Low cost desalination DEW Point Enquiry No. A0192
17th November 2008
Low cost desalination
Andy Bastable, Oxfam GB
Client contract No:
CNTR 07 8625
DEW Point Ref:
Contact and correspondence:
DEW Point, The Old Mill • Blisworth Hill Barns • Stoke Road • Blisworth • Northampton, • NN7 3DB • UK TEL: +44 (0)1604 858257 FAX: +44 (0)1604 858305 e-mail: [email protected]
Neil Noble and David J. Grimshaw (Practical Action Consulting)
Organisations undertaking Practical Action Consulting work:
Draft Final Report (this 21/11/2008 document) Final Report Reference:
Other, A.N, 2008, Report title. Version. DEW Point.
Task Manager :
Quality Assurance Internal:
Quality Assurance. DEW Simon Mercer Point:
Disclaimer This report is commissioned under DEW Point, the DFID Resource Centre for Environment, Water and Sanitation, which is managed by a consortium of companies led by Harewelle International Limited1. Although the report is commissioned by DFID, the views expressed in the report are entirely those of the authors and do not necessarily represent DFID’s own views or policies, or those of DEW Point. Comments and discussion on items related to content and opinion should be addressed to the authors, via the “Contact and correspondence” address e-mail or website, as indicated in the control document above.
Consortium comprises Harewelle International Limited, NR International, Practical Action Consulting, Cranfield University and AEA Energy and Environment
Table of Contents
Description of enquiry
The basics of desalination
Potential technologies and approaches to desalination
1. Description of enquiry Dear DEW Point, Across Oxfam GB's programmes we are facing increasing problems with saline water for poor rural communities. What we'd like to do is support communities facing salinity problems construct their own - household or shared - low cost desalination units. My question to DEW point is what are the latest developments in the low cost desalination sector and is there any technology previously trialled (or needing trialling) that Oxfam could install in poor communities in Turkana, Kenya , for example.
2. Methodology The response to this enquiry is mainly based on comments and informations kindly supplied by Neil noble and David J. Grimshaw from Practical Action Consulting (see part 4).
3. The basics of desalination WELL technical brief on desalination (technical brief No.40): http://www.lboro.ac.uk/well/resources/technical-briefs/40-desalination.pdf Sourcebook of Alternative Technologies for Freshwater Augmentation in West Asia, published by UNEP (see http://www.unep.or.jp/ietc/Publications/TechPublications/TechPub8f/index.asp). The document contains a chapter on water quality improvement and on water desalination technologies: http://www.unep.or.jp/ietc/Publications/TechPublications/TechPub-8f/B/Desalination1-1.asp
4. Potential technologies and approaches to desalination There are a number of existing and some potential new technologies that claim to tackle desalination of water. Traditional approaches have mainly been large scale and there has been a high demand for energy. A key challenge for new and emerging technologies is to reduce the demand for energy and be appropriate for adoption at the point of use.
4.1 Forward osmosis 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. 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. In developing countries the most readily available source of power is human muscle. The water purification pillow uses human energy to create clean from contaminated water by positive osmosis. Currently the concept has been proven but product development and marketing depends on further interest from users/consumers. Further details can be obtained from: Professor Richard Jones, Department of Physics, University of Sheffield, UK. (Source: David J. Grimshaw - personal contact).
4.2 Carbon nanotubes Researchers at Lawrence Livermore National Laboratory have created a membrane made of carbon nanotubes and silicon that may offer, among many possible applications, a less expensive desalination. “Membranes that have carbon nanotubes as pores could be used in desalination and demineralization. Salt removal from water, commonly performed through reverse osmosis, uses less permeable membranes, requires large amounts of pressure and is quite expensive. However, these more permeable nanotube membranes could reduce the energy costs of desalination by up to 75 percent compared to conventional membranes used in reverse osmosis. Source: https://publicaffairs.llnl.gov/news/news_releases/2006/NR-06-05-06.html
Scott Dougherty, LLNL Artist’s rendering of methane molecules flowing through a carbon nanotube less than two nanometers in diameter.
