Salt water cities - Nanotechnology and desalination plants

Materials World magazine
,
1 Dec 2015

With 1.8bln people expected to be living in areas of water scarcity by 2025, reverse osmosis desalination plants are seen as a way to solve supply issues. Can nanotechnology make these power-hungry plants more efficient, and feasible for developing nations? James Perkins reports. 

Tired of fighting over water with rival Californian cities, San Diego’s water authority took the decision 12 years ago to build a large reverse osmosis (RO) desalination plant at Carlsbad to ensure its own supply. It is due to begin operation before the end of 2015, as the state battles a record drought.

San Diego, the eighth-largest city in the US, has followed the lead of countries such as Saudi Arabia, Israel and the United Arab Emirates in commissioning large-scale desalination plants. The number of people worldwide living in water scarce areas is growing and these plants are being seen as a way to quench the thirst of both people and industry. According to Bloomberg, China plans to quadruple its desalination capacity to 3 million cubic metres of purified seawater per day by 2020.

This proliferation of desalination plants comes as governments the world over attempt to curb CO2 emissions to prevent catastrophic climate change. These plants are expensive to build and therefore not feasible for many developing nations, and they are expensive to operate, largely due to maintenance and power consumption. 

San Diego has invested US$1bln in the Carlsbad project, which will supply 204,412m3 of water daily to the city. It will use 30-35MWh for every hour of operation at full production, which is around 9,092m3/h. Of this energy load, 70% is for production of the water and 30% is related to delivery from sea level to the water authority delivery system in San Marcos, which is around 350m above sea level. The energy is needed to reach the pressure needed to push the water through the membranes used in the RO process. If the membrane technology used in RO plants is improved through nanotechnology, power consumption could be cut dramatically. Even improved processes, such as manipulated osmosis, developed by UK company Modern Water, still use membranes. 

As Tim Harper, CEO of UK-based G2O told Materials World, 'If you require less energy to clean water, then instead of having these huge 'megaplants' such as the one on the west coast of the USA, you could end up with much smaller plants powered by renewables such as solar, wind or tidal. You are reducing the energy requirements, which means you do not need those economies of scale anymore.' G2O is working on a graphene oxide coating that can be used on existing membranes, to increase flow rates and reduce fouling.

The competing technologies

Competing companies and researchers are taking various routes to achieve nano membranes. Scientists at the Massachusetts Institute of Technology (MIT) are working to fabricate a graphene film coating created through chemical vapour deposition (CVD), and Lockheed Martin has done a similar thing to create its patented Perforene technology. G2O and a rival team at the University of Manchester are using graphene oxide (GO) prepared with solution-based methods that are claimed to be easier to scale up than CVD.

In the lab, Harper has seen the G2O membrane coating give a flow rate increase of anywhere between 3–100 times, reducing the associated energy cost by around 90%. 'Generally, in water production, somewhere around 50% of operating costs is energy. That is even when you use the turbines downstream to recover 50% of that energy. Because the spacing between the graphene oxide platelets is just larger than a water molecule, you get wonderful preferential transport of water between a graphene platelet, and that increases the water flux rate, allowing us to create a very efficient filter,' said Harper.

Today's membranes are made of a range of substances, from polyamide, polysulfone, cellulose nitrate and ceramics. Polymers of intrinsic microporosity are also showing promise as a new type of substrate. It is not just desalination where improved membranes could prove useful. Industries from oil and gas to wastewater treatment, beer distilleries and pharmaceutical producers depend on this technology. G2O recently received a £700,000 grant from Innovate UK to work with the Nuclear Decommissioning Authority on scaling up its process for use in the clean up at Sellafield. 'If it works there it will work with pretty much any type of hazardous waste. If we can crack one of the top problems then the rest should be pretty simple,' said Harper.

