DESALINATION OPTIONS – FOR SMALL TO MEDIUM-

SCALE PLANTS IN DEVELOPING COUNTRIES

Otto Ruskulis Consultant

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DESALINATION OPTIONS FOR SMALL TO MEDIUM-SCALE PLANTS IN DEVELOPING COUNTRIES

1. Available Technologies

Desalination has been undertaken using various techniques. The choice of technique would depend on a number of factors including:- Salinity or brackishness of the water Scale of water requirement Level of availability of skills to operate and maintain the plant Energy use and power sources available Capital costs Operating costs

Salinity or Brackishness of WaterSalinity is determined by the amount of dissolved salts in the water, shown by the Total Dissolved Salts (TDS) value. The World Health Organisation recommends TDS values of between 100 and 1500 milligrams per litre for drinking water. Though a narrower range of 300 to 800 is more typical in areas where water supplies are not usually considered a problem. Brackish water has TDS values in the range 1000 to 5000. For moderately saline water and severely saline water TDS values are 5000 - 10,000 and 10,000 to 30,000, respectively. Typically seawater has a TDS value of about 35,000, which is considerably higher than most inland sources, though not always so. In one area of Botswana groundwater with a TDS value as high as 235,000 was being processed to produced drinking water (Yates, Woto & Tlhage, 1990).

WEDC (1994) have provided some guidelines on typical ranges of TDS values for water which can be processed with different techniques. These are :-Ion Exchange - 500 to 1000Electrodialysis - 500 to 3000Reverse Osmosis (standard membranes) - 500 to 5000Reverse Osmosis (high resistance membranes) - over 5000Distillation – 1000 to 100,000 +, but especially over 30,000

Additionally, electrodialysis and reverse osmosis require water that is otherwise clean, apart from containing dissolved salts. These techniques would do little to remove bacteria from water, and particulate solids would be especially damaging with reverse osmosis as they would clog up the membranes and make them ineffective. To remove solids and bacteria from water, additional processes would be required before the electrodialysis or reverse osmosis stage. In contrast, distillation would separate out water from solids, but the solids would need periodic cleaning out, and it can help to remove bacteria provided care is taken to keep the water input separate from the distillate.

A reservation on electrodialysis needs to be mentioned in the case of relatively poor and isolated communities. For such communities, although electrodialysis is cheaper and less complicated than reverse osmosis it might not be a realistic option as if the TDS value is above 3000, distillation would be much more effective. If it is below 3000 and in the range for which electrodialysis is normally used, local people are likely to be used to brackish water and might not consider it a priority in development terms to get treated water. In reality most people become accustomed to their source of drinking water, even if rather brackish and possibly causing some longer-term health risk. It is likely that in some areas people are making use of water with TDS values even in excess of 3000.

Scale of Water RequirementPersonal consumption of water is very variable. An absolute minimum, in hot climates, for survival is about two litres per day as drinking water. This leaves no water for personal washing, washing of clothes, cooking and cleaning of eating and cooking utensils, etc. A more realistic minimum water requirement per person is therefore generally taken as about

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eight litres per day. However, many millions of the poorest people, especially in arid or semi-arid areas have to manage on less than this amount of water for significant parts of the year. A figure for personal consumption generally taken to indicate absence of a shortage of water is 20 litres per day. This is the figure which will be used in this report. Of course in countries in the West many people are extravagant in personal consumption of water and figures in excess of 200 litres per day are not uncommon.

Therefore, a household of, say, eight people would have a water requirement of 160 litres per day, say 200 litres (0.2 cubic metres) for a bit extra. For a small settlement of about 100 people the requirement would be about 2000 litres (2 cubic metres), or a little bit over, and for a larger settlement of 500 people the requirement would be 10,000 litres (10 cubic metres) per day. If the desalination unit is based around a water pump or well, a typical output would be up to 5,000 litres per day with manual water lifting, although the output from a powered pump would be considerably higher. A manual pump or well would therefore be comfortably used by no more than 200 or 300 people. A desalination unit on about this scale would then also be appropriate. Of course a standard water pump can be used by a community of less than 200 to 300 people, in which case its output would be lower. However, if there are too few people using the facility, probably less than 100 in many places, it would be too difficult for a small number of relatively poor people to be able to maintain a pump.

Below are given approximate maximum and minimum production rates with various desalination technologies. The consumption based on the number of people using the facility and personal consumption of 20 litres per day can be used to size the facility, then a technology chosen for which the total consumption fits within the specified range.

Technology Minimum Production – m3

per dayMaximum Production – m3

per daySolar stills 0.002 2001

Electrodialysis 0.12 15,0003

Reverse Osmosis 0.54 100,000?Multi Stage Flash Evaporation

4,0005 45,0005

Multi Effect Distillation 16 10,0007

1Based on large-scale plants of around 30,0000 m2 which were operating in Greece in the 1960s. It is not known if these are still in operation and no references to any larger plants were found. Large-scale plants are very expensive to maintain and most plants still operating are much smaller than this. Some companies also market solar stills, for householders, of a few to about 25 litres per day.

2Units of this size and upwards are made in Kazakhstan (APCTT)

3Started operating in 1978 in Corfu, Greece (Commonwealth Science Council)

4Used on sea-going boats (see e.g. Spectra Watermakers), for land-based applications units start at around 2 m3 per day.

5 (Teplitz-Sembitzky, 2000), though maximum now likely to be higher

6 For laboratory and hospital use

7 (Wade & Callister, 1997)

It needs to be noted, though, that reverse osmosis, multi-stage flash and multi-effect distillation are relatively complex technologies the cost per unit output of which would increase sharply at the smaller scale of their range of operation, so smaller scale operation might only be viable in particular situations, e.g. where there are no other viable sources of water and a settlement is small but relatively wealthy, for example on the Gulf of Arabia.

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Operating SkillsAll types of desalination technology would require people with particular degrees of skill to maintain them. Medium or large-scale reverse osmosis and electrodialysis plants and plants based on multi stage flash and multi effect distillate would require a dedicated team of trained technicians, mechanics, electricians, engineers and managers in the same way as any other type of technology-intensive industrial process would.

It could, however, be of interest to consider the possibilities for community-based maintenance, possibly linked to some form of more specialist back-up – e.g. by a local research institution or private company, in the case of small-scale electrodialysis, reverse osmosis or solar distillation. Operators of electrodialysis and reverse osmosis plants would need to have a range of technical skills e.g. understanding of electrical circuits, repair of pumps, or at least knowing what the problem is when pumps don’t work, ensuring proper operation of filters, dismantling of membrane units or electrodes for cleaning or replacement, maintenance of battery units, maintenance of photovoltaic arrays, wind turbines or generator sets or, particularly in the case of reverse osmosis, hydraulics and repair of leakages, especially at joints in areas of high pressure operation. The operators would also need to have a realistic view of the cost of the services they provide and the costs of operation, management, maintenance and repair, and a process needs to be in place of recovering these costs from the users, who themselves would need to be in a position to pay these costs, which could be impossible if many of the users are very poor. Here a specialist local institution could be of assistance. It could undertake a preliminary draft cost analysis at the planning stage to decide whether a project is viable and identify or assess possible local expertise to run a plant.

There is some experience from India of small to medium-scale (10 to 300 m3 per day)

operation of reverse osmosis plants in rural areas (Misra, 2002). These seem to have been set up in partnership between national and state governments, research institutions, private companies, which provide most of the plant, and local organisations. Plants are set up where there is usually a cluster of villages dependent on a brackish water supply, and water is piped from a central plant to households or standpipes in the villages. Local people have been trained in day to day running of the plants and routine maintenance. More extensive repairs requires an engineer or technician from a private company or research centre to be called out. This has often caused delays and non-operation of the plant, as has a shortage of spare parts. It is not known if there are any operational reverse osmosis plants of smaller than 10m3

are operating anywhere, other than as pilot or test plants.

Solar still plants are technically much simpler than other types of desalination plant. Most significantly, the still units themselves contain no moving parts – though pumps might be involved for water lifting from the ground or to a storage tank or reservoir. It would be reasonable to assume that solar still plants could be operated and maintained largely by communities themselves, even if they contain no-one with some technical education or experience. Availability of skills would therefore not be likely to be critical factor in determining the viability of an operation, and factors such as levels of salinity and brackishness, suitability of other types of water supply and availability of construction and repair materials would be likely to be more critical.

A parallel can be drawn between small-scale water pumping technologies and desalination using solar stills in terms of sustainability of operation. Small-scale water pumping is a mature technology which has much wider application than solar desalination or distillation. With regard to the suitability of particular pumping systems to operation and maintenance in rural communities, a system called Village Level Operation and Maintenance (VLOM) has been set up. The Afridev and the Indian Mark II are two very common types of pump. However, the Afridev is considered a VLOM pump whereas the Indian Mark II is largely not, mostly because its operating components are more complex and a specialised pump technician would be required to diagnose many operating problems with it and carry out repairs. There have therefore been some poor experiences with the Indian Mark II pump in relatively poor communities without access to specialist skills, but it has been more successfully applied in relatively more wealthy and diverse communities. These communities or households have been able to contract a specialist mechanic, from a private company or

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development agency, to carry out non-routine maintenance and repair.

The bases of VLOM, in outline, are considered to be (e.g. Davis & Brikke – 1995 or Noppen – 1996):- Project Planning

- Community participation in selection of site and technology- Community agrees on and commits to financial contribution- Community agrees on maintenance and servicing arrangements and who is to carry

them out- Community agrees on management arrangements- Risks, problems and constraints are identified and addressed- Evaluation undertaken after project had been running to identify lessons learnt and

changes to practice Social Aspects

- Gender sensitivity, especially in involving women in planning and management as women are usually the main users of water resources

- Improved water supplies are an expressed priority need of the community- Linking of promoting safer water with improved sanitation and primary healthcare

processes- Ensuring an enabling environment, i.e. that local and regional organisations are

supportive of improving water supplies- Support provided to development of key skills and capacities- Sensitivity to local traditions, cultures and power structures- Water supply management aims to build on local traditions of water resource

management Economic Factors

- Operation and maintenance is affordable to local users- Reasonably accurate cost estimates can be made and discussed at the planning

stage- Charging is seen as fair and reasonable, good records are maintained and operators

are diligent in collecting charges Appropriate Technologies

- Pump can be easily maintained by a local caretaker with basic skills and tools- Pump is ideally manufactured in the same country, but especially the spare parts- Design is robust and suitable for field conditions- Pump is of a standard type which is widely used in the area, so that maintenance

manuals, training and spare parts can be standardised Institutional Aspects

- Preferable if introduced as part of a larger-scale programme, e.g. by a national government, local authority, large NGO, international agency or donor, which can also organise any necessary support services

- Skills in participation are available and used by institutions- Local institutions are committed to project- More specialist support services, e.g. for non-routine repair or training, from local

institutions are available and accessible- A local management committee, mostly drawn from community members, is usually

set up to make decisions on pump operation and maintenance- Effective inter-agency collaboration so that, for example, pumps and spare parts are

available and can be installed without excessively long waits and delays

On this basis, solar still-based distillation might be considered to be VLOM in particular situations. The main constraints would be likely to be :-- The community might not choose a desalination technology and prefer an alternative, e.g.

a deeper borehole or building a reservoir further away with water trunked in- Ground conditions and other land use might constrain choice of site, which might not be

the community’s preferred location- There is little documented experience of long-term operation of solar stills, so it would

initially be difficult to assess risks, problems and constraints - Local institutions and agencies might be skeptical about desalination, especially as there

is only limited knowledge of viable small-scale operation

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- It would not be possible to develop a larger-scale programme for a number of years, and so achieve the benefits a larger programme could provide, as the technology is largely unproven in the South, and so skills, experience, optimisation of operation and testing of viability could only be developed through a series of trials and pilot projects.

- Affordability would depend on particular local contexts and could not easily be assessed without operating experience

- Solar stills would be maintenance intensive, requiring extensive cleaning every few days- Some components, such as sealants and valves might not be readily available locally- Most still designs are quite fragile, so operators would need to take special care when

using or maintaining them. Glass, seals, joints and openings can easily be damaged, and if damaged the efficiency of the stills decreases severely. Children and animals would need to be kept away, but it might not always to practicable to do so.

If pilot project experiences can prove the viability of small-scale solar desalination at pilot scale operation it would be important to document these experiences and to develop a set of guidelines for operators and managers, as very little precedent for such guidelines exists currently.

Energy Sources and RequirementsDistillation is an energy intensive technology, as energy is required to evaporate water. Reverse osmosis uses much less energy, the main requirements being largely for applying pressure - between 15 and 100 atmospheric pressure depending on the salinity of the solution and type of membrane used, for overcoming the osmotic pressure of the solution across the membrane, and to pump water around the system. Electrodialysis, somewhat surprisingly, generally needs more energy than reverse osmosis, despite not requiring to be operated at very high pressures. This is probably due to the inherent inefficiency of the process making it necessary to pass the solvent through the unit at least three times, whereas the efficiency of the reverse osmosis process is much better. Also, for electrodialysis there is a need to heat the input solution to 30 to 35ºC to improve ion removal efficiency, which also adds to the energy requirement. Some figures, by way of comparison, are given below. These are only quite crude as direct equivalence is not generally possible, with actual consumption depending on very many factors e.g. ambient temperature, wind speed, altitude, size of plant, compactness of plant, extent of heat or pressure recovery incorporated, age of plant and how well it has been maintained.

