Even though the volume of the earth's water is vast, less than 10 million of the 1 400 million cubic metres of water on the planet is of low salinity and suitable for use after applying conventional water treatment alone. The other 97.5% of the water on our planet is to be found in the oceans, where it is officially classified as seawater.
Desalination provides a means of tapping this resource, and in this special feature, Filtration + Separation takes a look at some of the driving forces behind desalination worldwide...
Today, over 15,000 desalination facilities operate in more than 120 countries worldwide producing in excess of 3 500 million gallons per day of potable water, a dramatic rise compared with 30 years ago, when virtually no desalination took place at all. Indeed, some countries, such as Spain, Saudi Arabia and the United Arab Emirates, now rely on desalinated water for more than 70% of their water supply.
This increased acceptance of desalination for municipal, industrial and commercial applications has been driven by a reduction in cost brought about by newer and more efficient technologies, especially in the Reverse Osmosis (RO) field. And this has also brought about a modernisation in the way plants are delivered. Most of the large seawater desalination facilities built in the past 10 years (or currently undergoing construction) are delivered under public-private partnership arrangement – using the build-own-operate-transfer (BOOT) method of project implementation. The BOOT project delivery method is preferred by municipalities and public utilities worldwide because it allows cost-effective transfer to the private sector of the risks associated with the variables affecting the cost of desalinated water; namely intake water quality and its effects on plant performance, which are difficult to predict; permitting challenges; start-up and commissioning difficulties; dealing with the fast-changing membrane technology and equipment market; and limited public sector experience with the operation of large seawater desalination facilities.
Desalination v. Conventional processes
People define desalination in different ways, says Antonia Von Gottberg, director of municipal technology at Koch Membrane Systems – a supplier of spiral wound RO membranes used to desalinate brackish water and seawater. “Most conventional water treatment plants use a combination of settling, filtration and disinfection to treat water sources. In many cases these water sources are ‘fresh water', and the main requirements are simply the removal of suspended solids, bacteria and viruses. In some cases, this might require precipitation chemistry to precipitate and dissolve species such as hardness and arsenic, followed by the clarifying or filtering of the water to remove them.”
But these conventional technologies do not remove salts and most soluble non-organic and organic substances, and cannot be used to produce fresh water out of seawater or brackish water, adds Nikolay Voutchkov, senior vice president and corporate technical director for Poseidon Resources Corp. Desalination sees these dissolved salts removed through either thermal processes (where water is evaporated then freshwater is condensed) or membranes (where salts are retained by the membrane barrier).
Nowadays, RO membranes are capable of making seawater fresh, and such RO processes are widely acknowledged to have overtaken thermal techniques in terms of cost effectiveness, and the trend is likely to continue. “RO plants can remove practically all soluble salts and fine soluble or insoluble inorganic and organic materials in the source water,” says Voutchkov.
But it is important to recognise that depending on the specifics of a project, the desalination aspect (thermal or membrane) may only be one part of a plant's process:
“If the seawater desalination processes (both thermal and membrane based) are relatively standard regardless of location/seawater type, it is the pre-treatment that tends to vary,” continues Chris Howorth, market development manager for Aquious PCI Membranes (a division of ITT Advanced Water Treatment). “Pre-treatment can be performed by a vast array of technologies – including those that would be considered ‘conventional' for treating freshwater, together with others, such as pre-coat filtration, that are rarely used for drinking water applications. It is all about feed water quality and variability, particularly in relation to organics when RO is employed.”
Voutchkov continues, “RO plants typically consist of two types of treatment facilities in series – a pretreatment system and RO membrane system. The purpose of the pretreatment system is to remove particulate matter (mainly suspended solids) from the source water. For that reason, a conventional water treatment plant and the pretreatment system of a typical desalination plant are very similar – i.e. they use the same treatment technologies such as sedimentation and granular media (sand/garnet) filtration. The purpose of the RO membrane system, which is located downstream of the pretreatment system, is to remove soluble materials in the source water (salts, natural or manmade organic materials, soluble metals) and all very fine particulate materials (fine solids, silt, bacteria, viruses and other pathogens), which the pretreatment system was incapable of removing. Brackish water RO membranes are used for low salinity (below 15,000 mg/L TDS) source water. Seawater RO membranes are used for desalination of ocean and seawater.”