The team at the University of Manchester is also working with graphene oxide, though Rahul Nair, Reader at the School of Physics and Astronomy, was not able to go into detail, as the team's recent developments in this field are not published. While defect-free graphene film is a perfect barrier membrane, the Manchester scientists have previously shown that water flows through a layered film of GO. Nair said, 'Pure one atom thick sheets of graphene do not allow anything to pass through. If you want to allow water and block salt, you have to make very small pores. The pore size would be 0.5nm, and the challenge there is to make that very uniform controllable pore. 'He continues, 'In our case, using multi-layer film, we are depending on the space between graphene, which is around 1nm at the moment. If you can control that down to a certain lower limit, we will also be getting salt rejection and desalination. Currently, we are in the process of developing this with our industrial partners Lockheed Martin and BGT Materials.'

The MIT team, in partnership with the Oak Ridge National Laboratory and King Fahd University of Petroleum and Minerals (KFUPM), is using CVD to create its graphene film, with defects repaired in a two-step process, before holes are punched in the material to allow the permeation of water. It is a complex affair. First, intrinsic defects were healed using atomic layer deposition conducted in a vacuum chamber. This involved pulsing a hafnium-containing chemical that does not normally interact with graphene, but in this case it plugged the defects upon contact. A second process used interfacial polymerisation, where the graphene was submerged at the interface of two solutions – a water bath and an organic solvent – containing the ingredients for nylon, and this plugged the remaining holes. At this stage, salts are still passing through the punched holes. Professor Rohit Karnik, Associate Professor of Mechanical Engineering at MIT and leader of the Microfluidics and Nanofluidics Research Group, told Materials World that the next step for his team is to improve their understanding of the transport properties and pore creation in graphene. 'One of the key goals is to better control pores such that monovalent ions are also rejected. In parallel, there is an effort to commercialise these membranes through an MIT spin-off.'

The scale up

For the parties working on the graphene oxide process, there is a lot of confidence in the ability to scale it up. Nair said, 'Currently we are working on the scale of centimeters, or tens of centimeters and we are already in communication with potential partners about further scale up of our process..' 

Harper is also bullish on the potential for the G2O technology to be produced in larger volumes and this is a key reason he backs the technology. He has seen nanowires, fibres and carbon nanotubes used in filtration membranes but 'they all seem to have fallen over just on the scalability issue'. The G2O technology is different, he says, 'We have already scaled up from a few square centimetres to tens of square centimetres with no problem, and over the next six months we should be scaling up to square metres and moving that into more of a roll-to-roll process.' Harper claims the GO membrane coating is easier to fabricate than the single-layer graphene films. As well as scaling the process up, the teams must perfect the substrate bonding process. Harper says G2O chemically combines with its substrate with a covalent pi-pi bond. 'The way we do it is we use vacuum filtration - we take the existing membrane, we apply a dispersion of graphene oxide and the vacuum pulls that into the pores, into the body of the membrane, and it is just a question of letting it dry. It is a water-based solution.' The University of Manchester team uses a similar process. Nair said, 'We use a solution-based technique to coat graphene, or graphene oxide, on various porous supports, using normal spray painting or coating techniques.'

The MIT team has just achieved proof-of-concept in the laboratory and Karnik said scale-up manufacture, tailoring of the membranes to specific applications, and packaging of the membranes will need to be addressed. 'How much longer it takes for this technology to be used in industry depends on the amount of support it receives, and the application.' He continued, 'It could be as little as a year or two for small-scale filtration applications to several years for realising large-scale desalination.' Nair echoes Karnik's observation about support. There the new technology needs to outperform existing and competing membranes on a large scale to have a chance. 'Competition is tough and even if, let's say, we come out with a nice membrane, I am not sure that the filtration plant owners will be happy to replace that membrane. This is a huge challenge.'

Of all the applications of graphene, permeable and barrier films appear to be two with the most potential, with each having the ability to improve a number of industrial processes. With the right levels of support, these filtration membranes could be only two years away, but will they get that support?

Water – in demand

According to Water.org, 'Only 2.53% of earth’s water is fresh, and some two-thirds of that is locked up in glaciers and permanent snow cover.'

It is predicted that 1.8bln people will live in areas affected by water scarcity by 2025.

663 million people worldwide lack access to safe drinking water.

The water crisis is the top global risk based on impact to society, according to the World Economic Forum

400 of China's top cities suffer from water scarcity.

It takes 130,000 gallons of water to drill a horizontal shale gas well and a further 2,700,000 gallons for hydraulic fracturing.