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The Magic of What do Perth, Saudi Arabia and cruise ships have in common? They all rely on desalination for fresh water. And the Gold Coast, Sydney and Adelaide are about to join the club! by Geoff Graham T urning salty water into fresh, drinkable water is not new. In the early Australian gold rush days large areas of woodlands were stripped to feed “condensers” that boiled salty water and trapped the condensation for sale to thirsty miners. These days a large cruise ship will generate over a million litres of water a day from the sea using either flash evaporators or reverse osmosis, while Middle East countries such as Saudi Arabia produce over 70% of their drinking water using various forms of desalination. Australia is not left out. The advent of a drying climate has triggered a flurry of desalination plants either planned or under construction with the first in Perth, Western Australia, running since 2006. There are a number of technologies used for desalination but most modern large scale plants are based on reverse osmosis. These plants are expensive to build but, in the longer term, cheaper to run. This technology is quite recent – it only got its start in the 1970s and 1980s when efficient reverse osmosis membranes were first manufactured in quantity. In Australia Small desalination plants have been operating across Australia for many years, providing drinking water for towns such as Penneshaw, Coober Pedy and Marion Bay in South Australia. The new plants on the drawing boards are on a much larger scale and represent a major infrastructure investment. In total six plants are running or currently planned. All are destined Fig.1: the layout of a typical desalination plant. It looks simple – salt water is filtered and passed through the reverse osmosis process. However, as with most things, the reality is more complex with the magic happening in the reverse osmosis section. (courtesy Sydney Water) 12  Silicon Chip siliconchip.com.au DESALINATION Part of a $2 billion project, this aerial picture (taken in March 2009) shows the Sydney Desalination Plant, currently under construction at Kurnell. It is now more than 80% complete, with mechanical and electrical work well underway and will become operational this coming summer. (courtesy Sydney Water) to serve major population centres and will supply a significant amount of our water needs. The first was a plant at Kwinana, south of Perth, built three years ago for the WA government by a French consortium. A similar plant, built by another French consortium, has just been completed on the Gold Coast. Sydney is not far behind with a monster plant nearing completion at Kurnell that is planned to supply 15% of the city’s water requirements. Others preparing for construction include a second plant for Perth and the first plant for Adelaide, at Pt Stanvac, both of which will be built by separate Spanish consortiums. Finally, Victoria is in the early planning stage for an installation on the Bass Coast near Wonthaggi. Desalination is not cheap. The Perth plant cost $387 million to build in 2006 while the Sydney plant is expected to cost almost $2 billion, including the connecting pipeline. The amount of water produced is large by any measure. The Perth plant produces 130 million litres a day while Sydney is projected to produce 250 siliconchip.com.au million litres of water each day. A typical plant On paper a desalination plant looks relatively simple. You suck seawater in, filter it to remove sand etc and then pass it through reverse osmosis membranes to obtain your clean water. As always, the complications lie in the details. The inlet system is where the process starts. Typically a plant will suck in 20 million litres of water an hour through large concrete intakes on the Fig.2: osmosis occurs when water migrates through a permeable membrane towards the more salty solution. The level on the less salty side will then decrease. seabed. This is an enormous amount of water and you might think that it could also suck in fish and other ocean life, even including someone who was enjoying a cooling dip. This cannot happen because the inlets have grates across them and are designed with a very large intake area to keep the flow to less than 0.1 metre per second. At this rate the flow is less than a typical ocean current and does not affect marine life which can swim around the inlets as normal. The water then goes through screen- Fig.3: reverse osmosis occurs when pressure is applied to the salty solution forcing the water through the membrane to the less salty side. July 2009  13 ing and filtration stages to remove sand, algae and similar impurities. The technology varies but typically, as in the case of the Perth desalination plant, sand filters are used. These are a similar technique to the sand filter used in a home swimming pool. All this is normal technology but the water then enters the high-tech reverse osmosis section where the magic begins. Reverse osmosis Reverse osmosis can be best explained by looking at the phenomenon of osmosis first, then explaining the reverse part. Osmosis is the ability of water to migrate through a permeable membrane while leaving dissolved components behind. This can be observed with two solutions, one saline and the other not, separated by a suitable membrane. By osmosis the water will move slowly through the membrane from the less saline solution to the more salty solution. Contrary to what you might first assume, this action will raise the level of the salty water above