Process Total Energy Consumed – kWh/m3

Multi stage flash (distillation) 60 to 701

Multi effect distillate About 501

Multi effect distillate with vapour compression About 102

Solar still About 1000Reverse osmosis 4 to 103

Electrodialysis About 10

1 Teplitz-Sembitzky, W. (2000)

2 Wade, N. & Callister, K. (1997)

3 Teplitz-Sembitzky, W. (2000), energy used is nearer to the lower figure if a device such a Pelton wheel is used for energy recovery

One feature to note about the above table is the very large figure for energy consumption with solar stills. Distillation without any heat recovery as the distillate condenses is not an efficient process. Its attraction is the direct use of solar energy which is, in effect, free energy. For this reason distillation by direct application of heat from a fuel source on a small scale is not an attractive option. In some areas where water is brackish or saline indigenous stills fired with wood or charcoal are sometimes used, for example in Botswana (Yates, Woto & Tlhage, 1990). Previous dissemination of higher yield fired stills based on traditional models in Botswana had resulted in rapid depletion of firewood reserves around the villages where they were used. The decision was then taken to develop solar-based stills.

The larger-scale industrial distillation processes (multi stage flash and multi effect distillate)

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incorporate recovery of the heat as the distillate water is condensed. With vapour compression much of the energy used to evaporate the solution is recovered when the vapour is compressed making the process highly efficient.

Multi stage flash and multi effect distillate utilise electricity from a generator or grid system or steam from a separately fired industrial boiler. Most of the energy requirement is to raise steam which is needed to drive the process. At present it would be likely that only desalination plants of these types at the smaller end of their ranges could be powered wholly or in part by energy from renewable sources.

Solar stills, on the other hand, by definition use renewable energy. They might need to incorporate pumps to extract water from the ground or reservoir or to take treated water to a tank or reservoir, but with small-scale systems the pumps can be manually powered or use electricity from solar photovoltaic or wind sources. Solar stills can have a fan installed in the unit to increase output and so reduce the area needed to be covered by solar collectors. There have also been trials, for example by the Ben Gurion University in Israel, on achieving some heat recovery with solar stills on condensing of the distillate water.

With reverse osmosis and electrodialysis the main energy requirement is electricity for pumping and also, in the case of electrodialysis, to maintain an electric potential gradient to cause ion migration out of solution. Also with electrodialysis, a separate heat source might be required in cooler climates to raise the temperature of the water input to above 30ºC at which temperature the efficiency of the ion removal process improves. There is therefore considerable potential for use of renewable sources – solar photovoltaic and wind, to provide some or all of the electrical input. A number of organisations have been developing and trialling relatively small systems, generally of 0.5 to 3 m3 capacity, powered by renewable energy. In 1995 an electrodialysis unit of approximately 3.5 m3 capacity per day, powered by solar photovoltaic or wind at Spencer Valley, New Mexico, USA, constructed for Navajo inhabitants had been in operation for three years (USBR, 1995). A fairly large dispersed wind-powered reverse osmosis plant was built in 1998 on the island of Syros in Greece. This contains several reverse osmosis modules near the sea and a series of wind propellers and a wind turbine on a hilltop site 1.5 kms away (Assimaccopoulos, 2001). This produces between 60 and 900 cubic metres per day, depending on wind conditions. So far, this has been a one-off plant and the technology involved relatively complex, so it is not known if such a plant could become more widespread. Dulas Engineering in the UK (see contacts list) have developed a small efficient reverse osmosis plant of about 3 cubic metres per day capacity which is powered by solar photovoltaics and uses an electrical inverter (changes direct voltage into alternating voltage) instead of a battery.

Economics of Installation and OperationIf there is a problem of brackish and saline water in a particular area, desalination would not always be the most economic option. Other possibilities could be :-- Bringing in water from another area by vehicle- Supporting informal processes already operating, e.g. water carriers who make a living

from supplying water, and extending their scope- Drilling a deeper borehole, if it is known that water at greater depths is cleaner- Improved rainwater harvesting and storage- Piping in water from more distant sources- A comprehensive water improvement programme if the area affected is large,

incorporating one or more of the above, and which might also include building of water storage tanks or reservoirs.

The best option would be very dependent on local conditions and factors, and these would need to be assessed when attempting to make a costing estimate of the various possible options. It has been estimated that in industrialized countries a desalinated water supply is 2 to 50 times more expensive than water from other sources (Commonwealth Science Council). However in some areas in developing countries, where water shortages can be acute, the cost difference might not be so great.

In the project on improved solar stills in Botswana, Yates, Woto & Tlhage (1990) carried out a cost comparison between solar distillation, water trucking and reticulation (piping) based on

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local costs in 1985. These were based on different numbers of years the stills would have operated, up to 20 years. However, five years would probably be a more realistic operating time of a basic solar still. At five years the cost of having operated a Mexican type solar still would be equivalent to water being trucked in at just under 500 km/month or reticulation from just under 20 kms away. With the much cheaper brick still the equivalent figures would be that utilising the still would be cheaper than trucking for any distance and cheaper than reticulation from more than 7 or 8 kilometres away. Desalination does have the disadvantage also that capital costs are high compared with trucking, so after an initial high cost outlay it would be some years before the costs can be recouped compared with trucking.

Desalination at small or medium scale could offer particular advantages if :-- The community is relatively self-contained and not highly dependent on interacting with

other communities- Alternative water sources are impracticable – e.g. they are very distant, very deep

underground or rainfall is very limited. This might be the case with some relatively dry islands where the only realistic option to supply water for most people is seawater desalination

- There is already some desalination being undertaken elsewhere in the vicinity- There is political, and possibly donor support to enable testing and optimising operation,

possibly initially as a subsidised operation- Local skills can be readily adapted to building, operating and maintaining desalination

units- More specialised technical support is available, e.g. from a university, research centre,

private company, NGO or government agency- Materials for building and spare parts can largely be sourced locally.

If it is decided to go ahead with a desalination option then to a large extent the preceding considerations would determine which would be the most suitable type of process. By way of comparison between the processes the following is a very approximate estimate of costs.

Process Investment Cost US$ per m3

Operating Costs US$ per m3

Total Costs US$ per m3

Multi stage flash – about 50,000 m3 per day

About 0.311 About 0.741 1.051

Reverse osmosis - about 50,000 m3 per day

About 0.301 About 0.641 0.951

Solar still2 About 25Multi effect distiller3 About 10Electrodialysis 0.25 to 14

1 Teplitz-Sembitzky, W., 2000

2 Teplitz-Sembitzky, W., 2000, very small household still, 5-8 litres per day

3 Teplitz-Sembitzky, W., 2000 – based on relatively small experimental units, comparative cost of larger scale commercial units likely to be considerably less, e.g. about 4.5 for a commercial plant in the USA where also most costs, e.g. labour costs, are much higher than in developing countries

4 UNEP / SOPAC – note costs are comparatively low with electrodialysis partly because brackish water with a relatively low TDS value is treated. Electrodialysis is not suitable for treating saline water.

2. Extent of Desalination Use

Desalination is becoming increasingly important for water supply. However, this has largely been through technological development of large-scale plants and the growing importance of

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desalination in particular situations. Among these situations include :-- Arid and semi-arid areas experiencing relatively rapid population growth, but where,

nevertheless, the majority of the population cannot be considered as poor. This is the case, for example, on the Arabian Gulf and some other Middle Eastern and North African countries.

- Tropical islands, where water from rainfall and other natural sources is not adequate to supply steadily growing populations and tourist influx – e.g. Canary Islands, Aruba, Corfu – Greece, the Virgin Islands, Antigua, the Cayman Islands and Malta.

- Reservations of native people, mostly in the USA and Australia, with the units often installed as part of development projects or provided as gifts

- Areas of countries in the North, such as the USA, Spain and Greece, with seasonal or erratic rainfall, relatively wealthy stable or growing populations, growing personal consumption of water and expectation of water availability, and damaged or vulnerable eco-systems which can no longer support the same level of accumulation of water as they once did.

Common factors amongst at least three of these areas of development are increasing demand and expectation, lack of suitable alternatives and a willingness and capacity to pay comparatively high prices for water supplies.

In developing countries in many areas there are more compelling water needs due to :-- People attempting to make a livelihood from increasingly marginalised land- Increasing desertification and ecological degradation- Increasing numbers of people having access to insufficient water, below generally

accepted standards, or to water which is considered unsafe- Water sources, especially groundwater, which once provided safe water becoming

increasingly contaminated. This is particularly the case with increasing brackishness or salinity, largely as a result of increasing extraction. Water supplies which were being used sustainably are now deteriorating as extraction is exceeding replenishment by a wide margin.

- Poor people are having to use up more time and energy, or money, to obtain water and could be using these resources more effectively to address their other poverty constraints

- Poor people having little capacity or power to improve their environment on a significant scale or, more specifically, to improve their access to water.

Could desalination significantly improve access for poor people living on marginalised land to improved water supplies? This possibility would need to be looked at with regard to the criteria outlined in the previous section. However, on three of the above aspects where there could be some impact would be on accessing cleaner and safer water, being able to continue to make use of deteriorating water resources or be able to make use of new resources such as seawater, and also having the possibility that the time and distance they take to obtain water being significantly reduced. Additionally by having a level of control over water provision, as incorporated in the VLOM approach, this could contribute towards empowerment and local people having greater decision making capacity over their own environment.

Some possible problems with desalination introduced in poor rural communities are that :-- Maintaining the unit could impose a cost burden on users and could be non-sustainable- Maintenance of the unit could be difficult to fit into the livelihood strategies of the

community, and would again be difficult to sustain- Desalination could be contributing to increasing the deterioration of a groundwater

sources due to increasing extraction and increasing the salinity or brackishness of the groundwater. This would be doing little to enhance the long-term sustainability of maintaining a particular aquifer. It could also marginalise some people further as, if the aquifer is large, a community using desalination on one part of it could be making matters worse for other people dependent on it elsewhere.

- It can continue to maintain a short-term problem while obscuring a longer term solution. A particular scenario is that many of the poorest people in rural areas are dependent on shallow wells and many of these would be saline. The long-term solution could be a deeper borehole to tap into better reserves, improving rainwater catchment, or tapping into surface ponds, reservoirs or streams. Desalination might be continuing to make a deteriorating situation worse.

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With regard to marginalised communities and environments, it could be most useful to consider an integrated or holistic viewpoint of the situation of poverty, marginalisation, risk and environmental degradation of a particular environmentally related area. This would consider not just problems related to water, but also other issues such as farming, enterprises, health, other natural resources management, markets, community organisation, etc. By taking such a holistic view it would enable the underlying problems of the area to be better identified and solutions outlined. These might or might not bring out desalination as a possible course of action. On the other hand, if desalination is identified as priority then a stronger case would have been made for it than if the analysis had only been undertaken through considering only the water needs. The integrated approach is implicit in the Sustainable Livelihoods Framework (Ashley & Carney, 1999), which could be particularly useful to applying in situations where water supply problems are significant.

In global terms it has been estimated that there are about 11,000 desalination plants in around 120 countries (Commonwealth Science Council, 1999). 60% of these are in the Middle East. Approximate production is about 4 billion gallons (20 million cubic metres) daily. This would provide about 4% of the world’s population with 75 litres per day, but taking all water consumption into account, including in agriculture and industry, this represents only 0.25% of the world’s water needs. Most desalination plants process seawater rather than brackish water inland, though there are also a smaller number of inland plants, notably in India and the United States. Two thirds of the plants use vapour condensation technologies, while the remaining third are mostly based on reverse osmosis.

By far the largest producer of desalinated water is Saudi Arabia with about 5 million cubic metres per day. The majority of drinking water in Saudi Arabia is provided through desalination. The United States are also a significant producer, but of all the water consumed in the USA, that from desalination would be only a very small fraction. Although production is smaller in countries, mainly island states, such as Cyprus, Malta, Gibraltar, the Canary islands, Cape Verde Islands and the United Arab Emirates, it accounts for a significant proportion of the water used in those countries.

Small plants of less than 20 cubic metre capacity are likely to account for only a very small proportion of the world’s desalination capacity. It is estimated that in the early 1990s there were just over 100 solarthermal plants at this level of operation, excluding an unknown number of very small units of several litre per day capacity, throughout the world (Teplitz-Sembitzky, W., 2000). There would also be even fewer of this capacity operating on reverse osmosis or electrodialysis. Most of these are likely to be one-off operations, without significant institutional support or private sector capacity to supply plants of this scale in the commercial market.

The most significant obstacle to the development of small-scale plants is their cost relative to their level of output. They are also demanding on maintenance relative to their output. Small reverse osmosis units are used on boats, where the alternative of carrying large tanks of freshwater would also be costly, as well as inconvenient, but in terrestrial applications there are generally alternative sources of water supply in many areas.

3. Desalination Operating Principles

Large-Scale SystemsThe types of plants described in this section are generally developed on a large or very large scale, i.e. several thousand cubic metres per day, or greater, capacity. Details of their operating principles are given only briefly. More detailed descriptions are given in Wade and Callister (1997).

