Echoing the theme of IDA’s upcoming World Congress in Tianjin, China, desalination is a promise for the future in our increasingly water-challenged world, while today it is also the source of fresh water for some 300 million people around the globe. A major player in desalination, the IDA celebrates its 40th anniversary this year.
Thirty six percent of the global population lack access to proper sanitation and 780 million people live without a supply of safe drinking water, according to the World Health Organization. They estimate that at least 3.6 million people die each year from water related diseases (see Figure 1). Saltwater accounts for 96.5% of the water available to us, and humans have been utilising that supply in various ways for centuries. The promise for the future, as articulated by the IDA, is that desalination can be used as one of the essential tools to bridge the safe drinking water gap.
Before we look back at the last 50 years, to set the scene let’s go way back to early classical times when one of the first mentions of desalination was by Aristotle, who wrote about seawater distillation around 300-400 BC. Different techniques have been used through the centuries, with successful efforts reported by explorers such as Jean De Lery. He wrote about the distillation of seawater during a voyage to Brazil in 1565, and James Cook regularly desalinated seawater during his circumnavigation of the world in 1772.
The major businesses involved in the commercialisation of desalination through the 19th century, such as Caird & Rayner (a brand which still exists today), were located in the UK, France, Germany and the USA.
“The first steam engines for motive power operated at very low boiler pressures,” Birkett explained, “and usually used whatever water was available for boiler make-up. As high pressure steam and compound engines became more popular for reasons of fuel efficiency, pure water for boiler feed became necessary to reduce corrosion and the likelihood of boiler explosions."
“There was usually plenty of excess steam available for the modest make-up requirements and so simple single effect stills became the most popular; even the Titanic had single effect stills on board.”
These systems utilised the waste heat to produce fresh water for boiler feed, drinking and cooking.
“Through the mid-1900s,” Awerbuch explained, “the most commonly used techniques involved evaporation and distillation. The development of desalination processes took a major step forward in the 1940s during World War Two, when military establishments operating in arid areas needed a way to supply their troops with potable water.”
The first major technical advance in desalination technology was the development of multiple effect distillation (MED). Preheated feed water flowing over tubes in the first effect is heated by prime steam, resulting in evaporation of a fraction of the water content of the feed. The water vapour generated by brine evaporation in each effect flows to the next effect, where heat is supplied for additional evaporation at a lower temperature. There the vapour condenses, giving up its latent heat to evaporate an additional fraction of water from the brine. The process of evaporation-plus-condensation is repeated from effect to effect, each at successively lower pressure and temperature. The combined condensed vapour constitutes the product distillate water.
The first MED system for maritime use was probably installed on the Queen Mary in 1934 but, for land-based applications (such as military establishments), energy efficiency was more important as there was likely to be no excess steam available. Normandy in the UK was perhaps the first company to supply MED systems for potable water production in the late 19th century.
The last 50 years
In the years following World War Two, scientists began looking at alternatives to thermal desalination by studying membrane processes. Electrodialysis (ED) was the first of these processes to be developed commercially in the 1950s.
“Since ED could desalt brackish water more economically than distillation,” Awerbuch explained, “more interest was focused on using desalination as a way to provide water for municipalities with limited fresh water supplies but the availability of brackish water sources.”
Birkett added: “ED opened up brackish water markets all over the world. It was highly successful but too expensive for operation on full-strength seawater, energy consumption being a direct function of the amount of salts driven through the membranes.
“However, in brackish situations, ED was chemically and mechanically rugged and thrived on hot feed water. Membranes lasted practically forever and if seriously fouled or scaled, could be removed and scrubbed with a brush before reassembly. ED actually gave membranes a good name.” The introduction of electrodialysis reversal (EDR) greatly reduced scaling problems associated with ED.
“There is, of course,” Hill explains, “one outstanding development. In 1963 Loeb and Sourirajan at the University of California, in Los Angeles, developed the first synthetic reverse osmosis membranes.” “And, previously, the first reported general use of the term reverse osmosis, now a popular desalination technology, appeared in the US Department of Interior’s Office of Saline Water 1955 annual report,” Awerbuch added.