the level of the less salty solution (see Fig.2). Membranes are common in nature; your skin is a membrane and water will move through it via osmosis while you are sitting in the bath. Reverse osmosis, as implied in its name, is the reverse of osmosis and occurs when you force the water through the membrane in the opposite direction as shown in Fig 3. The pressure applied to the salty side must first overcome the tendency of the water to move via osmosis to the salty side. Then, with increasing pressure, the water will reverse direction towards the less saline side leaving the salt behind in the increasingly saline solution. Special membrane This process requires a special type of membrane that is permeable to water but not dissolved salts. It is Reverse Osmosis pressure vessels. Each contains seven reverse osmosis membranes tightly wound in coils. The pressure used to force the water through the membranes is vey high, up to 1000 psi. At this pressure salt water is very corrosive so high quality stainless steel is used (courtesy Water Corporation WA). tempting to think of the membrane as a fine filter which traps larger particles (salt) while letting water through - but that is not correct. The osmosis mechanism is not fully understood but one explanation is that the water works its way through the membrane by packing into an ice-like A Serendipitous Discovery In 1959 Sidney Loeb was researching for his master’s thesis with Srinivasa Sourirajan when together they discovered the first practical membrane for reverse osmosis. That discovery is credited with being the foundation of modern desalination technology. While working on membranes in their laboratory they hit upon a formula which was an unexpected success in that it allowed a practical flow of water while stopping most salt. 14  Silicon Chip structure (at room temperature) and “melting” away on the other side. The ions from the salt cannot fit into the ice-like matrix and get left behind. Unlike a filter, in osmosis it is not the membrane pore size or the particle size that matters. Osmosis itself was first observed But a second test (from the same sheet of membrane) did not work. Subsequent tests were either good or bad “as if flipping a coin” according to Dr Loeb. Finally they figured out that the membrane was anisotropic (directionally dependent). The side facing the air when the membrane was cast on a glass plate had to be installed in contact with the saline solution to work correctly. In Dr Loeb’s words, “I sometimes wonder if I would have continued testing that membrane sheet if the first test had been a failure.” siliconchip.com.au Fig.4: the construction of a reverse osmosis module. The clean water permeates through the membrane and collects in the centre while the water that does not pass through (the concentrate) carries away the salt and other impurities. (courtesy Water Corporation WA) 250 years ago and since then researchers experimented with reverse osmosis. Despite these efforts, reverse osmosis remained a curiosity because the water flow through the membrane was so low that the process was impractical for large scale use. The breakthrough came in 1959 when Sidney Loeb and Srinivasa Sourirajan in the USA discovered a membrane that was much more efficient (see the sidebar A Serendipitous Discovery). The modern membrane used in reverse osmosis is a wonder of materials science and is normally a thin-film, composite membrane consisting of a thin polymer barrier layer formed on one or more porous support layers. Membranes have different characteristics and it is common for desalination plants to need two stages of reverse osmosis to remove everything. For Fig.5: this diagram better shows the flow of water to the centre of the membrane coil. The outer porous layer allows the salt water to flow over the surface of the membrane. Water that passes through the membrane then flows via the inner porous layer to the centre where it is drained. (courtesy Water Corporation WA) example, the first stage will remove salt while the second targets boron or in some cases, bromide. The pressure vessel The membranes sheets are wound into large rolls held inside pressure vessels. These vessels are the long (generally white) tubes that you see in a photo of a typical desalination plant. Inside a pressure vessel the sheets of membrane are rolled up (see Fig.4) with the desalinated water (permeate) collecting in the central spine. At least half of the intake water does not go through the membrane but instead runs out and is eventually discharged back into the sea. It is this flow of discarded water across the membranes that keeps them clean and prevents them from clogging up as a sieve would. As shown in Fig.5, the membrane A sea water intake. A desalination plant can suck up to 20 million litres per hour but the design of the intakes ensures that the flow into the intake is mild enough to have little effect on marine life (including divers!). (courtesy Water Corporation WA) siliconchip.com.au spiral is separated by a porous material that allows the seawater to contact every part of the membrane with an inner porous layer allowing the clean water (permeate) to flow to the centre. A pressure vessel would hold a number of these rolled membrane sheets and a typical plant would use almost 20,000 membranes at a cost of about $1,000 each. Nothing in desalination is cheap. Long Term Trend You would have to be a hermit or living overseas, if you did not know that Australia is in a prolonged period of drought. Falling rainfall levels and rising water consumption across