The nanotubes, special molecules made of carbon atoms in a unique arrangement, are hollow and more than 50,000 times thinner than a human hair. Billions of these tubes act as
the pores in the membrane. The super smooth inside of the nanotubes allow liquids and gases to rapidly flow through, while the tiny pore size can block larger molecules. This previously unobserved phenomenon opens a vast array of possible applications. The team was able to measure flows of liquids and gases by making a membrane on a silicon chip with carbon nanotube pores making up the holes of the membrane. The membrane is created by filling the gaps between aligned carbon nanotubes with a ceramic matrix material. The pores are so small that only six water molecules could fit across their diameter.” Further details from: Olgica Bakajin, The Lawrence Livermore National Laboratory, Livermore, CA. ([email protected]
) Source: http://www.greenoptimistic.com/2008/06/09/low-energy-water-desalination-throughnanotechnology/ For further information, the following websites and references can be consulted: The move from research through to practical products that embody new knowledge can be time consuming. Scientists at UCLA have teamed up with entrepreneurs to form a new company called nanoh2o (http://www.nanoh2o.com/index.php5). Based in California the agenda for the company is likely to be influenced by their own local needs. Affordable Desalination Collaboration, http://www.affordabledesal.com/ International Desalination Association, http://www.idadesal.org/ Desalination Directory, http://www.desline.com/home.cfm Hille, T., Munasinghe, M., Hlope, M. and Deraniyagala, Y. (2006) Nanotechnology, Water and Development, Meridian Institute, accessed 6 December 2006. Meridian Institute, (2006) Overview and Comparison of Conventional Water Treatment Technologies and Nano-Based Technologies, accessed 6 December 2006. Jason K. Holt, Hyung Gyu Park, Yinmin Wang, Michael Stadermann, Alexander B. Artyukhin, Costas P. Grigoropoulos, Aleksandr Noy, Olgica Bakajin (2006) Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes, Science 19 May 2006:Vol. 312. No. 5776, pp. 1034 - 1037
4.3 Freezing A 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 vaporises 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.
4.4 Small-scale multi effect distiller The Centre 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 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).
4.5 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 now available for commercial development. 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. For further information on the Seawater Greenhouse visit http://www.seawatergreenhouse.com/
4.6 Solar troughs Solar 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) & GarcíaRodríguez and Gómez-Camacho (1999).
4.7 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 ability to achieve 100% salt rejection. Owing to its low energy requirement MD, coupled with solar energy, can achieve cost and energy efficiency. The Singapore government recently showed its commitment to solarenergy technology by allocating S$170 million towards its research and development. 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 that 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. 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. See also: http://www2.hawaii.edu/~nabil/solar.htm.
4.8 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 undergoing 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. Further information: 6 The first solar powered RO in India The Barefoot College Tilonia CSMCRI Gujarat http://uk.youtube.com/watch?v=Bx3sEppnU3c Central Salt & Marine Chemicals Research Institute, Bhavnagar, Gujarat, India 6
Foundation for Water Research Allen House, The Listons, Liston Road, Marlow, Bucks, SL7 1FD Telephone +44(0)1628 891589 Facsimile +44(0)1628 472711 E-mail [email protected]
Solar electric powered reverse osmosis water desalination system for the rural village, Al Maleh: design and simulation, Marwan M. Mahmoud, Renewable Energy Research Centre, An Najah National University, West Bank, Palestine. 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.
4.9 Solar powered reverse osmosis plant for the treatment of borehole water Reverse Osmosis, a process where an external hydraulic pressure is applied to a concentrated solution thus forcing pure water through a permeable membrane, requires a high energy input for the high pressure feed pumps. 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 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 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. (Adapted from Development of a solar powered reverse osmosis plant for the treatment of borehole water, http://www.fwr.org/wrcsa/1042101.htm).
4.10 Wind powered reverse osmosis Wind powered reverse osmosis is being researched in a number of locations and the extract below highlights a few examples although there are probably many more: A Wind-Powered System for Water Desalination, Eyad S. Hrayshat, Tafila Technical University, Tafila, Jordan, International Journal of Green Energy. A 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. For further information, see also: http://www.desline.com/articoli/4119.pdf. and Canary Islands Institute of Technology (ITC Canarias) www.itccanarias.org Contatc: Mr. Gonzalo Piernavieja [email protected]
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 three different 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 power; Electro Dialysis Reversal (EDR): 1 plant with a production capacity of 200 cubic meters per day, using as feed water artificially produced brackish water; Vapour Compression (VC): 1 plant with a production capacity of 50 cubic meters per day.
4.11 Bullock-driven reverse osmosis process for desalination of brackish water in coastal villages It is well known that the groundwater in many coastal and inland areas is brackish and not 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. 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 can cater to the cooking and drinking water needs of 1000 villagers when operated for 6-8 hours per day. For further information on the CSMCRI Bullock-Driven RO Unit: Central Salt & Marine Chemicals Research Institute Gijubhai Badheka Marg, Bhavnagar-364002, Gujarat (INDIA) E-Mail: [email protected]
, [email protected]
Tel: 0278-2567760 / 2568923 / 2565106 Fax: 0278-2567562 / 2566970 E-mail: http://www.csmcri.org/