Saltwater works

In brief

• Thames Water has the go-ahead for a £200m desalination plant in East London
• The plant will be able to supply 140ML/day of drinking water using traditional reverse osmosis technology
• Developments in forward osmosis technology could reduce energy consumption in desalination processes by up to 50%

Mention desalination and most people think of arid lands in the Middle East and North Africa. But last month’s announcement by UK utility company Thames Water that it has got the go-ahead for a new £200m desalination plant in east London means that, from 2009, many in the UK capital could soon be receiving some of their water supplies via the technology. The plant will be the first of its kind to be built in the UK and will be able to provide up to 140m L of drinking water a day – enough for nearly 1m people, which should make the prospect of future water restrictions, such as hosepipe bans, less likely, Thames Water claims.

The decision to build the new facility, however, was not without controversy. London mayor Ken Livingstone originally opposed the proposed plant as wasteful and an energy guzzler that would contribute to global warming. Livingstone claims the plant is expected to release approximately 25 000t of CO₂ annually. Thames Water has confirmed that it will run only during times of drought or periods of low rainfall, or as back up in the event of an incident at one of its other water treatment facilities. It will also be powered entirely by renewable energy.

High energy consumption has long been the thorn in the side of modern desalination processes, preventing greater uptake of the technology. Like most major desalination plants now in operation in other parts of the world, the London facility will rely on a process of reverse osmosis, the major method of desalination for the past four decades. Reverse osmosis involves overcoming high osmostic pressure and moving water through a semipermeable membrane, typically made of polyamide. The high energy requirement results from moving water against an osmotic pressure of typically approximately 420psi in a conventional desalination unit.

Forward osmosis

New technology to reduce the energy consumption of desalination processes, however, is already being developed. At the UK’s University of Surrey, Adel Sharif and his colleagues have developed a desalination process using forward osmosis, now being commercialised by specialist water technologies firm, Modern Water.

‘Our technology could reduce the specific energy consumption of the desalination process by up to 50%,’ says Sharif, who directs the university’s centre for osmosis research and applications.

The team has been operating a desalination unit based on forward osmosis at Surrey for a year. Sharif’s work builds in part on research done by Jeffrey McCutcheon and his colleagues at Yale University, US, and the process involves using a ‘draw solution’ to manipulate osmotic pressure, thus allowing water to move from the source, ocean water, leaving the salt behind. ‘The draw solution contains osmotic agents that have osmotic potential higher than the source water,’ Sharif explains.

While the identity of the osmotic agents is proprietary, pulling the desalinated water out of the draw solution uses less energy than in reverse osmosis, Sharif says: ‘The properties of the osmotic agents allow ease of separation and hence lower energy consumption.’

Modern Water, which has recently raised £30m in a public offering, is moving ahead aggressively with the technology, says Neil McDougall, the firm’s executive chairman. The company is planning to buy existing desalination plants and retrofit them with the forward osmosis technology. ‘We are in discussions with a number of people to acquire existing plants,’ McDougall says. ‘Our major focus is on the Mediterranean and the Middle East.’

Eventually, the plan is to move into the market in North America, South America, and Australia. Modern Water has committed itself to ‘identify and acquire our first plant within 12 months of the initial public offering (IPO). As to the actual retrofit we’ve given ourselves for our first one 12 months, but we would hope that it would be rather faster than that,’ says McDougall.

Biofouling

Other efforts to reduce the energy consumption of modern desalination methods, meanwhile, are also focusing on the problem of biofouling. A major headache for desalination plants, biofouling occurs when bacteria form films that clog the membrane, thus increasing the energy demands to pump water still further. In Sharif’s case, the problem can be reduced by tailoring the ingredients in the draw solution, he claims, but elsewhere other researchers have more radical solutions in mind.

Erik Hoek, an assistant professor of environmental engineering at the University of California Los Angeles (UCLA), has focused on using nanotechnology to develop more energy efficient membranes – ones that can combat biofouling by themselves. But finding the right type of particles to perform this trick can be a challenge, he says.

‘One particle, which might kill bacteria, if you trap it in a polymer film, may no longer retain that activity. Another particle that might work well in solution might not have the right structural characteristics to make a better membrane,’ he says.

Hoek has found materials that can be fashioned into 100nm particles, which are porous, and can be integrated into existing membrane polymers. ‘Some of the early [lab-based] data that we have suggests that we can make membranes that are two to three times more permeable and more energy efficient without sacrificing salt rejection,’ says Hoek.

Based on these results, ‘We project up to a 25% savings in the cost of desalinated water based on a more energy efficient and fouling resistant membrane,’ says Jeff Green, ceo of NanoH₂O, a Los Angeles-based firm that is working to commercialise Hoek’s work.

‘It’s so rare that you see this kind of leap in technology without having to invest in a whole new technology platform. The combination of what Eric’s doing of using nanomaterials and the existing thin film polymer polyamide, [enables us] to leverage industry infrastructure,’ he says.

‘What’s interesting about adding the nanomaterial to the polymer membranes, is that you have now unlocked the degrees of freedom to change the performance characteristics of these membranes in a way that the industry can’t do right now.’

In some applications, for example, all the salt might need to be removed, but in other instances, ‘having semi brackish water might not be a big deal, in which case you could tailor the membrane and salt rejection, so you could get a much more efficient membrane because you don’t need to reject all the salt,’ he says.

he company is currently field-testing the nano-membrane in terms of scale up for manufacturing, says Green. Should that testing go well, it plans to commercialise the membrane in 2009.

If successful, this would be the first big change in membrane technology for several decades, experts say. ‘The membranes used for desalination were developed about 30 years ago and haven’t changed much in the meantime. They’ve gotten better incrementally, but the basic idea hasn’t changed,’ says Thomas Mayer, a project director for membrane research at Sandia National Laboratory in the US.

Hoek’s colleagues in the water technology research center at UCLA, meanwhile, have also developed sensors that can be embedded in desalination membranes that provide information on the condition of the membranes, such as biofouling and scaling that could impair their efficiency. The information would allow the central control system to decide ‘what to do; how to change operating conditions, whether or not any type of scale of fouling mitigation is needed, and for the most part actuate that,’ says Yoram Cohen, the center’s director.

He estimates such sensors can improve the efficiency of desalination plants by up to 20%. The sensors are currently being tested at the ‘bench scale’, and will be tested on pilot plants now being built.

The sensors are also being field tested in Israel, and will be tested in Australia. ‘I can’t tell you when it will become commercial,’ he says, adding that implementation depends on the interest of the market.

While Thames Water appears focused on using traditional reverse osmosis technology in its plant, the developers and commercialisers of the new technologies clearly have high hopes for them in the marketplace. These technologies promise increased efficiency and reduced energy demands, but the crucial challenges of scale up, adoption, and meeting those expectations remain.