Multi-stage Flash (MSF)

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This is based on heated saline water ‘flashing’ rather than boiling as it enters a chamber or stage at less than atmospheric pressure. The plant contains several chambers, each at a lower pressure. Thus the heated water cools as it flashes in the first chamber due to the heat taken out to cause vapourisation, then is passed to a second chamber at lower pressure where it flashes and cools again, then to several further stages where the same happens, before a highly saline effluent is pumped out at the final stage and heat recovered from it. Meanwhile the vapour from the stages is taken to a condenser to change back to water. Heat exchangers are used to extract some of the heat given out by condensation and to use this to preheat the input water.

The process is relatively complex, so best suited to large-scale operation. Typical plants produce 10,000 to 100,000 cubic litres per day. Well over 50% of the output of water from desalination is from MSF plants. Corrosion is a problem with MSF plants and various measures, including use of special anti-corrosion additives in the seawater feed, have been used to try to control this, which considerably increase the expense of operations.

Multi-effect Distiller (MED)Seawater is heated to raise steam for the process. The steam is then used to pass through a series of heat exchange tubes, or effects. Each effect is coated by a film of seawater. The steam passing through the inside of the tubes vapourises the water from the films, which is then condensed. The steam itself also condenses after it has passed through the effects and also becomes part of the product. Unlike with MSF there is no use of reduced pressure to facilitate evaporation. Nevertheless to achieve significant levels of heat recovery large-scale plants are more suitable than small or medium scale ones. Plants are typically in the range 500 to 20,000 cubic metres per day. Scaling is a particular problem with MED plants and special measures have been developed to remedy this. Another problem is drying out of the evaporate film on the effect tubes, which reduces efficiency and aggravates scale growth.

Vapour CompressionThe efficiency of MED plants can be increased quite significantly by recovering more of the heat given out when water vapour is condensed. This can be achieved through vapour compression. In Thermal Vapour Compression high pressure steam is used to achieve rapid compression, and in Mechanical Vapour Compression a mechanical device such as a large piston is used to achieve the compression. The rapid release of heat through a compression stroke, rather than the steady release obtained in conventional condensation, can be more easily captured and used elsewhere in the process.

Processes Which Can Also Be Utilised At Small or Medium Scale

Solar DistillationThis is the simplest method of desalination. Essentially water is evaporated by the sun and condenses on a cooler surface from where it is collected. Dissolved salts remain in the solution. A common design of a solar still is shown below. A glass or plastic plate is fixed on top of the still to increase the temperature in the still, and the bottom of the still is lined with a black material such as bituminous paint, butyl rubber, epoxy enamel, fibreglass painted black or aluminium painted black, to act as a heat absorber. It is also important that the whole still is well-insulated to improve efficiency. The sides and base of the still are typically brick or concrete. Moulding of stills from fibreglass was tried in Botswana (Yates, Woto & Tlhage, 1990). This was more expensive than a brick still and more difficult to insulate sufficiently, but has the advantage of the stills being transportable.

For the collector plate, glass is preferable to plastic because most plastic degrades in the long term due to ultra violet light from sunlight and because it is more difficult for water to condense onto it. Tempered low-iron glass is the best material to use because it is highly transparent and not easily damaged (Scharl & Harrs, 1993). However, if this is too expensive or unavailable, normal window glass is a satisfactory alternative. This has to be 4mm or more thick to reduce breakages. It is important for greater efficiency that the water condenses on the plate as a film rather than as droplets, which tend to drop back into the saline water. For this reason the plate is set at an angle of 10 to 20º. The condensate film is then likely to run

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down the plate and into the run off channel.

condensate runoff glass or plasticchannel plate

rainwater catchmentchannel

base incorporating insulationblackened surface

saline water

The efficiency of solar stills which are well-constructed and maintained is about 50%. Some problems with solar stills which would reduce their efficiency include :-- Poor fitting and joints, which increase colder air flow from outside into the still- Cracking, breakage or scratches on glass, which reduce solar transmission or let in air- Growth of algae and deposition of dust, bird droppings, etc. To avoid this the stills need

to be cleaned regularly every few days- Damage over time to the blackened absorbing surface.- Accumulation of salt on the bottom, which needs to be removed periodically- The saline water in the still is too deep, or dries out. The depth needs to be maintained at

around 20mmIt is important that stills are regularly inspected and maintained to retain their efficiency and reduce deterioration. Damage, such as breakage of the collector plate, needs to be rectified.

The output of a still will depend on its efficiency and the intensity of solar radiation. The latter would vary throughout the day, at different locations, with the extent of cloud cover and from season to season. In tropical countries a typical yield is 2-3 litres per square metre per day, although close to the equator in arid areas yields can be double this. However, at very high air temperatures such as over 45ºC, the plate can become too warm and condensation on it can become problematic, leading to loss of efficiency. By placing a fan in the still it is possible to increase evaporation rates. However, the increase is not large and there is also the extra cost and complication of including and powering a fan in what is essentially quite a simple piece of equipment. Fan assisted solar desalination would only really be useful if a particular level of output is needed but the area occupied by the stills is restricted, as fan assistance can enable the area occupied by a still to be reduced for a given output.

The above design incorporates provision for rainwater collection, which can be one fifth to one third the volume of water collected by desalination. Some alternative types of solar desalination units do not include such provision for collecting rainwater.

In practice, if relatively thin glass sheet is used for the collector plate, this limits the size of an individual still to a few square metres. If more area needs to be covered for higher outputs, then more stills are used.

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Some companies, e.g. in the United States, Russia, India and South Africa, sell solar stills, largely for household use to produce a few up to about 50 litres per day. Costs range from a few hundred pounds to, perhaps a thousand pounds for a top of the range model.

Some construction details on simple solar stills are provided in Yates, Woto & Tlhage (1990) and in McCracken & Gordes (1985)

The above is a basic design. However some variants have also been developed :-

The Mexican stillIn the Mexican still two stills such as the above are fixed together to form a triangular tent shape. The glass plates can be supported from below at the apex where they join, but if they are not and just lean against each other, fixed with sealant, this increases the fragility of the still and limits the area even further of each of the glass plates.

The Brace Research Institute stillThis is essentially a still as shown in the above drawing. However the stills are placed next to each other over the width of say 10 metres of the distillation plant. Lengthwise, the unit such as shown is built over a considerable distance, such as 15 metres. Glass plates are placed along the length of the still and simply joined with sealant. Units of this size also have two small weirs lengthwise to encourage saline water to flow along the full length of the still. A project of this type was set up by the Brace Research Institute, McGill University, Canada in Haiti. The scale of the unit requires caretakers to be trained in the maintenance of it, and maintenance requirements are quite considerable.

Solar Wick typeSome designs have been developed which incorporate absorbent or film-type materials to increase the surface area of evaporation – e.g. an article on the design developed by G.N. Tiwari of the Indian Institute of Technology, New Delhi, was published in the New Scientist. However, it is not known what levels of efficiency improvements have been achieved.

Use of ReflectorThe inside walls of the still can incorporate a reflective coating, such as aluminium foil, to increase the reflection of heat energy onto the evaporating water. It is not known how far this has helped to improve the efficiency of the still

Spherical StillIn a design developed by the Thermal and Solar Laboratory at Claude Bernard University, Lyons, France, a trough, where the saline water is placed, is positioned in the centre of a hollow transparent plastic sphere. Distillate water condenses on the inside surface of the sphere and is collected by a mechanical windscreen type wiper blade which forces the condensed water to fall to the bottom of the sphere to be collected. There seems to be a small improvement in efficiency compared with a conventional solar still, but the greater cost of this still compared to conventional models might cancel out this advantage. (World Water)

Personal Survival ProductsA number of small flexible plastic stills have been developed for personal survival at sea or on long treks. The need for them to be lightweight and capable to be folded away has influenced their design. A typical design is a plastic cone with a water absorbing material at the bottom. The unit is placed in the sea or on a stream or pond and water is evaporated and condenses on the plastic surface, flowing down it to a plastic channel around the bottom edge, which is separated by a plastic barrier from the saline absorber. Outputs are a few litres per day when it is sunny, less in cloudy conditions, sufficient for one person’s survival.

Inclined StillsThe aim of inclining a still is to increase the solar radiation, by catching it head on, rather than at an angle as with stills which lie flat. To do this constantly, as the sun rises and sets, would need someone to monitor the sun and turn the unit regularly, or a sophisticated automatic tracking and turning mechanism.

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Condensate Heat RecoveryHeat recovery from the energy given out when water vapour condenses has generally not been attempted with small-scale solar distillation, unlike with larger-scale systems. It is known that the Ben Gurion Institute, and more latterly the Technion Institute in Israel has undertaken some experiments with heat recovery. In the simplest system, saline water is made to flow over the outside of the condensation plate before entering the still, but then this would reduce the amount of solar radiation passing through the plate. There may be scope for further research to overcome current difficulties with attempting heat recovery from solar distillation.

Reverse OsmosisSome polymer materials demonstrate the phenomenon of osmosis when used to separate a solvent from a solution. So if a solution of salt and water is placed in a chamber and in an adjoining chamber is placed the solvent, water, with the osmotic membrane forming the barrier between the chambers, ions from the salt in solution will migrate across the membrane to the solute until an equilibrium state is reached with a more dilute solution of equal salinity in both chambers. The migration of the ions is driven by a pressure, called the osmotic pressure, which exists across the membrane.

Osmosis is a reversible process, so if there are two chambers with the same solution in each, applying a pressure in excess of the osmotic pressure to the solution in one of the chambers will cause ions to migrate from it, across the membrane, and into the solution in the other chamber. This is the basis of the reverse osmosis process of desalination. However, the osmotic pressure of a saline solution is high, so high pressures are needed to achieve reverse osmosis, and the higher the saline concentration, the higher the osmotic pressure. Pressures are typically 15 to 100 bar or atmospheres, needing robust, high specification and relatively complex types of pumps and accounting for most of the energy used in reverse osmosis desalination.

Polymer materials such as polyamides or cellulose acetate have been used for membrane materials in reverse osmosis. However, there has been considerable research into improved membrane formulations and now a number of proprietary materials have been developed for high performance operation to enable desalination to be undertaken faster, at higher pressures, with higher solute removal rates and with lower levels of brine reject. Further research would be likely to lower the cost of the process, though this would probably be more applicable at the higher volume end of the production scale.

For production of 10 cubic metres or greater per day it is now possible to buy kits from commercial suppliers, who would also be likely to install the plant and train operators. Smaller systems, from about 0.5 cubic metres per day, have also been developed for specialist applications such as on boats, for hydroponic irrigation and for providing distilled water for hospitals. Such small systems would generally be uneconomic for supplying drinking water on land, as the operating cost of a small system per unit volume of water produced can be five times or more of a large one. For a 30 cubic metre per day plant in India – a rate of production at which reverse osmosis can be considered economically viable in suitable locations, the cost is estimated at around £22,000. A similar plant supplied from a Western country could cost considerably more. A small experimental solar powered reverse osmosis plant of 400 litres (0.4 m3) per day set up by Murdoch University in Western Australia cost about £8,500 (Mathew et. al., 2001).

The critical element in the reverse osmosis process is the membrane. It is particularly important that the water passing through the membrane is free from solid materials, or the pores in the membrane would get blocked up and the membrane lose efficiency. A separate filter is therefore required with reverse osmosis to remove small particulate matter from the saline water before it undergoes reverse osmosis. Slow sand filtration, rather than an expensive proprietary filter might be suitable provided the water has only low to medium turbidity. With care, the membrane element can last five years without much decrease of efficiency. It is, however, costly to replace. Usually it comes as a pre-packaged element, and the old element can be relatively simply be removed from the unit and the new one put in place. Membranes are usually packaged spiral-wound, with a hollow core into which treated

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water will pass, or as bundled hollow fibres, with treated water emerging from the end of the hollow. Such arrangements of membranes are usually supplied in a metal casing, to which a pipe supplying high pressure saline water is connected at one end, and at the other end pipes or ducts to remove treated water and brine. When treating seawater, or very saline groundwater, it may be necessary to pass the water through two, or sometimes three, reverse osmosis stages.

Other precautions with reverse osmosis are the need to add chemicals to prevent scale formation on the membranes, chlorination to reduce risk of algael or bacterial growth on the membranes and coagulation if there is a significant presence of flocculent materials.

More detailed information on reverse osmosis is contained in Wade & Callister (1997), Mathew et. al (2001), UNEP (1997), Commonwealth Science Council, & Thomson – Miranda – Infield

A further membrane-based technique which also needs mention is nano-filtration. Pores are slightly more sizeable for nano-filtration membranes than for those used in reverse osmosis, but still only a few nanometres (10-9 metres) across. Osmotic pressures across such membranes are less than those across membranes used in reverse osmosis, so pumping pressures are lower, saving on cost and technical complexity, but typically allowing a 30 to 60% passage of monvalent salts and 5 – 15% passage of divalent salts (Wade and Callister, 1997). Nano-filtration would therefore not be effective for seawater because of its high TDS and because sodium chloride is the main dissolved salt, and both sodium and chloride ions are monovalent. However, it could be more useful for brackish water inland, especially if it contains significant calcium or magnesium-based salts as their ions are divalent. A small solar-powered nano-filtration unit has been developed by the Engineering faculty at the University of New South Wales in Australia (UNSW, 2001). This is claimed to be more cost-effective and appropriate to remote communities in Australia, where groundwater is more brackish than saline – TDS generally in the range 3000 to 6000, and chloride contents of the water are relatively low.

