The UK weather is mostly predictable; it can be relied upon to rain. However, last summer, the country faced a prolonged period of dry weather coupled with extreme heat, which resulted in a drought across most regions. Now a year later, southern England is still recovering its reservoir levels and considering investing in a desalination plant to make up the deficit. As we anticipate these hotter, drier spells to be the new normal, finding alternative and weather-resilient solutions to water security is of increasing importance for the UK.
50 years of innovation
Our national water story began in the 1970s as the country launched its first Water Master Plan to develop diversified water sources and a resilient water system, which my colleague William Yong explored in his discussion of Singapore’s journey to water excellence. As these so-called national taps were developed – first, importing from our neighbours, and second, building reservoirs – it became apparent that much of the national water provision relied on the occurrence of rain. It was the case then as it is now that rain does not always come when you need it. This focused attention on developing further taps that we had greater control over. One of these was desalination.
Beginning the desalination journey
Any desalination plant is run by one of two mature processes – evaporation or reverse osmosis. For evaporation to be successful at scale, a significant amount of energy is required, which has cost and carbon implications. Reverse osmosis, while still requiring large amounts of energy, has a lesser demand than evaporation and is more efficient. For this reason, reverse osmosis is used at all five of Singapore’s desalination sites.
The reverse osmosis process works by forcing water under pressure through a semi-permeable membrane. As the water passes through, the membrane blocks the salt, separating it from the water. In the context of a climate crisis, which is accelerating the need to consider these alternative water resources, technologies that have a large energy demand present a second set of challenges. Because of the sheer scale of this energy need, it is unlikely that renewable sources will be able to generate sufficient amounts. This is in part owing to the limited availability of land for solar panel or wind turbine installations, though of course the former can be utilised on site roofs to run other elements of the plant’s activity carbon free.
Because of this energy challenge, significant investment has been made to improve, and continue to improve, the energy efficiency of the process. One such mechanism that has been developed utilises the energy recovery nexus to drive energy back into the process. Energy recovery devices that operate at a constant pressure with more than 95% efficiency can recover otherwise lost energy from the high-pressure rejected water back into the feed stream, significantly reducing the overall energy required for desalination. This development has the benefits of substantial savings in operating expenses, reducing the overall costs of desalination processing and lowering the barrier to its adoption by the industry.
What can we learn?
With the addition of desalination and recycled used water (known as NEWater), Singapore now has four national taps for its water supply. We continue to utilise the first two taps, imports and reservoirs, to supply us with our day-to-day needs. This is because of desalination’s high energy requirements, which means it has a high carbon footprint, as we have explored. As we work to tackle climate change, we need to consider carbon reduction as much as we consider solutions to the challenges we face, and utilising desalination in emergency circumstances ensures that balance is respected. To integrate renewable solutions at the capacity required, a large land area will be needed. While Singapore is far more restricted in available land than the UK, utilising land appropriately remains an important consideration.
Because Singapore is a small country with a dense population, we have found intuitive ways to optimise the space we have to meet our needs. Utilising every inch available has led to the development of highly compact sites that also remain very functional. The Keppel Marina East Desalination Plant in the bay area has been built by locating its treatment facilities entirely underground, while a green roof on the surface has been integrated with surrounding parkland and is accessible to the public. The approach makes better use of available space, which is vital not only in a country as land scarce as Singapore but also in the context of cities. Developing this technology, in any region, will need it to be close to the populations it will serve; therefore, it is likely to be in a space-restricted area.
One challenge that we continue to face, and is an evolving development priority here in Singapore, is the question of energy efficiency. As we have explored, the desalination process is highly energy intensive, and we now need to find ways to reduce that energy requirement whether the plant is in service here in Singapore or in the UK. As the technology has evolved and developed, we have seen that achieving greater efficiencies improves system delivery and makes it a more sustainable solution. Currently, PUB is undertaking research to understand how we can achieve a more efficient process. Researchers are aiming for up to a 50% reduction in energy use based on current levels, which will secure a more resilient resource for the future.
As this research is undertaken, we hope for a Singaporean breakthrough. However, what we do know for certain is that, until that point, desalination will remain an emergency solution. We engineers across Singapore have seen the growth in this technology and the vital contribution it makes to our national water security, but we must also learn from the challenges in achieving efficiency and energy reduction that mean it is also future-proof.
Jason Tan is director at Binnies Singapore, an RSK Group company, and was involved in the development of three out of the five desalination plants in Singapore as the design manager, engineering manager and project manager.