In 1963 virtually all the world’s seawater desalination capacity (about 1,000 m3/d) was in the Middle East and was met by multistage flash (MSF), a technology that was independently and more or less simultaneously invented in the UK and the USA. Professor Silver at J & G Weir Ltd, the Westinghouse, Bethlehem Steel and Agua-Chem had contributed to rapid development in the late 1950s to use the waste heat from steam turbine power generation.
Hill explained, “Most of these plants were in the Middle East and were constructed as part of power and water facilities with the power station fired by oil.”
In MSF, seawater (after mixing with the recycle stream) is pressurised and heated to the maximum top brine temperature (TBT). When the heated brine flows into a stage maintained at slightly below the saturation vapour pressure of the water, a fraction of its water content flashes into steam. The flashed vapour passes through a mist eliminator and condenses on the exterior surface of heat transfer tubing. The condensed liquid drips into trays as a product water. The unflashed brine enters a second stage, where it flashes to vapour at a lower temperature, producing a further quantity of product water. The flashing-cooling process is repeated from stage to stage until both the cooled brine and the cooled distillate are finally discharged from the plant as blowdown brine and product water.
By the late 1960s, commercial desalination systems producing up to 8,000 m3/d were beginning to be installed in various parts of the world. Most of these installations used the thermal processes. Development continued, and in the 1970s larger scale commercial RO and ED systems began to be used more extensively. Initially, in brackish applications, RO had to compete against the now established ED and EDR technologies. “This took time,” Birkett explained, “as early RO was complicated and not always reliable. But continuous incremental improvements in RO ultimately won the day. Early membrane guarantees were for no more than three years but then crept to five and now we see maybe seven.”
“The growth of RO once the initial engineering problems had been overcome,” Hill explained, “was due to the standardisation of the spiral wound membrane module, which resulted in increased competition amongst suppliers and reduced costs.”
“The inexpensive spiral wound format combined low production cost with high packing density, and was ideal for the interchangeable approach to be adopted by RO makers from the early 1990s,” Pearce added.
The introduction of thin film composite (TFC) membranes to replace cellulose acetate was the next giant step. “One can point to individual installations,” Birkett explained, “with original membranes in place for twelve or more years. Membrane developments occur so regularly that by the time you need to replace your membranes, you will probably upgrade the system to the next generation of membranes instead.”
In the late 1970s and 1980s the development by Dow of the FilmTec TFC polyamide membrane brand resulted in process improvements including lower operating pressures, higher fluxes and higher salt rejection, which helped reduce energy consumption.
“The critical enabling development for the modern RO industry was not made until the development of these TFC membranes,” Pearce confirmed, “but even then, it was the late 1990s before the industry took off with a series of major plants.”
“Back in the 1970s,” Hill added, “the introduction of energy recovery turbines significantly reduced the operating costs of seawater RO.”
“The introduction of energy recovery techniques has been tremendously important, “Birkett added. “If we look at just the RO separation stage, including high pressure pumps, membranes and energy recovery system, we can now, using commercially available components, desalt seawater at about 1.5 times the minimum theoretical energy of separation.”
This practical separation level is cited by Birkett at approximately 6 KWh/Kgal.
“We can’t get much lower than this,” Birkett explained.
“Of course there are many other energy consuming items in the total plant as well, which drives total plant consumption to double the separation level. We will need to reduce these peripheral loads if we want to get much further.”
“By the 1980s,” Awerbuch concluded, “desalination technology had become a fully commercial enterprise and by the 1990s, the use of desalination technologies for municipal water supplies was commonplace.”
However, the spiral wound RO format had some important limitations affecting energy consumption. Firstly, the hydrodynamic inefficiency of the design imposed a varying back pressure, dependent on the length of the membrane sheets.
“Due to the relatively low fluxes used in RO,” Pearce explained, “this constraint was not too serious. A potentially more serious limitation was that the spiral wound format is highly susceptible to fouling, and this requires effective pre-treatment.”
