Desalination Options for Small to Medium-Scale Plants in Developing Countries

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

Otto Ruskulis

Consultant

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 Water

Salinity 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 1000

Electrodialysis - 500 to 3000

Reverse Osmosis (standard membranes) - 500 to 5000

Reverse Osmosis (high resistance membranes) - over 5000

Distillation – 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 Requirement

Personal 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 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 day / Maximum Production – m3 per day
Solar 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.

Operating Skills

All 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 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

-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 Requirements

Distillation 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 1000
Reverse 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.