The emergence of desalination
Apart from a general increased acceptance of desalination as an alternative means of generating useable water sources, what other drivers do those operating within the desalination field pinpoint?
“Although every country, region and even specific water supply area will have its own unique set of circumstances,” explains Howorth, “the drivers of desalination often tend to be similar across the globe. The principal driver is limited freshwater – this is common to all projects (including the one in London, UK, though this was also driven by the fact that there was nowhere to put a large reservoir.”
Accessible fresh water reserves are becoming harder to find and increasingly costly to exploit, he says, partly because the environmental cost of their exploitation is being increasingly considered. Over-exploited groundwater reserves are also becoming progressively more saline due to seawater intrusion. “Where sufficient freshwater resources are not available and demand cannot be reduced to meet capacity, the only alternative to desalination is generally water transfer (using tankers at small scale or conveyance infrastructure at larger scales). Seawater desalination offers an inexhaustible water resource.”
Along with limited freshwater, comes increasing demand. With populations growing and tourism to foreign climates becoming ever more affordable, demographic patterns are showing that dry coastal areas are growing particularly fast in many regions, and living standards improving around the world. This has made the demand for drinking water greater than ever. “Agricultural demand is also rising due to a trend of more intensive agriculture and the increasing use of irrigation,” Howorth adds.
Then there is global warming. “The changing climate appears to be making droughts more severe and also raising average temperatures, increasing the area considered to be water scarce.”
Another important area driving desalination is regulation. “In the more developed regions the very high level of treatment provides the advantage of compliance with increasingly strict drinking water quality standards,” says Howorth. “The European Union's (EU) Water Framework Directive, for example, is demanding a more holistic consideration of water supply, which will elevate the importance placed on protecting vulnerable and over exploited freshwater resources.”
In other places such as China, adds Koch's Von Gottberg, “regulations are preventing industries from using freshwater sources to meet their water demands, so industries need to find other water sources, which includes recycling of industrial and municipal waste, or seawater desalination.”
Other factors include cost effectiveness - recent developments in desalination technology have made desalination more cost effective – and hence more economically viable – than ever before; and then there is Politics: “Desalination at a significant scale is always intimately connected with politics, although this is a very subjective driver,” says Howorth, 'the short-term mentality of politicians (and indeed consumers) makes drought an extremely effective incentive for desalination installation – when consumers are facing usage restrictions, methods of alleviating the situation are likely to be attractive.
Consumers' perceptions of the value of water are also (slowly) changing, from a free resource that falls from the sky, to a precious resource that fundamentally underpins economic development. The United Nations (UN) suggests that 1500m3/year/capita of naturally renewable freshwater is required to support unhindered economic development. In Europe, both Malta and Cyprus are below this limit (74m3 and 979m3 respectively), as are specific regions of other countries (e.g. Spain's Canary Islands).”
Cost reduction in desalination – the membrane
Historically, one of the key obstacles limiting the use of seawater desalination in a large scale has been the high cost of water production, explains Nikolay Voutchkov: “However, a number of cost-saving innovations in seawater desalination technology over the last ten years are transforming this once costly option of last resort into a viable water supply alternative.”
A typical RO membrane desalination plant includes the following key components: source water intake system; pretreatment facilities; high-pressure feed pumps; RO membrane trains, and a desalinated water conditioning system. But the 'engine' of every desalination plant that turns seawater into fresh potable water is the RO membrane element.
The most widely used type of RO membrane elements consist of two membrane sheets glued together and spirally wound around a perforated central tube through which the desalinated water exits the membrane element.
A large seawater desalination facility usually has thousands of membrane elements connected into a highly automated and efficient water treatment system, which typically produces one gallon of fresh water from approximately two gallons of seawater. The membrane productivity, energy use, salt separation efficiency, cost of production and durability of the membrane elements by and large determine the cost of the desalinated water. Technological and production improvements in all of these areas in the last two decades are now rendering water supply from the ocean affordable. Membrane productivity - i.e. the amount of water that can be produced by one membrane element, has more than doubled in the last 20 years.
Innovations such as the recent introduction of spiral wound membrane elements with a larger number of membrane “leaves” and denser packing also offer increased efficiency as compared to older designs.