Though traditionally provided by conventional process technologies incorporating clarification and media filtration, attention shifted in the 1990s to the use of membrane filtration as a more effective pre- treatment technology for RO desalination.
Membrane filtration using ultrafiltration (UF) and microfiltration (MF) membranes was to ultimately represent a major breakthrough in the advancement of desalination technology. It was first proposed for large scale water treatment applications.
“But initially,” Pearce explained, “users were skeptical that membranes would ever become commonplace as pre-treatment because they were felt to be too expensive or unnecessary.”
A summary of the early perceptions of membrane filtration technology and a timetable for the eventual breakthrough of the technology as desalination pre-treatment is shown in Table 1.
The first major development for UF and MF came with the emergence of legislative drivers for cryptosporidium, giardia and virus removal, which led to a gradual adoption of the technology in drinking water treatment. As the number of projects increased,” Pearce explained, “prices fell, and eventually membranes became widely applied. Now they are used where an absolute particle barrier is required, and as a cost effective technology for wastewater reuse. Industrial applications have benefited as well with the rapid expansion of developing markets.”
Several critical choices were made in the early stages of membrane development for water treatment.
“The most important decision was to use hollow fibre membranes in pre-treatment rather than the spiral wound format used in RO,” Pearce explained.
“This improved module integrity and enabled a design that was testable and repairable. Also, the hollow fibre module had better hydrodynamic efficiency than other formats, ensuring stable performance."
“Secondly, early pioneers developed dead-end operation with intermittent backwash rather than cross-flow operation, and this had a dramatic impact on energy costs."
“Finally, there was the development of the air scour concept, which encouraged development of outside-in treatment flow with its membrane area advantage. Without air scour, outside feed membranes would struggle to achieve stable operation.”
This made the choice between inside feed and outside feed formats a finely balanced decision with both approaches offering potentially important advantages.
“But the most difficult water treatment duty to conquer for UF and MF membranes was seawater RO pre- treatment,” Pearce added.
“Membranes had just been considered too expensive and indeed unnecessary for this duty, and have only begun to make an impact since 2006. Now established, however, membranes are being used more widely. Membrane pre-treatment provides a particle barrier, which is an ideal pre-treatment for the spiral wound RO element since this format is highly susceptible to particle fouling.”
Membrane pre-treatment has been shown to be cost effective by Pearce in a total treated water cost analysis, since it reduces operating costs for chemical cleaning, improves the cost and operational stability of the RO desalination system, and reduces environmental impact.
A key issue, which has been difficult to resolve, is that membrane pre-treatment does not necessarily eliminate bio-fouling. Bacteria can cause bio-fouling problems if supplied with nutrients in the form of dissolved organics, and it has been important to develop membrane pre-treatment designs recently to reduce or eliminate bio-fouling potential.
“Enhancing the removal of dissolved organics with coagulants,” Pearce explained, “is an important part of the strategy, but needs to be exercised with care during periods of good water quality since overdosing can lead to coagulant fouling."
“Of equal significance, eliminating the feed tank and cartridge housing between pre-treatment and RO is important since this encourages bio-growth and should not be necessary except perhaps during commissioning. Recent plants that have implemented these steps have demonstrated a much lower incidence of bio-fouling than previously.”
“Membrane pre-treatment,” Pearce concluded, “has been demonstrated over the last five years or so as a cost effective pre-treatment technology for seawater desalination. However, designs need to be implemented with care to ensure the elimination of bio-fouling, but recent developments have demonstrated that this can now be achieved.”
“The challenge,” Birkett added, “is that the feed water is highly likely to change over time, both short and long term. Advanced instrumentation with direct feedback to the pre-treatment system is vital. I would say that improved instrumentation with feedback, including programmed corrective measures, would be an area for improvement.”
Another development has been the application of nanofiltration (NF) or membrane softening as a pre-treatment for MSF or MED. “The efficiency of most thermal systems,” Birkett explained, “is tied to top end operating temperature which in turn is limited by inorganic scale formation. It has been demonstrated that retrofitted NF softening can dramatically improve both productivity and efficiency of existing thermal systems.” ♦