ElectrodialysisElectrodialysis is generally used to treat brackish water with TDS values less than 3000.

An electrodialysis cell is shown below. This has positive and negative electrodes on either side applying a direct voltage across the cell. The inside of the cell is divided by a series of membranes, four in the above case. Two, shown as solid lines, allow cations (positive ions) but not anions (negative ions) to pass, while the other two, shown as broken lines, allow anions but not cations to pass. Thus cations (such as sodium, potassium, calcium or magnesium ions) become deposited on the membranes shown as broken lines and anions (such as chlorides, sulphate or nitrate ions) are deposited on the membranes shown as solid lines. In the above cases, going from left to right, chambers 1, 3 and 5 would contain a higher concentration of ions than the original solution, so the water from these chambers can be considered as effluent, while in chambers 2 and 4 the ionic concentration would be lower than in the original solution, so this is taken as the treated water, which will be chemically dosed before storage and use. Because anions or cations are not often precipitated out of solution at their respective membranes, but are just concentrated around them, and the concentration becomes an equilibrium process, so that after a certain concentration it becomes difficult for the membranes to hold any more ions, this limits electrodialysis to treating water with relatively low TDS values. Water with TDS values above 3000 can be treated, but this would involve passing it through several electrodialysis stages, which would make it no longer viable in terms of cost compared with reverse osmosis or distillation techniques. Electrodialysis works most efficiently at temperatures just above 30ºC, so if the water is at a lower temperature than this, it would need to be heated.

Solid precipitation, or scaling, around the membranes is nevertheless the most serious problem with electrodialysis. It causes uneven charge distribution around the membranes and loss of efficiency. This can be remedied with some systems by reversing the polarities of the electrodes, so that the chambers in which the effluent was concentrated become the ones which now contain the treated water, and vice versa. The precipitated salts then also get

15

washed away with the first flush of treated water.

Relatively small electrodialysis plants have been developed and operated, for example in Kazakhstan, at 100 litres to 2.5 m3 per day (APCTT), and at Spencer Valley, New Mexico, USA, at 3.5 m3 per day (USBR, 1995). Reference to these plants has already been made earlier in the report. The Kazakhstan plants are reputed to cost from US$1,000 to a few thousand.

Further details on water treatment by electrodialysis can be found in Wade and Callister (1997) and in Elmidaoui et al. (2001).

brackish water feed from filters

positiveelectrode

negative electrode

To discharge outlet

clean water for storage

Uncommon, Experimental or Pilot Processes

FreezingA refrigeration plant combined with low pressure chambers to cause evaporation of seawater is used. As seawater evaporates heat is extracted from it and this causes ice crystals to form. A refrigerant such as ammonia or butane is used to remove heat from the process. The refrigerant vapourises in direct or indirect contact with seawater, thus extracting heat from the seawater. The vapour is taken away to the collector area for the ice. Here it is condensed by contact with the ice and, as heat is given out, the ice is melted. The condensed refrigerant is then re-circulated to begin the process again. Freezing requires only about one seventh of the energy of distillation processes, and there is no problem with scale formation. However, plant operation is quite complex and it is not known how plant performance and cost compares with other processes. Some further details can be found on –http://web.singnet.com.sg/~ikeya05/methods.htm

Small-scale Multi Effect DistillerThe Center for Solar Energy and Hydrogen Research (ZSW) in Germany in collaboration with the National Centre for Coordination and Scientific and Technical Research in Morocco have installed small pilot multi-effect distiller plants in Morocco and are monitoring them. These are

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+ -

-

+

-

+

more efficient than solar distillation as significant heat recovery is possible, and solar heating in the day can be supplemented by heating with fossil fuel at night. The output range of the plants is 0.05 to 10 m3 per day. Possible applications are remote fishing villages on the Atlantic coast, remote settlements of cattle herders in the South of Morocco, and dedicated supplies to coastal tourist hotels, frequently affected by disruption of piped supplies (MEDRC). Additionally, a small experimental multi-effect humidification system of 100 litres per day was set up in the Canary Islands in 1992 by ZAE Bayern, a German company. The collector area was 8.5 m2, so giving an output of 12 l/ m2, more than three times that of conventional solar stills. A similar plant was also set up in Tunisia producing 500 litres per day from a collector area of 38 m2 (Childs et. al, 1999).

Seawater Greenhouse This is an innovative product which combines growing fruit and vegetables in the greenhouse with collection of desalinated water. An experimental unit has been set up in Abu Dhabi. The unit consists of strengthened thick porous cardboard walls which are kept wet by being doused with pumped seawater. The greenhouse has been orientated towards the prevailing wind direction, so the wind assists evaporation of the seawater into the greenhouse. The effect of the evaporation and the cooler seawater it passes through is to cool the air going into the greenhouse, which also saturates it with water vapour making a damp environment in the greenhouse. Thus, while the outside temperature might be 45ºC, inside it is about 30ºC. The unit, however, does not work as a conventional greenhouse, as the intention is to maintain it at a lower rather than a higher temperature than the outside air. The roof is polythene which is coated in a special infra red reflector to reduce heat transmission, but which is largely transparent to visible light to maintain photosynthesis of the plants. A fan is also used to assist the air flow through the greenhouse. At the end of it the air which still contains a high level of humidity is mixed with hot dry outside air, then passed through a second moistened porous cardboard wall to pick up more water vapour before being condensed on a surface cooled by seawater. Thus clean water is also an end product of the greenhouse.

The size of the greenhouse is 45 by 18 metres and the full unit costs about US$ 4,000,000. It has been estimated that when conditions are optimal about 20 litres water per day are produced for each square metre of greenhouse. The seawater greenhouse is marketed by a company called Lightworks. Contact details are given in the Further Information section.

Seawater Greenhouse

Commercial EnquiriesTim Mott - Commercial DirectorSeawater Greenhouse38 Hampton RoadTwickenhamMiddlesexEnglandTW2 5QBTel: +44 (0)20 8894 2288Fax: +44 (0)20 8894 2244email: timmott@btconnect.com < mailto:timmott@btconnect.com >

Scientific and Design EnquiriesCharlie Paton - Scientific and Technical DirectorSeawater Greenhouse2a Greenwood RoadLondon, E8 1ABTel: +44 (0)20 7249 3627Fax: +44 (0)20 7254 0306E-mail: lightworks1@compuserve.com Website: http://www.seawatergreenhouse.com/

The Seawater Greenhouse is now available for commercial development.

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The continued growth of demand for water and increasing shortages of water supply are two of the most certain and predictable scenarios of the 21st century. Agriculture, with a high demand for water for irrigation, will be a major pressure point.

The Seawater Greenhouse will help to address this crucial problem in a cost-efficient and sustainable way, saving scarce water supplies for human and industrial use.

http://www.mathworks.com/products/user_story/userstory2347.jsp?industry=6

Solar TroughsSolar radiation is concentrated in solar troughs, for example a parabolic reflective dish which reflects radiation onto a small area. Metal piping with saline water can be taken through this area, where the water heats up and evaporates to produce steam. The steam can be condensed directly, e.g. by mechanical compression or, more efficiently, used in another process e.g. multi-stage flash distillation or a multi effect distiller. On a small scale solar troughs would have no advantage over conventional solar stills and, in fact would be more complex and expensive. On a larger scale they have the advantage of being a convenient and renewable way of raising steam for another desalination process. However, the need to build a large solar collector would add considerably to the complexity and expense of a project. Further information on solar troughs is given in Teplitz-Sembitzky (2000) & García-Rodríguez and Gómez-Camacho (1999).

Forward Osmosis The term osmosis describes the natural diffusion of water through a semi-permeable membrane from a solution of a lower concentration to a solution with a higher concentration (Figure 1). Like reverse osmosis (RO), forward osmosis (FO) uses a semi-permeable membrane to separate water from dissolved solutes effectively. The semi-permeable membrane acts as a barrier that allows small molecules such as water to pass through while blocking larger molecules like salts, sugars, starches, proteins, viruses, bacteria, and parasites.

Instead of employing hydraulic pressure as the driving force for separation in the RO process, FO uses the osmotic pressure gradient across the membrane to induce a net flow of water through the membrane into the draw solution, thus efficiently separating the freshwater from its solutes. Driven by an osmotic pressure gradient, FO does not require significant energy input, only stirring or pumping of the solutions involved (Figure 3). FO offers the advantages of high rejection of a wide range of contaminants and lower membrane-fouling propensities than traditional pressure-driven membrane processes. In addition, for food and pharmaceutical processing, FO concentrates the feed streams without requiring high pressures or temperatures detrimental to the feed solution.

FO has thus drawn much attention with applications developed in various fields such as wastewater treatment, pharmaceutical and juice concentration, desalination, and even power generation and potable-water reuse in space. In view of the technique’s great potential, scientists from the National University of Singapore (NUS) Department of Chemical and Biomolecular Engineering, the Singapore–MIT Alliance, and the Agency for Science, Technology and Research (A*STAR) have been working together to explore the exciting possibilities FO research promises.

Membrane Distillation Membrane distillation (MD) is also known as transmembrane distillation, membrane evaporation, and thermo-pervaporation. It combines both membrane technology and evaporation processing in one unit. It involves the transport of water vapour through the pores of hydrophobic membranes via the temperature difference across the membrane. For almost three decades, MD has been considered an alternative approach for conventional desalination technologies such as multistage-flash vaporisation and RO. These two techniques involve high energy and high operating pressure respectively, which result in excessive operating costs if oil prices continuously move up. MD offers the attractiveness of operation at atmosphere pressure and low temperatures (30o – 90oC), with the theoretical

18

ability to achieve 100% salt rejection.

Owing to its low energy requirement, MD coupled with solar energy, geothermal energy, or waste heat can achieve cost and energy efficiency. The Singapore government recently showed its commitment to solar-energy technology by allocating S$170 million towards its research and development. Earlier, the city-state had positioned itself as a global hydrohub to supply up to 5% of the total world water market. The ideal proposition is to combine both MD and solar technology to realise a cheaper and more energy-efficient system for obtaining potable water.

However, the industry has not fully embraced MD for several reasons: low water flux (i.e., productivity) and shortage of long-term performance data due to the wetting of the hydrophobic microporous membrane. Materials breakthroughs on new microporous membranes with desired porosity, hydrophobicity, low thermal conductivity, and low fouling are essential to bring MD closer to commercialisation. Opportunities therefore beckon membrane researchers to improve the flux in the process and increase its durability by fabricating highly permeable super-hydrophobic membranes and/or modifying the MD module configurations.

Dual-Layer Hydrophilic-Hydrophobic Hollow Fibres To enhance the flux in an MD process, one possible approach involves utilising hydrophilic–hydrophobic membranes. Minimising the thickness of the hydrophobic functional layer and using a hydrophilic layer as support for the thin functional layer will reduce water vapour mass-transfer resistance through the membrane.

For the first time, the team fabricated dual-layer hydrophilic–hydrophobic hollow fibres especially for direct-contact membrane distillation (DCMD) by co-extruding two different physically modified polyvinylidene fluoride (PVDF) solutions. Incorporating hydrophobic particles in the outer-layer dope solution makes the outer thin hydrophobic functional fibre layer. The mixed-matrix functional layer exhibits a greater hydrophobicity with a contact angle of about 140° compared with that made of neat PVDF, which is about 75°. Blending PVDF dope solution with hydrophilic particles and polyacrylonitrile polymer allows the fabrication of the inner hydrophilic support layer, permitting a contact angle as low as 50°.

The morphologies of the successfully spun dual-layer fibres (Figure 5) show the achievement of a desirable sponge-like and highly porous structure. The researchers tested the fabricated fibres in a DCMD process to obtain fluxes as high as 55kg/m2hr at 90°C, which is much higher than most existing data published in the open literature. A*STAR and NUS have filed a patent for this invention.

Baffled Module Designs Because of the effect of temperature polarisation or temperature drop crossing membrane (Tf –Tp), heat transfer across the boundary layer from the bulk to the membrane surface often limits the rate of flux transfer in MD. Thus to improve mass transfer of flux in MD, researchers must minimise this phenomenon. An alternative approach to the flux enhancement in MD application lies in the modification of module design by spacers/baffles/turbulence promoters. Spacers can aid in the increase of both the heat and the mass-transfer coefficients via the generation of turbulence flow, a change in the characteristics of which will enhance boundary-layer heat and mass transfer, resulting in increased flux transfer.

The Singapore researchers employ a series of systematic module configurations in an attempt to enhance the total flux. They have incorporated module designs such as the baffle, external helix, inner helix, and sieve during the process of module fabrication. They have also introduced two special configurations (spacer and twisted modules) into the module configurations.

By implementing these different module designs, the investigators observed an 11–49% increase in flux performance at 75ºC with respect to the original, unaltered module. It is interesting that the highest flux attained (49% increase) combined two plastic sieves and the inner helix configuration at 75ºC. The generation of turbulence, the increase in effective

19

membrane-surface contact, and the effects of cross-flow possibly account for the improvement in MD performance.