Australia have reduced the level of water in our dams and forced our politicians into making some expensive decisions. The trend is most apparent in Western Australia where the inflow of water to Perth’s dams has been steadily falling over the past 50 years to one third of the previously typical levels. To make it worse, demand has increased by three times during this same period. Perth introduced its first water restrictions in 1960 and tapped into other sources such as groundwater but the trend has been inexorable. Three years ago the state government built Australia’s first desalination plant, the largest of its kind in the southern hemisphere and now a second plant for Perth is about to start construction. Sydney and Melbourne felt the effects of the big dry later but their dam levels have also been steadily falling since 1998. With traditional sources of water such as new dams being ruled out for environmental and other reasons planners across the country are turning to desalination. July 2009  15 This installation uses six high pressure centrifugal pumps drawing 2600kW each and pumping 1144 cubic metres/hour. They are made from super duplex stainless steel and need to be very well balanced during installation. Manufacturer was Clyde Pumps in Scotland. (courtesy Water Corporation) Recycle Instead? Another approach to the crisis is to recycle water. The technology used in recycling is similar to desalination – you filter the water to remove the big stuff and then use reverse osmosis to remove everything else. In planning for the Sydney desalination plant Sydney Water made a detailed comparison of the two systems and the differences are instructive. The cost of building identical capacity plants was about 50% higher for the recycling plant with the running costs also more expensive. This makes sense if you think about it. Both desalination and recycling take in dirty water and clean it but recycled water is dirtier and needs more cleaning. Also, salty water is easier to get; you just suck it in from the ocean, whereas water for recycling must be piped from the sewage plants. Apart from the cost, it is difficult to sell the notion of recycled water to the public, so it is no wonder that the planners chose desalination. 16  Silicon Chip Due to the spiral construction the membrane does not rupture under pressure but rather is slowly compressed. It is this compression which limits the life of a membrane which is about five to seven years. During its lifetime the performance of each membrane is monitored by measuring the flow rate and testing the quality of the desalinated water. Membranes are also cleaned two to three times a year using caustic, acid and detergent solutions. Practical issues The principle of reverse osmosis works well, but implementing it in a plant that must produce millions of litres a day is not easy. To force water through the membrane enormous pressures are required. In the Perth desalination plant there are six large centrifugal pumps which move millions of litres an hour at pressures up to 70 bar or in layman’s terms, about 1000 psi. These are made from super duplex stainless steel and must be very well balanced during installation to cope with the high speeds involved. Each consumes 2600kW, enough electricity to power hundreds of homes (see the sidebar Where Does the Electricity Come From?). The energy used to drive the pumps is a large part of the cost of running a plant, and for this reason a lot of attention is paid to energy efficiency. The water that passes through the membranes loses its pressure on the way through. However, the salty water destined for discharge retains the high input pressure and rather than let that energy go to waste, a modern desalination plant tries to recover as much as it can. The technique used in many plants is called isobaric or “pressure-equalising” energy recovery. This technology works by allowing the high pressure water to directly contact and push against the incoming water in pressure equalising or “isobaric” chambers. These chambers are inside spinning rotors that limit the contact time to avoid mixing; the result is a stream of high speed hammer blows against the incoming stream that transfer most of siliconchip.com.au Isobaric or “pressure-equalising” energy recovery devices. These transfer the energy contained in the discharge water to the incoming water and can reduce energy consumption by up to 96%. Inside each cylinder is a high speed spinning rotor made from tough ceramic that allows the outgoing water to hammer against the incoming water and thereby transfer the energy. (courtesy Water Corporation) The only time it will be stopped is for maintenance and environmental reasons (for example, the salty outflow is not dispersing). Even in these circumstances the plant maintains a small output by continuously rotating a small production through each bank of membranes to prevent a full shutdown being forced on them. Discharge the energy held in the outgoing stream. The energy recovery can be as high as 96% although in practice the actual percentage is rather lower. Regardless, this efficiency makes a huge difference in the amount of electricity required to drive the high pressure pumps and therefore the plant’s running costs. Another issue in plant design is corrosion. As anyone with a boat knows, sea water is very corrosive and at the high pressures used for reverse osmosis, it is positively destructive. As a result high grade stainless steel and ceramics are used in many places and this is part of the high price tag of a desalination