGY
Shortage of fresh water is a very important problem that is continuously increasing, due to population growth and changes in weather conditions, and affects many countries in the world. These countries usually have abundant seawater resources and a good level of solar radiation, which could be used to produce drinking water from seawater. Although everybody recognizes the strong potential of solar thermal energy to seawater desalination, the process is not yet developed at commercial level. The main reason for this is that the existing technology, although already demonstrated as technically feasible, cannot presently compete, on produced water cost basis, with conventional distillation and reverse osmosis technologies. Nevertheless, it is also recognised that there is still important room to improve desalination systems based on solar thermal energy. Among low capacity production systems, solar ponds represent the best alternative in case of both low fresh water demand and land price. For higher desalting capacities, it is necessary to choose conventional distillation plants coupled to a solar thermal system, which is known as indirect solar desalination.
Distillation methods used in indirect solar desalination plants are multi-stage flash (MSF) and multi-effect distillation (MED). MSF plants, due to factors such as cost and apparent high efficiency pushed out MED systems in the sixties, and only small size MED plants were built. However, in the last decade, interest in multi-effect distillation has been significantly renewed and currently MED process is competing technically and economically with MSF technology. Recent advances in research of low temperature processes have resulted in an increase of the desalting capacity and a reduction in the energy consumption of MED plants, providing long-term operation under remarkable steady conditions.
SOLAR DESALINATION PLANT
The system operates with synthetic oil that is heated as it circulates through the solar collectors. The solar energy is thus converted into thermal energy in the form of sensible heat of the oil, and is then stored in the thermal storage tank.
In the last years, a considerable reduction in fresh water cost from desalination plants has been achieved. On the other hand, water produced in conventional treatment plants has risen due to over-exploitation of aquifers, contamination of ground water and saline intrusion. In countries such as the Persian Gulf region seawater desalination is, since many years ago, a fully competitive and used technique, and this situation is also increasingly close to become a reality in many other world areas due to continuously water demand increase and a parallel reduction in water availability due to previously mentioned reasons.
Water scarcity is an increasing problem around the world and everybody agrees that seawater desalination can help to palliate this situation. Among the energy sources suitable to drive desalination processes, solar energy is one of the most promising options, due to the coupling of the disperse nature and availability of solar radiation with water demand supply requirements in many world locations. This technology cannot currently compete, from an economic point of view, with other conventional desalting technologies, without further improvements.
A new project has been initiated in 2002 trying to improve the existing system. The Project objective is the development of a least costly and more energy efficient seawater desalination technology based on Multi-Effect Distillation process with zero brine discharge.