The FO research has received attention from the Public Utilities Board’s (PUB’s) Centre for Advanced Water Technology in Singapore. Negotiations are underway to have a joint project and pilot-scale trials at PUB. A Saudi Arabia university and a Middle Eastern research institute have expressed interest to develop a collaborative research on the MD project, while a US company is looking at licensing FO membrane technologies.

Membrane DistillationThis is a relatively new process which utilizes a specialist membrane which allows water vapour to pass through, but not water in liquid form. The membrane is placed over an area of water. As the water evaporates through the membrane it is condensed on a nearby cooler surface. The gap between the membrane and condensing surface is typically a few millimetres, and a porous spacing material can be inserted to avoid the two surfaces touching. In a South African experiment to make a portable still based on membrane distillation (Sanderson et. al., 1994), a small unit was built with a plastic (pvc) cover containing a membrane bag (Teflon coated cloth), resting on a spacer to the plastic surface where it condenses. A canvas evaporative bag is placed behind the unit to cool the plastic condensing surface. Saline or brackish water needs to be heated before going into the membrane bag, where it is further heated by solar radiation passing through the plastic cover. Experiments were undertaken with heating the water before it enters the bag. At 50ºC preheating, production of treated water was about half that of a conventional solar still of equivalent area, at 60ºC production would be about equal for both processes and at 75ºC the membrane distiller would produce over twice the amount. Thus at the latter temperature it would be possible to have a distillation unit of less than half the area of a conventional solar still, but with equivalent output. Efficiency could be further improved by having several units in series with each other. Note, membrane distillation is a distinct effect based on the affinity of a membrane to release water vapour from warm or hot water it is in contact with. Simply putting hot water into a conventional solar still would not achieve the same level of efficiency improvement. It is not known, however, whether there are any in service desalination plants operating based on membrane distillation.

A separation method in which a nonwetting, microporous membrane is used with a liquid feed phase on one side of the membrane and a condensing, permeate phase on the other side. Separation by membrane distillation is based on the relative volatility of various components in the feed solution. The driving force for transport is the partial pressure difference across the membrane. Separation occurs when vapor from components of higher volatility passes through the membrane pores by a convective or diffusive mechanism.

Membrane distillation shares some characteristics with another membrane-based separation known as pervaporation, but there also are some vital differences. Both methods involve direct contact of the membrane with a liquid feed and evaporation of the permeating components. However, while membrane distillation uses porous membranes, pervaporation uses nonporous membranes.

Membrane distillation systems can be classified broadly into two categories: direct-contact distillation and gas-gap distillation. These terms refer to the permeate or condensing side of the membrane; in both cases the feed is in direct contact with the membrane. In direct-contact membrane distillation, both sides of the membrane contact a liquid phase; the liquid on the permeate side is used as the condensing medium for the vapors leaving the hot feed solution. In gas-gap membrane distillation, the condensed permeate is not in direct contact with the membrane.

Potential advantages of membrane distillation over traditional evaporation processes include operation at ambient pressures and lower temperatures as well as ease of process scale up.

SOLAR AND WASTE HEAT DESALINATION

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BY MEMBRANE DISTILLATIONCollege of EngineeringUniversity of Texas at El PasoEl Paso, TX 79968

http://www2.hawaii.edu/~nabil/solar.htm

Solar Powered Desalination Plant Using Reverse Osmosis

RADG - Remote Areas Development Group Murdoch University Perth AustraliaAreas of research; solar powered reverse osmosis desalination unit, low cost bacteriological water testing

Solar-powered Reverse Osmosis Desalination

The Remote Area Developments Group (RADG) at Murdoch University in collaboration with a local manufacturer, Venco Products Pty Ltd have developed a solar-powered reverse osmosis desalination unit ('Solarflow') specifically designed for remote areas. Initial research examined several renewable energy power supply options and due to portability, low maintenance and an output which matches demand, solar power was selected.

The unit is available in a 400 litre/day version with two possible recovery ratio options of 16 or 25% (Solarflow 40016 and Solarflow 40025). It has been designed to operate from a two panel photovoltaic array with built in maximiser to keep the solar panels at their optimum voltage of 30 volts. Efficacy can be improved by up to 60 percent with the use of a solar tracker.

The solar panels power a DC motor coupled to a high quality industrial gearbox which is capable of providing sufficient torque to run the unit even at low currents. The efficiency of the unit is also greatly enhanced by the novel energy-recovery system which allows the unit to operate with the minimum number of solar panels: the high pressure reject water is returned to the back of the piston to reduce the load on motor and gearbox, rather than going to waste.

The unit has recently been commercialised with twenty five units presently in operation through Australia and South East Asia. The 400 litre/day unit has been designed with small communities of up to 40 people in mind. A unit capable of meeting the requirements of larger communities of up to 150 people which can provide 1500 litre/day is currently in the prototype stage and under going performance monitoring before entering commercial production. The unit has received an energy-efficiency award from the Alternative Energy Development Board (AEDB) of Western Australia. The AEDB have also provided funding for the current research and development of this project.

Research at Murdoch University's Environmental Technology Centre (ETC) is currently underway to determine the performance of the units both under laboratory and field conditions over the longer term with marginal feed waters. This will allow an assessment of power supply, maintenance requirements and membrane life to enable the units to perform reliably in remote areas.

The Solarflow unit is now manufactured by Solar Energy Systems Ltd.

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The first solar powered RO in India The Barefoot College Tilonia CSMCRI Gujarathttp://uk.youtube.com/watch?v=Bx3sEppnU3cCentral Salt & Marine Chemicals Research Institute, Bhavnagar, Gujarat, India

Canary Islands Institute of Technology (ITC Canarias)Mr. Gonzalo Piernaviejae-mail: gpiernavieja@itccanarias.orgURL: www.itccanarias.or ITC has developed many projects at this location, with the main objective being the development of stand alone renewable energy-driven desalination systems which are able to produce fresh water at any location that has renewable energy potential. SDAWES (Sea Water Desalination (SWD) by means of an Autonomous Wind Energy System)

The majority are designed for coupling to a reverse osmosis plant only. One exception is the SDAWES (Sea Water Desalination by means of an Autonomous Wind Energy System) project: it consists in an off-grid wind farm with two wind generators which have 230 kW nominal power each, supplying electricity to threedifferent kinds of desalination systems:

Reverse Osmosis (RO): 8 plants with 25 cubic meters per day nominal production each, being connected or disconnected depending on the available wind powerElectro Dialysis Reversal (EDR): 1 plant with a production capacity of 200 cubic meters per day, using as feed water artificially produced brackish waterVapour Compression (VC): 1 plant with a production capacity of 50 cubic meters per day

Foundation for Water ResearchAllen House,The Listons,Liston Road,Marlow, Bucks,SL7 1FDTelephone +44(0)1628 891589Facsimile +44(0)1628 472711E-mail office@fwr.org.uk

DEVELOPMENT OF A SOLAR POWERED REVERSE OSMOSIS PLANT FOR THE TREATMENT OF BOREHOLE WATERReverse Osmosis, a process where an external hydraulic pressure is applied to a concentrated solution thus forcing pure water through a permeable membrane, is a novel technology used to provide purified water to industry and people. The process requires a high energy input for the high pressure feed pumps and has made it difficult to use the alternative energy sources such as those named in the first paragraph. The development and implementation of a solar powered RO unit will not only be of great benefit for communities in rural areas, but is also seen as a cost effective method of supplying potable water from brackish sources in disadvantaged and or remote areas. The decision was made to develop a pilot demonstration unit to evaluate the feasibility of the* combined technologies; as well as the operation, application and commercialisation in the local market.

The concept is relevant to areas where small communities are spread over large areas, where the high cost of erecting large desalination plants and reticulation of desalinated water, or alternatively the piping of fresh water from other sources, is neither practically nor economically viable. The use of solar panels, which generate the power required to drive the

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RO unit, constitutes an initial capital investment that can be written off over the lifetime of the. unit. Results gained from the test runs with the demonstration unit will significantly contribute toward the optimisation of future units and plants of increased capacity.

Wind Powered Desalination Plant Using Reverse Osmosis

Windwater.com trialed a churn (heater) made with polycarbonate and while it proved that it could heat the water we found it needed more strength. We are currently seeking quotes for a slightly larger heater made out of marine aluminium from which it could well be possible that a larger quantity of good water may be produced. May be able to produce 5000 litres a day instead of the original 1000.  Brenton CornishCharlie Madden DirectorsE-mail: charliem@bigbutton.com.au

A Wind-Powered System for Water DesalinationEyad S. HrayshatTafila Technical University, Tafila, Jord anInternational Journal of Green Energy, Volume 4, Issue 5 September 2007 pages 471 - 481A wind-powered reverse osmosis desalination system is proposed in order to assess the potential of the development of water desalination in Jordan. A simulation model for the prediction of the power delivered for a given value of wind speed is adopted. Based on the average wind speed data and salinity of the feed water, the amount of water that can be produced at eight different sites is calculated. According to the annual amount of water produced, the selected sites can be divided into three different categories. The first one, which includes Hofa and RasMuneef, is considered to be “adequate” for wind-powered reverse osmosis desalination. Its annual amount of water output forms about 57% of all water produced at all the eight sites combined. The second category, which includes Safawy, Twaneh, and Tafila, is considered to be “promising”. Its water output adds up to about 30% of all water produced at all sites. The third category, which includes Jurf AlDaraweesh, Aqaba, and Shoubak, is considered to be “poor”. Only about 13% of the water produced from all sites combined can be obtained from these three sites.

http://www.desline.com/articoli/4119.pdf

Solar electric powered reverse osmosis water desalination system for the rural village, Al Maleh: design and simulation Author: Marwan M. Mahmoud a Renewable Energy Research Centre, An Najah National University, West Bank, PalestinePublished in: International Journal of Sustainable Energy, Volume 23, Issue 1 & 2 March 2003 , pages 51 - 62 Desalination of brackish water by using reverse osmosis (RO) system powered by solar PV has not been tried and examined in Palestine until now. This paper proposes rural village Al Maleh for erection and testing of the first PV-powered RO system. Al Maleh is highly qualified for testing of such systems since it has a lot of mineral hot water springs of about 3400  ppm salinity. Based on the climate conditions in Al Maleh, the paper presents the design of the PV-powered RO water desalination system. The obtained design results can be used for an economic feasibility study of this technology [Mahmoud, M. Techno-economic feasibility of PV-powered water desalination in Palestine. Special Case: Al Maleh Village (to be published).]. The performance of the designed system is investigated by software simulation. The obtained results show that a daily production of 1m3 from the brackish water in Al Maleh would require about 820 peak watt of PV generator.

Bullock-Driven RO Process for Desalination of Brackish Water in Coastal VillagesCentral Salt & Marine Chemicals Research Institute, Bhavnagar, Gujarat, India

It is well known that the ground water in many coastal and inland areas is brackish and not

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potable. A need for appropriate desalination technology was therefore felt. Since many villages have bullocks with insufficient work during non-agricultural periods such as summer months, when the need for water is especially acute, and further given that electricity is perennially in short supply, a bullock operated reverse osmosis (RO) unit was considered to be an attractive option. Central Salt & Marine Chemicals Research Institute (CSMCRI), Bhavnagar, a national laboratory under Council of Scientific & Industrial Research, India, has developed such a unit. The idea was conceived by the Director of the Institute and designed by N. Pathak and his team. The photograph below shows the prototype unit in operation at the Institute’s premises.

A pair of bulls is connected to one side of a 4 m long mechanical link while the other side of the link is coupled to the input shaft of a gear box, comprising three sets of bevel helical gears. The gearbox is designed to convert bullock power in the form of low rpm (ca. 2 rpm) and high torque at the inlet shaft into mechanical power (equivalent to ca. 1.2 hp) of high rpm (200 rpm) and low torque at the output shaft. The output shaft is coupled to the crankshaft of the reciprocating high pressure pump, which discharges 20 LPM feed water at 30 bar hydraulic pressure. This hydraulic pressure is adequate to carry out desalination (the Institute uses its indigenously developed thin film composite spiral RO membrane elements) of feed water with up to 5000 mg/L TDS (total dissolved solids) to deliver 6-8 LPM permeate water containing < 500 mg/L TDS. Harmful elements such as fluoride, arsenic, nitrate and heavy metals are simultaneously removed along with other salts during the desalination process and the permeate water is also free from bacteria.

The unit, which costs approximately Indian Rupees 0.25 M (~ 3500 Pd. Stg.), can cater to the cooking and drinking water needs of 1000 villagers when operated for 6-8 hours per day. Two such units—one funded by the Indian Department of Biotechnology and the other by the Pepsi Relief and Rehabilitation Trust for earthquake affected villagers in Kutch (Gujarat)—will be installed in the coming months. The Institute is continuing its efforts to improve the performance of the unit and to further drive down capital cost.

CSMCRI’s Bullock-Driven RO Unit

Central Salt & Marine Chemicals Research InstituteGijubhai Badheka Marg, Bhavnagar-364002, Gujarat (INDIA)E-Mail: salt@csir.res.in, salt@csmcri.org Tel: 0278-2567760 / 2568923 / 2565106Fax: 0278-2567562 / 2566970E-mail: http://www.csmcri.org/

4. Use of Water from Desalination Plants

- Drinking WaterThe majority of output from desalination plants is used for drinking water. Some processes, such as distillation or reverse osmosis, will produce almost pure distilled or de-ionised water.