plant. Starting and stopping a plant can take some time (up to a day) and the membranes need special preservation arrangements to prevent damage when not being used. Accordingly, the engineers like to keep the plant running continuously at full capacity. The water discharged from the plant is about double the normal salinity of sea water and this could be a problem for marine life if it was simply dumped back into the sea. Some sites, such as the Gold Coast and Sydney, can rely on strong ocean currents to help disperse the salty water but other locations are not so convenient. For example, the Perth plant discharges into Cockburn Sound which does not have strong currents. Because of this the outlets were designed to Where Does the Electricity Come From? Former NSW Premier, Bob Carr, once famously dismissed the whole idea of desalination as “bottled electricity”. Desalination can be thought of as: salt water + electricity = drinking water On average it takes about 5kWh of electricity to produce one thousand litres of fresh water. For plants producing millions of litres this adds up to a lot of electricity. As we do not want to compound the environmental effects that are blamed on burning fossil fuels, renewable energy is a popular source for the electricity. Consequently Perth, Sydney and others have decided to go with wind farms. As with the renewable electricity that you can purchase at home, the electricity for desalination is drawn from the general power grid. However, it is purchased at a higher than normal price, even if the wind farm is becalmed at that time. The extra money is then paid to the wind farm when they do generate some electricity and feed it into the grid, as that means that less power is required to be generated from fossil fuel. The result is the same as transmitting the power directly to the desalination plant but avoids the cost of building a duplicate transmission system. The Perth desalination plant has a continuous power draw of 24MW and this is nominally supplied by the Emu Down Wind Farm located 100km north of the city. This facility cost $180 million siliconchip.com.au to build and has 48 wind turbines capable of generating a peak 80MW of power. 40MW of that is reserved for the desalination plant which, given the variability of wind power, means that the desalination plant will end up paying for the equivalent of 24MW of continuous renewable energy. For the Sydney desalination plant a wind farm will be built at Bungendore (near Canberra), with a capacity of 140MW. The second desalination plant for Perth will go one step further with 20% of its power to come from what is called “speculative energy sources”. This covers technologies such as geothermal, wave power and other experimental sources and accordingly an even higher price will be paid for this electricity. July 2009  17 sources of water (such as dams) on days of light water consumption to favour water from the desalination plant. In extreme cases they will even pump the desalinated water into dams for storage. As the overall aim of the desalination plant is to conserve the water in our dams this arrangement will even out in the long term. In the unlikely event that the dams approach overflow the desalination plant would then be shut down, probably for a long time. The overall cost of desalinated water can vary considerably, depending on many factors, but it is still affordable. When constructed the Perth plant had running costs of about $20 million per year and the cost of water produced was close to $1.20 per kilolitre. This can be compared to the cost of water from traditional sources at the time of 80c to 90c per kilolitre. Other plants currently under construction have projected production costs that range from $1 to $3 per kilolitre. If you are in a serious situation like Adelaide, even that price is a bargain. With so much effort going into producing the water in your tap, you should appreciate a glass of water SC even more. The salty discharge water on its way back to the ocean. The salt level is double normal levels but it quickly disperses in the ocean. In the background you can see the sand filters that are used to clean the incoming sea water by removing large particles such as sediment and algae. (courtesy Water Corporation) shoot the outflow upwards from the sea bed to encourage mixing. Before construction this design was tested by the University of NSW in a large swimming pool. Overall, the designers aim to mix the outflow to such an extent that the salinity of the water reaches normal levels at 50 to 75 metres from the outlets. Drinkable water The water produced by reverse osmosis technology is essentially pure but still needs processing. So a desalination plant must include a post 18  Silicon Chip treatment stage which adds components such as fluoride that we expect in out drinking water. This stage also adds alkalinity to the soft processed water. A similar treatment stage is used for soft dam waters as this prevents corrosion in the distribution system. In keeping with other treatment methods, chlorine is also added for cleansing and maintenance of the distribution system. Finally the water is fed into the municipal water reticulation system. Because the plant is run continuously the engineers will throttle back other Monitoring buoys are used to monitor salt concentration, dissolved oxygen and many other parameters. If these exceed safe levels the desalination plant will be shut down until the ocean currents can return the sea water to acceptable levels. (courtesy Water Corporation) siliconchip.com.au