Today's most efficient elements have more than twice as many membrane leaves compared to older designs. Higher productivity means that the same amount of water can be produced with significantly less membrane elements, which has a profound effect on the size of the membrane equipment, treatment plant buildings, and the footprint of the desalination facility – all of which ultimately reduce the cost of water production. Koch Membranes Systems, for example, has recently introduced a MegaMagnum RO element, which the company describes as the world's largest spiral wound element. “This has seven times the membrane area of typical 8-inch by 40-inch elements, and so fewer elements and housings are needed for an application. This offers the customer potential savings due to fewer connections and smaller footprint,” explains Koch's Antonia Von Gottberg.
Membrane performance tends to naturally deteriorate over time due to a combination of material wear-and-tear, and irreversible fouling of the membrane elements. Typically membrane elements have to be replaced every five years to maintain their performance in terms of water quality and power demand for salt separation. Improvements in membrane element polymer chemistry and production processes over the last 10 years have made the membranes more durable and have extended their useful life. The use of elaborate conventional media pre-treatment technologies and ultra and micro-filtration membrane pre-treatment systems prior to RO desalination is expected to prolong the membrane's useful life to 7 years and beyond, thereby reducing the costs for its replacement, and therefore the overall cost of water.
Today, the RO membrane technology and elements are highly standardised in terms of size, productivity, durability and useful life. There are a number of manufacturers of high-quality seawater RO membrane elements that provide interchangeable products of excellent quality, proven track record and performance. Many of the leading membrane manufacturers are investing in R&D to support the water desalination market and its advancing membrane technology and science – at a pace no other water technology can compare with. “It is expected that an oxidant resistant membrane will be developed in the future,” explains Aquious' Chris Howorth, “overcoming the significant limitation of current polyamide films in terms of their inability to use common biocides to control biofouling (and indeed their susceptibility to damage by very small concentrations of oxidants should they be present for whatever reason).”
Another issue Howorth highlights is the growing importance of Boron passage through membranes: “Acceptable product water concentrations are being progressively reduced. Product cost reduction is a major objective. Advances in membrane performance are also ongoing, in terms of flux rate, fouling resistance, longevity, and salt rejection.”
Cost reduction in desalination – energy
In seawater desalination facilities, salts are separated from the fresh water applying pressure to the seawater, which is 60 to 70 times higher than the atmospheric pressure. After the salt/water separation is complete, a great portion of this energy stays with the more concentrated seawater and can be recovered, and reused to minimise the overall energy cost for seawater desalination.
Dramatic improvements in the membrane element materials and energy recovery equipment over the last 20 years, coupled with enhancements in the efficiency of RO feed pumps, and reduction of the pressure losses through the membrane elements have allowed a reduction in the use of power needed to desalinate seawater – to less than 14 kWh/1,000 gallons of produced fresh water today. Taking into consideration that the cost of power is typically 20 to 30% of the total cost of desalinated water, these technological innovations have contributed greatly to the reduction of the overall cost of seawater desalination.
Novel energy recovery systems working on the pressure exchange principle (pressure exchangers) are currently available in the market and use of these systems is expected to further reduce the desalination power costs by approximately 10 to 15%. The pressure exchangers transfer the high pressure of the concentrated seawater directly into the RO feed water with an efficiency exceeding 95%. Future lower-energy RO membrane elements are expected to operate at even lower pressures and to continue to yield further reduction in cost of desalinated water.
RO technology advancements compare to computer revolution
Nikolay Voutchkov explains that the advances in RO desalination technology are closest in dynamics to those of computer technology: “While conventional technologies, such as sedimentation and filtration have seen modest advancement since their initial use for potable water treatment several centuries ago, new and more efficient seawater desalination membranes and membrane technologies – as well as equipment improvements – are released every few years. And similar to computers, the RO membranes of today are many times smaller, more productive and cheaper than the first working prototypes. Although no major technology breakthroughs are expected to bring the cost of seawater desalination down further dramatically in the next few years, the steady reduction of desalinated water production costs coupled with increasing costs of water treatment – and driven by more stringent regulatory requirements – are expected to accelerate the current trend of increased reliance on the ocean as an environmentally friendly and competitive water source.”
This trend is forecasted to continue in the future and to further establish ocean water desalination as a reliable drought-proof alternative for many communities worldwide.