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Others, such as electrodialysis will produce water which still contains some dissolved salts but which is, nevertheless, safe to drink. Most people do not like to drink distilled water, as it can reduce the taste of beverages made with it, and a small amount of some mineral salts dissolved in water is said to have beneficial health attributes. Hence it is usual to add a small amount of mineral salts to distilled water if it is to be used as drinking water. With some types of inland brackish or saline water, if this is otherwise clean, it might be possible to add a small amount of this to the distillate. With seawater and some water from inland sources chemical dosing might be required.

Although animals would drink distilled water, it would also be better for them to drink water with a small dissolved content as well as this can be helpful to reduce illnesses.

- IrrigationDesalination is unlikely to be viable for irrigation purposes. This is because it is relatively expensive to treat water by desalination, most crops require large amounts of water for irrigation, and the price at which crops could be sold at would be unlikely to cover desalination costs. It has been estimated, for example, that to irrigate an area of a quarter of a hectare typically twice a month would require the same amount of water as for supplying drinking water for 1000 people, 250 cattle and 500 sheep (Stern, 1979). However, irrigation from desalination might be viable if this is for high value crops and uses efficient, although expensive, techniques such as drip irrigation

- HydroponicsHydroponics is the process of growing crops or plants without soil. Plants are grown in an inert mineral medium which is flushed periodically with water containing nutrients. Hydroponics is generally undertaken indoors and, sometimes, under artificial light. It is a suitable, although expensive, growing technique in areas where water is scarce as it has been estimated that it requires about a tenth of the water for hydroponics compared with conventional farming of the same crop. Serious hydroponics growers use distilled water as the base for making up the nutrient and mineral feed for plants, as then it is possible to determine the amount of minerals fed to the plants precisely. A number of proprietary small-scale desalination plants, generally based on reverse osmosis, have been developed specifically for the hydroponics market.

- Vehicle BatteriesDistilled water used to be in high demand for topping up lead-acid batteries for vehicles. However, with the more widespread use of sealed for life batteries, it is not known if this demand is still as high. It used to be possible for a small entrepreneur with a solar still to make a reasonable income from making and selling distilled water for batteries. In Sudan in the 1980s, after researchers visited a pilot desalination project several years after it had been set up, they found that the project was no longer operating, but a garage owner had built and set up his own still for distilled water and, apparently was barely able to keep up with demand for water for batteries.

- Health ServicesDistilled water is used by hospitals in a variety of applications, but often in blood transfusions and, laboratory testing of blood and cells, and for preparation of microscope specimens.

- LaboratoriesDistilled water has a variety of uses in laboratories, but especially in chemical analysis. Small-scale proprietary units for making distilled water for laboratories, generally based on reverse osmosis, have been developed.

- CosmeticsDistilled, or at least almost pure, water is specified in many cosmetic formulations to achieve products with the required characteristics.

- Use of the Solid ResidueThe solid product left behind after distillation processes might in some cases be too impure to be used for flavouring food for human consumption, or for industrial or chemical processes,

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though salt from seawater desalination would generally be acceptable for human consumption. The residue would be suitable, in most cases, for animals, for example as cattle lick.

5. Further Information

Some further details on information sources, projects, products and contact details, not already referred to on desalination are given here. This is a selection of information, and by no means a comprehensive list.

Articles on desalination

There was a very good article in the July/August 2001 issue of ReFocus called Water Water Everywhere. Desalination Powered by Renewable Energy Sources written by Professor D. Assimacopoulos.

He talks about the potential of renewable energy sources for desalination and gives a description of desalination options similar to the information I've included below.

The contact details provided in the articleProfessor D. AssimacopoulosNational Technical University of AthensDepartment of Chemical EngineeringSection II, Heroon Polytechniou St, 9,Zografou Campus, GR 15780, Athens, Greece.

We also have within the Library reports on solar stills produced in Sudan which describes the manufacturing process.

Waterlines Technical Brief on desalination

Appropriate Technology Vol.19 No 3 page 34 Multiple-wick solar still

New Scientist. 26 Jan 2002 page 41 Growing vegetables in the desert…

VITA Technical Paper No 37 Understanding Solar Stills

http://www.cdc.gov/safewater/manual/1_toc.htm

Technical Brief No 40 from Waterlines covers Desalination and, it covers some low-technology and high-technology solutions. It can be downloaded fromhttp://info.lut.ac.uk/well/resources/technical-briefs/40-desalination.pdfHe may also be interested in the two Technical Briefs on Household Treatmentwhich are at:http://info.lut.ac.uk/well/resources/technical-briefs/58-household-water-treatment-1.pdfandhttp://info.lut.ac.uk/well/resources/technical-briefs/59-household-water-treatment-2.pdf

- General Reference Sources

1. Desalination is a journal which describes developments, applications and research on desalination. It is published by Elsevier Science. A small number of articles, including the ones cited in the References section can be downloaded as .pdf files from the Internet without charge. For an index of articles and contributors see :-http://www.desline.com/index

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2. The Encyclopedia of Desalination and Water Resources (DESWARE) is a comprehensive sourcebook on almost all subjects relevant to desalination. It is available on-line or on CD-ROM, but there is a subscription fee, currently US$ 180 for two years for a single user, $540 for five users and $1200 for an unlimited number of users. For further information :-http://www.desware.net/desware/des1.asp

3. The Source Book of Alternative Technologies for Freshwater Augmentation in Latin America and the Caribbean, contains detailed presentations on desalination technologies as well as other water resourcing techniques such as rainwater harvesting, fog harvesting, wastewater reuse, disinfection and filtration, as well as a number of case studies. The sections on distillation and reverse osmosis are quite informative. It is downloadable free of charge :-http://www.oas.org/usde/publications/Unit/oea59e/begin.htm

4. International Desalination Association has a comprehensive Website which includes a database, publications list, links and details of programmes and events :-www.ida.bm

5. Desalination Directory Online offers a comprehensive Website which includes details on consultants, companies and suppliers, events, associations, publications and news :-www.desline.com

6. European Desalination Society, has a Website which lists companies, consultants, events, news, etc., mostly related to Europe :-www.edspc.com

7. Middle East Desalination Association, has a Website with details on events, research in progress, publications, links, etc. :-www.medrc.org.om (note, this address is correct)

8. US Bureau of Reclamation - use the Search facility on the Website Homepage with the term Desalination to bring up about 70 references to links or documents on desalination, some of which are more useful than others :-www.usbr.gov

9. Water Research Commission (based in South Africa) – use the select-term Search facility of the Website for several references on desalination in the Publications and Databases areas :-www.wrc.org.za

10. World Wide Water, some limited details on desalination; the Database area does not seem to be available currently :-www.desalination.com

11. Sasakura Engineering Company Ltd., a Japanese based company which has installed a number of mostly large or very large-scale desalination plants throughout the world. Also has offices in Bahrain, Hong Kong and Indonesia. Main products are multi-stage flash, reverse osmosis, multi-effect distiller, vapour compression and emergency water making plants :-7-32 Takejima 4-chomeNishiyodogawa-kuOsaka 555-0011JapanTel. +81-3-5566-1212; Fax. +81-3-5566-1233; http://www.sasakura.co.jp/sasakura2/html

The Desalination Directory www.desline.com

Books on desalination

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Advance Solar Distillation SystemsG.N. TiwariCentre for Energy StudiesIndian Instituet of Technology Haus KhasNew Delhi – 110016 India

The various organisations involved in desalination

The Energy Group,Department of Engineering,The University of Reading,Whiteknights,Reading,Berkshire RG6 6AY UKTel: 44 (0)118 931 8565, or 44 (0)118 987 5123 (ext. 7560)Fax: 44 (0)118 931 3327E-mail: Energy. Group@Reading.ac.ukMaria Vahdati comes from a Chemical Engineering background and has worked on both membrane distillation and clean combustion systems.

CREST - Centre for Renewable Energy Systems TechnologyDepartment of Electronic & Electrical EngineeringLoughborough University of TechnologyLeicestershireLE11 3TUUnited KingdomTel: +44 (0)1509 223466Fax: +44 (0)1509 222854Contact: Hilary Thompson E-mail: H.A.Thompson@lut.ac.ukCREST undertake training and research into renewable energy systems including photovoltaics and water distillation.

Solar stills have been promoted by the Rural Industries Innovation Centre based in Botswana.

Brace Research Institute, Macdonald Campus, PO Box 900, SteAnne de Bellevue, Quebec HDX 1C0, Canada E-mail: AE12@musica.mcgill.caContact for solar disinfection and distillation: T.A.Lawand,

The Pakistan Desalination Association(PakDA), E-16/2, Block 7, Gulshan Iqbal, Karachi 75300, Pakistan Tel: & Fax: 011-9221-497-3429 They are affiliated to the InternationalDesalination Association (IDA) at http://www.ida.bm and should be able to offer advice.

Bharat Heavy Electricals LtdElectronics Division Post Box No. 2606 Mysore RoadBangalore 560 036IndiaTel: +91 812 624 283Fax: +91 812 623 137Products: Cells, modules, systems for signaling, lighting, tele-communications, desalination, water pumping, battery charging

RADG - Remote Areas Development Group

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Murdoch University Perth AustraliaTel: +61 (0)8 9360 7310 E-mail: etc@murdoch.edu.au Website: http://wwwies.murdoch.edu.au/etc/pages/radg/radgprojs/rproj21.html

Areas of research; solar powered reverse osmosis desalination unit, low cost bacteriological water testing.

We received some literature commercial designs produced by NPO mashinostroyenia of Russia.

Solar Distillation

AquaPak Solar Solutions9950 Scripps Lake Drive, Suite 105San Diego, California, 92131USATel. 858-695-3806; Fax. 858-695-3807; Email: contact@solarsolutions.info; http://www.solarsolutions.info/products/productsright.htmlA small lightweight cone-shaped solar distiller of typical output 1.5 to 2.5 litres per day.

Avmar Ltd.12 Church StreetSouthportMerseyside, PR9 0QTUnited KingdomTel. +44 (0)1704 532649; Fax. +44 (0)1704 543557; Email info@avmar.co.uk; http://www.avmar.co.uk/stills.htmMake a small lightweight plastic still for marine use or personal survival.

Bangladesh University of Engineering & Technology (BUET), pilot solar desalination plants in Bangladesh (Rahman et. al. 1997), reports on tests undertaken on low cost brick or clay stills. Yields, however, were relatively low, 1.4 and 0.7 litres per day, respectively for the brick and clay stills.

Cussons Solar Still 102 Great Clowes StreetManchester, M7 1RHUnited KingdomTel. +44-(0)161-833-0036; Fax. +44-(0)161-834-4688; Email: sales@cussons.co.ukhttp://www.cussons.co.uk/p7130.htmlbasin and thin film types with outputs of around 5 litres per day :-Cussons

El Paso Solar Energy Association (EPSEA), has an informative Website on solar stills and other solar and green technologies, including details of the projects on installation of solar stills undertaken in Texas and Mexico, images of stills and useful background information on solar distillation (downloadable as a .pdf file). Detailed plans and construction details of solar stills can be ordered at a small cost.EPSEAP.O. Box 26384El PasoTexas 79926USAhttp://www.epsea.org/

Lion Energy, A Greek company which has developed a solar-based system of saline water

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distillation which incorporates mechanical vapour compression for improved efficiency. The plants are large-scale for the type of technology used, typically in the range 100 to 500 cubic metres per day. Plants are 1000 square metres or more in area. Cost details are not provided on the Website, though the process is claimed to be cheaper than reverse osmosis on a similar scale.Liapis Corporation7A Pentelis Ave.15235 AthensGreeceTel. +301-68-48-882; Fax. +301-68-48-849; Email: support@liapis.com; http://www.lionerergy.net/od22_m.htm

Mauritius – Solar Water Desalination in Preselected Coastal Villages in Rodrigues – This is a project of the United Nations Development Programme (UNDP) GEF (Small Grants Programme) fund to assess the viability of solar stills to provide water requirements in coastal villages of Rodrigues, particularly to relieve the burden of women, who spend 3 to 5 hours per day collecting water.http://www.undp.org/sgp/cty/AFRICA/MAURITIUS/pfs143.htm

McCracken Solar Company – designers and installers of small to medium-scale solar stills, e.g. a three units of approx. 1 x 3 metres each, with a total capacity of 50 litres per day for seawater distillation for a household on the Bahamas. Pumps and storage tanks are included, if necessary. Contact :-Horace McCracken329 West CarlosAlturasCA 96101USATel – 916-233-3175http://www.ibiblio.org/pub/academic/environment/alternative-energy/energy-resources/homepower-magazine/archives/10/10pg29.txtSee also under McCracken in the list of references below

MSS (Pty) Solar Stills – South African company which supplies solar stills of size 2 x 1 metres, and upwards. Stills can be supplied for production up to 1000 litres per day, if a number of stills are installed together. MSS (Pty) Ltd.,P.O. Box 3067MatielandSouth Africa – 7602http://home.intekom.com/canichem/html/stills.html

