Water Reuse: It’s a Membraner!

  • 24 February 2021
  • Environment, Technology
  • Robert Field and Albert Weale

If you travel to Singapore and drink water from the tap, the chances are that some of what you drink will have been reclaimed from wastewater, rather cleverly named NEWater. Much of this NEWater is unavailable for human consumption, not because its quality is poor, but because its quality is so good that it is used by the electronics industry for cleaning, a purpose for which the water needs to be ultrapure. Singapore has its own reasons, closely rooted in regional security concerns, for wanting to be self-sufficient in water supply. But it also provides a global example of how membrane technology applied to water treatment can contribute to meeting one of the most basic of human rights, access to safe and reliable water supplies.

In engineering, synthetic membranes are typically extremely thin polymeric filters of around 0.2 microns in size, about one hundredth the diameter of the finest human hair. Just as infused water passed through a coffee filter leaving the grounds behind, so fluids, like water, can pass through a suitable membrane whilst other material is retained.  In fact, membrane filtration is even more stringent. The small size of the pores means that membranes work as an effective barrier for almost all forms of bacteria found in water, being more effective than sand or other types of traditional filter. Additional treatment using reverse osmosis filtration, as in the NEWater process, will remove from the extracted water most dissolved chemicals, for example calcium phosphate, as well as viruses. Whilst microfiltration filters out particles and bacteria, and ultrafilters with small pores retains macromolecules, reverse osmosis membranes are hyper filters filtering out the molecules themselves.

Water reuse can take various forms, depending on whether it is used for drinking or not (potable versus non-potable) and whether it is recycled directly for use or indirectly via a reservoir or river. An early form of direct reuse was developed over fifty years ago, in 1968, in a plant in Windhoek in Namibia to supplement a community’s water supplies. However, in the last forty years, developments in membrane technology have revolutionized the possibilities of obtaining safe drinking water at reasonable cost and the Windhoek plant was retrofitted with membranes in 2000. Water reuse projects involving an element of direct or indirect potable reuse can now be found in California, Texas, Australia and South Africa, alongside Namibia and Singapore. All advanced reuse plants deploy at least one form of membrane technology, because they are so effective in filtering out contaminants.

Singapore and Namibia in their different ways may seem remote from the UK, but parts of the UK are also confronted with an increasingly urgent problem of ensuring the supply of safe drinking water to households and businesses, particularly in the south and east of England. Population growth and global climate change suggest that by 2030 some 27 water resource management zones will have a deficit in supply relative to demand. Ideally this deficit would be met by better management of existing supply, including leakage reduction and more frugal consumer use brought about by metering and information campaigns. However, policy makers doubting that these measures will be sufficient to close the gap foresee the need to plan for increased water supply, alongside augmenting demand management, in what is known as a ‘twin-track’ approach.

The supply problem is compounded by the need to protect the ecology of chalk streams and rivers that are found in the south and east of England. Those streams and rivers depend upon the water that is captured in chalk aquifers from which much water is currently abstracted. There are around 200 chalk streams in the world, with over 80% found in the areas of chalk deposits running from Dorset to Yorkshire and further east. They offer a unique habitat arising from the purity of their water, but for many years there has been considerable concern among conservationists and anglers over their poor state arising from abstraction, highlighted in the dry period between May 2018 and May 2019.  The Environment Agency has acknowledged the problem and has started to restrict water abstraction, as well as developing various remedial programmes. However, its work will be fruitless if in future years there is pressure to meet the supply gap by increased abstraction.

Decision time on water supply is looming for the UK government. Planning law provides for government to designate nationally significant infrastructure projects to facility development consent and the raising of capital.  The policy framework within which individual designations are made is set out in a National Policy Statement. A draft National Policy Statement on Water Resources Infrastructure from DEFRA, issued in 2018, failed to put water reuse on a level footing with other water infrastructure types of project, which included expanding large reservoirs and dams, transferring water from one region to another and desalination. This decision was heavily criticised by the Commons Environment, Food and Rural Affairs Committee in its 2019 report on the subject, suggesting that the final National Policy Statement should encourage reuse projects. A similar view is also held by the Chartered Institution of Water and Environmental Management . When the technology of water reuse is so well-developed, it seems counterproductive to give it a second-class status.

No one is saying that membrane technology and the water reuse it makes possible is a panacea. Like other infrastructure projects, it has land use and energy costs. However, its advantages and disadvantages needs to be set squarely alongside those of the other technologies. Large reservoirs and dams make significant demands on land use. Desalination is about two to three times more expensive to operate and requires significantly more energy than water reuse, as well as creating a disposal problem due to the saline residue. Water transfer involves disruptive work on piping, sometimes in sensitive locations like national parks, for example the Lake District as recently reported by The Financial Times. Moreover, water transfer contains the danger of invasive non-native species being moved from one place to another, a particular danger arising from the threat from non-native species to the white clawed crayfish found in chalk streams. 

Governments may fear public opposition to water reuse, particularly direct reuse, based on the ‘yuk’ factor, and this fear may be behind the reluctance to put the technology on a par with other forms of supply. But governments have a role to lead as well as educate public opinion, particularly where natural resources are at risk. Folklore has it that the water abstracted from the Thames and drunk by the average Londoner has already passed through seven bladders. How much safer might people feel if it had passed through membranes instead. 

Image sourced from: "File:Semipermeable membrane (svg).svg" by Freemesm is licensed under CC BY-SA 3.0, and edited to fit to size. 

Professor Robert Field FIChemE, CEng is Emeritus Professor of Engineering Science at University of Oxford and current member of Northumbria University. He has published widely on membrane science and technology and their application, especially in the area of water treatment and is a former Vice-President of the European Membrane Society.  He is currently working on advanced membrane processes and the concept of Water Factories for C21 with colleagues in Australia, Texas, Singapore and South Korea. Together with Albert Weale, he is engaged on a project examining the policy dimensions of Water Re-Use which is an APEX project that has been supported by the Royal Society in partnership with the British Academy and the Royal Academy of Engineering together with generous support from the Leverhulme Trust.

Professor Albert Weale CBE FBA is Emeritus Professor of Political Theory and Public Policy at University College London.  He has written widely on environmental policy in the UK and elsewhere including The New Politics of Pollution (Manchester University Press, 1992) and, with others, Environmental Governance in Europe (Oxford University Press, 2000), as well as the edited Risk, Democratic Citizenship and Public Policy (Oxford University Press, 2002).  His most recent book is Modern Social Contract Theory (Oxford University Press, 2020).  He is a former Vice-President of the British Academy, having had special responsibility for public policy.  He is currently working on the idea of prudence and the principle of precaution. The authors thank Professor Timothy O’Riordan FBA for his comments on an earlier draft.