NPO Mashinostroyenia, is a Russian company producing small solar desalination units, from 0.85 to 12 square metres in area, producing 7 to 80 litres of water per day. The smaller units can be demounted for ease of transportation.NPO MashinostroyeniaReutovGagarin Str., 33Moscow Region, 143952RussiaTel. +7 (0)95 528-5737 or 528-4066; Fax. +7 (0)95 302-5090

Seawater Greenhouse – described in text; contact Light Works Ltd.2a Greenwood RoadLondon E8 1ABTel. +44-(0)171-249-3627; Fax. +44-(0)171-254-0306

Solarshopee – suppliers from India of stainless steel stills of 3 to 5 litres per day, or masonry stills of around 60 litres per day :-

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Creative Marketing CompanyBungalow no. 85Wanowrie BazarPuneTel. (020) 6874735; Email: cmc@solarshoppee.comandSolartrap Solar Systems47 Birmal NagarShankar Seth RoadPune 411 037Tel (020) 6351228; Email: soltrap@solarshopee.comhttp://www.solarshopee.com/solar/sol_distillation.htm

13. Technion – Prof. Gershon Grossman of the Faculty of Mechanical Engineering, Technion, in Israel is undertaking a research project on utilizing air circulation within a special design of solar still to recover part of the heat evolved when water vapour condenses on a surface.Faculty of Mechanical EngineeringTechnion – IITHaifa 32000IsraelTel. +972-4-829-2074; Fax. +972-4-832-4533; Email: mergr01@tx.technion.ac.il; http://www.technion.ac.il/rdl/Solar_Energy.html

Reverse Osmosis

1. Aqua FX Reverse Osmosis Systems, a range of small-scale RO systems, mostly used for hydroponics or aquarium applications. Outputs range from 250 to 1000 litres per day, with a cost range of £130 to £650. Units incorporate from 3 to 9 RO stages. Note – cost of pump and power supply not included in price. Aqua FX Inc.7224 Sandscove Court, Suite 2Winter ParkFlorida 32792USATel. +1-877-383-3474

Or

Aquarium CityMaple RoadHalesowenBirmingham B62 8JPUKTel. 01934-624122http://www.btinternet.com/~aquariumcity/filters/Reverse_osmosis_4Stage.htm

2. Assimacopoulos, D. (Professor), contact for the experimental wind driven RO plant at Syros, Greece, mentioned in text :-National Technical University of GreeceDepartment of Chemical EngineeringSection II, Heroon Polytechniou St., 9Zografou CampusGR 15780, AthensGreece

3. Bhabha Atomic Research Centre, have been involved in installation of tens of small to medium scale RO plants in India, with capacities of 20 to 300 cubic metres per dayDr. B.M. MisraHead, Desalination DivisionBhabha Atomic Research Centre

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Mumbai 400 085Tel. 022-5505184; Fax. 022-5505151; Email bmmisra@magnum.barc.ernet.in

4. CDER (Centre De Developpement Des Energies Renouvelables) in Morocco are developing a 12 cubic metre per day RO plant, powered by solar photovoltaics and funded by the EU Intersudmed ProgrammeCDERRue Machaar El HaramQuartier IssilBP 509 GuélizMarrakechMorocco Tel. (212) (04) 30-98-14/22; Fax. 30-97-95; Email: cernet@cybernet.net.ma; http://www.jrc.es/projects/intersudmed/PROJECT11.html

5. CREST (Centre for Renewable Energy Systems Technology), are undertaking research on a small hybrid renewable energy RO system, which can operate on both wind or solar photovoltaic power, and the intention is also that the unit can operate without battery storage of electrical power. Output would be 2 to 3 cubic metres per day. See also the paper by Thomson, et. al., in the reference section.

CRESTDepartment of Electronic and Electrical EngineeringLoughborough University of TechnologyLeicestershire LE11 3TUUKTel. +44 (0)1509 223466; Fax. +44 (0)1509 222854; Email: M.Thomson@Lboro.ac.uk

6. Dulas Engineering, are developing a solar photovoltaic powered RO system of output of about 3 cubic metres per day, which uses no battery storage and has an innovative system of energy recovery :-

Dulas Ltd Unit 1, Dyfi Eco Parc, Machynlleth, Powys, SY20 8AX, Wales, UK Tel. +44 (0) 1654 705000; Fax. +44 (0) 1654 703000; Email: dulas@dulas.org.uk

7. Solarflow RO Units – small-scale RO systems powered by solar photovoltaics have been developed by Murdoch University and Solar Energy Systems in Australia. Outputs are 400 to 1500 litres per day and units have been installed with the Aboriginal communities in Australia and on Java in Indonesia. The units incorporate piston pumps and a power maximiser to increase efficiency.

Solar Energy SystemsPO Box 1650Osborne Park DC 6916 Australia Tel. +61 8 9204 1521; Fax. +61 8 9204 1519; Email: info@sesltd.com.au; http://www.sesltd.com.au/html/waterpure.htmOr :-

Remote Area Developments GroupInstitute for Environmental ScienceMurdoch UniversityWestern Australia 6150Email: mathew@essun1.murdoch.edu.auSee also Mathew in references list

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8. Spectra Watermakers, are a company which make small RO systems, generally for the marine market. Capacities range from just under 200 to just over 1000 litres per day. Use of the patented Clark pump improves efficiency.Spectra Watermakers20 Mariposa RoadSan Rafael, CA 94901USATel. 415.526.2780; Fax. 415.526.2787; Email: Spectra@SpectraWatermakers.com; http://www.spectrawatermakers.com/overview.html

9 Vari-RO Solar-Powered Desalting Technology; which is a system for desalination using the patented Vari-RO system of powering a RO unit using solar power. The unit has a solar dish or solar photovoltaic collector combined with a direct drive or Stirling engine. Solar trough collectors have also been used. Significant improvements in efficiency over conventional RO systems have been claimed.Vari-Power Company582 Rancho Santa Fe RoadEncinitas, CA 92024USATel. +1-760-753-19845; Fax. +1-760-753-2453; Email: eChilds@sdcoe.k12.ca.us(See also paper by Childs et. al. In the reference section)

- Electrodialysis / Nanofiltration

Electrosynthesis Co. Inc., suppliers of electrodialysis membranes.72 Ward RoadLancaster, NY 14086-9779USATel. 716-684-0513; Fax. 716-684-0511; Email: sales@electrosynthesis.com; http://www.electrosynthesis.com/ess/weid.html

University of New South Wales, have developed a small nanofiltration unit to treat water with TDS values of around 3000 TDS. For higher TDS values a RO system can replace the nanofiltration unit. Experimental units have been set up in the Australian desert where groundwater is often brackish. Dr. Andrea Schäfer, School of Civil and Environmental Engineering, University of New South Wales, AustraliaEmail: a.schaefer@unsw.edu.au; www.civeng.unsw.edu.au/research/solr

Note :- Electrodialysis is used for treatment of range of other liquids, e.g. waste effluents, so details on a number of suppliers are available on the Internet. General sources of reference, given above, would also be useful for details on electrodialysis equipment suppliers specifically for aqueous sources.

Academic Research on DesalinationThese are some institutions, not already mentioned, at which some research has, or is currently, taking place.

Low Pressure Solar Distillation Plant, a collaborative research project on a low pressure distillation plant using a 10m high evaporator to improve efficiency :-A.K. Lalzad, G. Maidment & T.G. Karayiannis, School of Engineering Systems and Design, South Bank University, 103 Borough Road, London SE1 0AA;I.W. Eames, Institute of Building Technology, University of Nottingham, University Park, Nottingham NG7 2RD;P. Panayiotou, Dept. of Mechanical Engineering, University of Sheffield.Paper on the above presented at 2nd International Heat Powered Cycles Conference (HPC’O1) – Cooling, Heating and Power Generation Systems, 2 – 7 September 2001, Paris, France.

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Renewable Energy for Desalination in Jordan, a collaborative project between institutions below; project comprises assessment of current situation in Jordan on water supply, choice of technology and site selection. This is currently ongoing :-- Research Institute for International Technical and Economic Cooperation, Aachen University of Technology (RWTH), Ahornstrasse 55, D – 52056 Aachen, GermanyTel. +49-241-88947-0; Fax. +49-241-8888-284Prof. Dr. W. Gocht, Dilp.-Ing. A Sommerfeld, Dipl.-Kfm. M. Herné- Solar Institute Jülich, Fachochschule Aachen, Ginsterweg 1, D – 52428 Jülich. GermanyTel. +49-2461-689236; Fax. +49-2461-689235Prof Dr. M. Meliss, Dr. A. Neskakis- Institute for Chemical Engineering, Aachen University for Technology (RWTH), Turnstrasse 46, D – 52056 Aachen, GermanyTel. +49-241-805470; Fax. +49-241-888-252Prof. Dr. R. Rautenbach & Dipl.-Ing. L. Eilers- Royal Scientific Society (RSS), Renewable Energy Research Centre, P.O. Box 9258198, Amman, JordanM. Kabariti, M. Eng.

University of ReadingThe Energy GroupSchool of Construction Management and EngineeringThe University of ReadingWhiteknightsReading, RG6 6AY, BerkshireUKTel: +44 (0)118 378 8563 Fax: +44 (0)118 931 3327 Email: Energy.Group@Reading.ac.ukHave undertaken undergraduate and postgraduate student projects on desalination, mostly on reverse osmosis.

University of SussexDr. C.A. LongSchool of EngineeringUniversity of SussexFalmerBrighton BN1 9QTTel: +44 (0)1273 678945 Fax: +44 (0)1273 678399 Email: C.A.Long@sussex.ac.ukSolar desalination plant

6. Summary and Conclusion

There has been renewed interest in desalination technologies in recent years due largely to development in areas where ensuring adequate water supplies by other means would be otherwise difficult. This has been due both to increasing tourist movements, especially the opening up of previously little visited areas, and general pressures from population growth and migration. The current level of urban and commercial development in many of the states around the Arabian Gulf would have been impossible without significant involvement in provision of water through desalination from seawater. An additional factor is the growing salination of some sources of groundwater due to too rapid abstraction, without allowing the resource to recover by natural replenishment.

Where other methods of freshwater supply, e.g. rainwater catchment and storage, abstraction from a river, lake or natural spring, or through lifting from a satisfactory groundwater supply, are readily available, desalination would be likely to cost several times utilising water from these sources, so would not be viable. Even bringing in water by truck can be more economical, unless there is a need to do this over long distances of more than tens of kilometres. However, in situations where there are little or no other viable options, then desalination can be a viable process. Such situations can include :- Islands which receive comparatively little rainfall and do not have adequate resources of

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clean groundwater Isolated communities in areas of sparse rainfall, particularly if located on coasts or where

the groundwater is saline or brackish Self-contained tourist developments, especially if relatively isolated and located on the

coast in arid areas Arid areas undergoing rapid economic development and population growth.Additionally, where some form of desalination is already taking place, e.g. crude distillation using wood or charcoal for fuel, as was the case in Botswana (Yates, Woto & Tlhage, 1990), the likelihood of introducing improved desalination successfully, such as with more efficient solar stills, is greater. Another factor, in the case of tourist and middle class developments, is that if there is some dependence on bottled spring or mineral water brought in from long distances away for drinking water, desalination would almost certainly be likely to be viable as it would then be the considerably cheaper option.

Although large-scale plants of around 100,000 cubic metres per day capacity, and more, are operating in a number of countries, the focus of this report is on smaller to medium scale operations, generally producing 0.01 (10 litres) to 50 cubic metres per day. At this scale of production, there are two, or possibly three possible methods – solar distillation, reverse osmosis, or for brackish water with a TDS (total dissolved solids) value of less than 3000 – electrodialysis. Although innovative developments, such as smaller-scale multi-effect distillation and use of solar troughs for distillation are taking place, which could become significant in a few years time, the current most promising areas of research and development at present are reverse osmosis or electrodialysis systems driven by renewable (wind or solar photovoltaic) energy and heat recovery and increased efficiency of solar-powered distillation systems. A number of experimental units have been set up based on these types of processes.

Conventional solar distillation has the disadvantage of needing large areas of solar collectors to obtain significant volumes of freshwater, so they can be relatively cost and maintenance intensive. On the other hand relatively small units of a few to tens of litres per day capacity can be built locally, and the level of knowledge and skills needed to maintain them is probably less than for a conventional handpump, which is undertaken quite successfully by local communities in a number of rural and urban locations. One particular niche application for small solar stills is where people depend on brackish water from shallow wells. The still could be used to treat this water. If this community eventually get access to supplies from a deeper borehole with cleaner water, the still can be used elsewhere, so the need for such a still to be relatively lightweight, demountable and transportable would be important. Another factor to consider is that, except where the system is very simple and people can simply carry water from the source to the still or it can be gravity-fed, a pump would be needed to take water to the still. Also most people would not want to drink distilled water, so some of the original brackish or saline water has to be mixed back in, or chemicals added to give an acceptable taste to the water. The solar collectors are glass or plastic. Glass is preferred.

Solar stills made of masonry would be the cheapest and easiest to build locally. Other materials such as stainless steel and fibreglass plastic, would also be possible, but would add to the cost and would need to be manufactures at a central workshop. A number of improvements to conventional solar stills have been trialled to improve efficiencies and reduce the areas occupied by solar collectors for a given water output. These improvements include, reflective coating of surfaces inside the still, fan-assisted evaporation, increasing the surface area for evaporation in the still and moving the collector to follow the solar trajectory. The improvements introduce additional cost elements which would only be justified if these are compensated for by an equivalent increase in water output.

There has been much recent interest and innovation in reverse osmosis. This has tended to improve efficiencies, reduce costs and increase the scope of the technique. This trend is likely to continue. Main developments are concentrated on better membrane formulations, increasing energy recovery from pumping and power supply systems, especially from renewable sources. Nevertheless, reverse osmosis is quite a sophisticated process and is unlikely to be suitable for many remote village communities in developing countries. Although maintenance requirements are not high, it would need a good understanding of the system to

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spot potential problems and take remedial actions. Maintenance problems would be likely in the pumping system, the membrane unit, and electrical and hydraulic connections and control systems. RO systems operate at very high hydraulic pressures, 15 bar or greater, which requires a knowledgeable person to look after the system. Additionally, RO systems need water to be fed to them which is free from sediment and biological matter, or the pores in the membranes become blocked up and efficiency falls. This requires the provision of an additional filter. Scaling in the system can also be a problem and periodic cleaning would be required. Due to installation and maintenance costs becoming proportionally higher the smaller the unit, RO was previously not considered a financially viable option at a scale of below about 10 cubic metres per day capacity, except in specialist applications such as on board boats. However, if the current prototype developments come on stream, smaller-scale systems would be likely to become more viable.

Electrodialysis is generally a cheaper and simpler system than reverse osmosis, principally because high pumping pressures are not required. However, energy consumption is not significantly less than with reverse osmosis, and can be higher. Electrodialysis is generally used for treating water with a TDS value of below 3000, which only covers brackish water. Several electrodialysis cells in series can be used for higher TDS values, but this greatly increases installation costs and makes the process less viable.

Desalination can be more viable if, in addition to producing drinking water, water, particularly distilled water, can be produced for other applications. It would be unlikely that desalination would be viable for conventional water for agriculture systems because of the high water needs for irrigation, but could be possible for more efficient and point-fed systems such as drip irrigation. Distilled water is also needed in hydroponic (without soil) cultivation, and in this application it is used much more efficiently than for conventional crop irrigation. Other possible applications for distilled water include in garages, e.g. for topping up batteries, in hospitals, health centres, for specialist aquariums and in making cosmetic products. Distillation and reverse osmosis will produce water that can be considered as distilled water. However, electrodialysis would produce water which would still have a small amount of distilled salts.

It would be advisable, though, to be cautious when considering introduction of desalination as part of a community-based development project. Costs in relation to other options would need to be considered, as well as how a system could be financed and maintained. Rather than opting for desalination as a first choice option immediately it would be important to look at needs, assets and problems of a particular community holistically, considering also impacts on eco-systems and political, institutional and community support structures, e.g. from NGOs. Such an analysis can highlight whether a desalination ought to be a priority and, possibly, foresee potential problems which can constrain such a project. One possible difficulty would be the availability of spare parts and components and specialist input locally. If these need to be provided from the North, the project might not be viable or sustainable. Another aspect of sustainability is on whether desalination is a short-term or a more longer-term solution. If, for example, an aquifer has become brackish or saline due to excessive extraction of water, introducing desalination can make the problem worse, leading to irreversible degradation of the aquifer and possible soil degradation leading to loss of agricultural output. However, small-scale low impact projects, such as small solar stills, can make up for water shortages for poor people and contribute to their survival strategies.

The table below presents some comparisons between different desalination techniques and aims to assess each technique against a number of criteria of applicability and suitability for small to medium scales of production – 20 litres to 50 cubic metres. On the various criteria the author has attempted to give a score for each technique ranging from 0 – completely unsuitable or rating very poorly, to 5 for being completely suitable and an extremely good option.

Critical Factor – score Desalination OptionSimple Solar

Distillation

Multi-Effect Solar

Distillation

Solar Trough

Distillation

Reverse Osmosis

Electro-dialysis

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Proven technology at small-medium scale

4 2 1 3 3

Potential for future development 2 2 2 3 3Applicability over a large range of scales of production

3 2 2 3 3

Range of salinity treated 5 5 5 4 2Use of renewable energy 5 4 5 4 3Space requirement 1 3 3 4 4Capital cost 1 - 41 2 2 3 3Operating cost 22 2 3 4 4Reliability 3 2 2 4 4Appropriateness for developing communities

3 1 1 1 1

Portability 3 2 2 4 4Extensive commercial or institutional support structure not required

4 2 2 2 2

TOTAL 36 - 39 30 30 39 361 Depending on whether an expensive material, e.g. stainless steel, or a cheap material e.g. masonry bricks, used for construction2 Relatively low figure due to high maintenance requirement

Note – not all the above criteria would be equally applicable in a particular situation where there is a water supply need.

7. References

Assimacopoulos, D., Water, Water Everywhere … Desalination powered by renewable energy sources, REFOCUS, July / August 2001, pp. 38 – 41, www.re-focus.net

APCTT (Asian and Pacific Centre for Transfer of Technology), Ref. ID: APC-0797-TO, http://www.apctt.org/database/to0797.html

Ashley, Caroline & Carney, Diana, Sustainable Livelihoods: Lessons from early experience, Department for International Development, London, 1999, ISBN 0 85003 419 1

Childs Willard D., Dabiri Ali E., Al-Hinai Hilal A. & Abdullah Hussein A., VARI-RO Solar-Powered Desalting Technology, Desalination 125 (1999) 155 - 166

Commonwealth Science Council, Background Information to the Discussion on Desalination, http://www.commonwealthknowledgenet.net/Desalntn/binfdsal.htm

Commonwealth Science Council, Summary of the Discussion on Desalination, 1999, http://www.commonwealthknowledgenet.net/Desalntn/sumdsion.htm

Davis, Jan & Brikke, François, Making your Water Supply Work: Operation and maintenance of small water supply systems, Occasional Paper Series, No. 29, The International Water and Sanitation Centre, The Hague, Netherlands, 1995

Elmidaoui A., Elhannouni F., Menkouchi Sahli M.A., Chay L., Elabassi H., Hafsi M. & Largeteau D., Pollution of Nitrate in Moroccan Ground Water: Removal by electrodialysis, Desalination 136 (2001), pp. 325 - 332

Mathew Kuruvilla, Dallas Stewart, Ho Goen & Anda Martin, Innovative Solar-Powered Village Potable Water Supply, Women Leaders on the Uptake of Renewable Energy Seminar, Perth, Western Australia, June 2001, http://acre.murdoch.edu.au/unep/papers/Mathew.pdf

McCracken, Horace & Gordes, Joel, Understanding Solar Stills, Volunteers in Technical Assistance (VITA), Arlington Virginia, USA, 1985,

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MEDRC, Innovative Small Desalination Systems Hybrid Fossil / Solar Heated Multi-Effect Still, http://www.medrc.org/technical/es/97-BS-016.html

Misra, B.M., Head, Desalination Division, Bhabha Atomic Research Centre, Mumbai 400 085, Email – bmmisra@magnum.barc.ernet.in, recent papers sent by author

New Scientist, 16 January 1993, pp. 21

New Scientist, 26 January 2002, pp. 41 - 43

Noppen, Dolf (Ed.), Village Level Operation and Maintenance of Handpumps, Experience from Karonga, Malawi, The International Water and Sanitation Centre, The Hague, Netherlands, 1996

García-Rodríguez, Lourdes and Gómez-Camacho, Carlos, Preliminary Design and Cost Analysis of a Solar Distillation System, Desalination 126 (1999), pp. 109-114

Rahman M.H., Mamtaz R. & Ferdausi S.A., Pilot Solar Desalination Plants in Bangladesh, in Water and Sanitation for All: Partnerships and Innovations, 23rd WEDC (Water and Engineering Development Centre, Loughborough University, UK), Durban, South Africa, 1997

Scharl, A. & Harrs, K., Solar Distillation, GATE / GTZ Technical Information W11e, 1993, downloadable from GATE Website – http://www.gtz.de/gate

Spectra Watermakers, http://www.spectrawatermakers.com/

Stern, Peter, Small-scale Irrigation, Intermediate Technology Publications Ltd., London, 1979, ISBN 0 903031 647

Teplitz-Sembitzky, W., The Use of Renewable Energies for Seawater Desalination – A brief assessment, GATE / GTZ Technical Information W16e, Sept. 2000, downloadable from GATE Website – http://www.gtz.de/gate

Thomson Murray, Miranda Marcos S. & Infield David, A Small-scale Seawater Reverse Osmosis Systems with Excellent Energy Efficiency Over a Wide Operating Range, CREST (Centre for Renewable Energy Systems Technology), Loughborough University, Loughborough LE11 3TU, UK, Email. M.Thomson@Lboro.ac.uk

UNEP / SOPAC, Source Book of Alternative Technologies for Freshwater Augmentation in Small Island Developing States, Small Island Water Information Network, http://www.siwin.org/reviews/swr0004/swr0004b42.html

UNEP – International Environmental Technology Centre, United Nations Environment Programme, Source Book of Alternative Technologies for Freshwater Augmentation in Latin America and the Caribbean, Unit of Sustainable Development and Environment, General Secretariat, Organization of American States, Washington D.C., 1997, Section 2.1 – Desalination by Reverse Osmosis, http://www.oas.org/usde/publications/Unit/oea59e/ch20.htm

UNSW, School of Civil & Environmental Engineering, University of New South Wales, Solar Desalination – A student driven project, 2001, http://www.civeng.unsw.edu.au/research/solar/background.html

USBR (United States Bureau of Reclamation), The Water Treatment Technology Program Newsletter, No. 3, Winter 1995, USBR, Denver Colorado, http://www.usbr.gov/water/newsletters/c_95win.html

Wade, N. & Callister, K., Desalination: The state of the art, J.CIWEM, 11, April 1997, pp. 87 - 97

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Water, Engineering and Development Centre (WEDC), Technical Brief No. 40: Desalination, in Waterlines, Vol. 12, No. 4, April 1994, pp. 15 – 18

World Water, July 1982, pp. 36

Yates R., Woto T. & Tlhage J.T., Solar-Powered Desalination - A case study from Botswana, International Development Research Centre, Ottawa, Canada, 1990, ISBN 0-88936-554-7

Progress review of solar desalinationThe MEDESOL project has released its public deliverable "Critical assessment of the state-of-the-art and bibliographic review on membrane distillation technology, solar collector technology and low-fouling heat transfer modified surfaces". The document's purpose is to address the project's relevant aspects of conventional desalination technologies, costs of reverse osmosis plants connected to the grid, renewable energy powered desalination, solar resources and databases available for EU and Mexico, solar collector technology, membrane distillation technology, thermal storage systems for storing solar heat and state-of-the art of low-fouling heat transfer surfaces. The document is available on http://www.psa.es/webeng/projects/medesol/results.html  

"State-of-the-Art of Reverse Osmosis desalination", C. Fritzmann, J. Lowenberg, T. Wintgens and T. Melin (Aachen, Germany)A sound and extensive state-of-the-art review of all scientific and technical aspects related to reverse osmosis desalination, including process theory and operational issues, has been conducted within the FP6 project “Technology enabled universal access to safe water” — TECHNEAU. The 76 pages article has been published in the journal Desalination 216(2007)1-76

http://www.circleofblue.org/waternews/2010/world/british-company-creates-cheap-small-scale-desalination-for-agriculture/British Company Creates Cheap, Small-Scale Desalination for AgricultureThe new system, which uses sub-surface pipes to remove salts and deliver water to plants on demand, grew 200 Prosopis trees in the United Arab Emirates’ desert during a test-run.A British company has developed an irrigation system that allows saline and brackish water, which contains more salinity than freshwater, to be used for growing crops, Wired reports.The Dutyion Root Hydration System uses a network of underground pipes to deliver water directly to a plant’s roots. Water then diffuses through the walls of the polymer pipe because of differences in moisture levels, which act as filters and leave contaminates behind. Almost any water source can be used–-even industrial wastewater–-without the need for secondary purification.

“What we’re looking to do is take our irrigation system and move to places where it’s not possible to irrigate today,” said the system’s designer Mark Tonkin of Design Technology and Irrigation. “[We] stumbled across a way of effectively desalinating water. We put pipe in the ground which lets water vapor to escape and the waste element is what gets left in the pipe.”The overall system is gravity fed and needs minimal maintenance while the pipes must be periodically flushed to clean out accumulated salts and dirt, Tonkin told Wired.Approximately 70 percent of the world’s freshwater is used for agriculture. Wide use of salt water for irrigation would free freshwater for other uses and increase food security for people living in dry coastal areas.

Brackish water is already being used to grow saltwater-tolerant plants for biofuels, but DTI is growing plants to eat–such as tomatoes, strawberries, peppers and beans as well as cherry and olive trees.

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“There are no plants that we’ve tried to grow that can’t survive simply by using water vapor as

opposed to having wet water put on them, and that is a major change,” Tonkin said in a

promotional video for LAUNCH, a forum for innovation. “It’s made it possible to put an irrigation system in the ground where there is no freshwater and no likelihood of anybody

building a desalination plant and grow plants where you couldn’t grow them today.”

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