Current research and innovation trends in desalination
The Middle East is home to nearly half the world’s desalination capacity and demand for desalination across the region continues to grow. Burgeoning population multiplied by high water consumption per capita are important drivers behind this upward trend. Growth in capacity has also been helped by the steady decline in costs. Desalinated water costs of 0.5-1 USD/m3 are readily achieved, on par with many conventional water sources. Several Middle Eastern countries, including Bahrain, Kuwait and Qatar, are entirely reliant on desalination today.
The continued desire to reduce costs has caused a technology shift in desalination. The shift has been away from the traditional thermally-driven technologies of multistage flash (MSF) and multi-effect distillation (MED) towards electrically-driven process of reverse osmosis (RO). RO uses several times less energy than MSF and MED, which means lower costs and lower emissions. Nevertheless, the environmental impacts of RO remain significant – not only because of CO2 emitted to the atmosphere, but also because of the massive quantities of brine discharged to the sea. Even state-of-the-art RO desalination plants give rise to >1 kg of CO2 and >1 m3 of brine for each cubic meter of freshwater produced.
What does the future hold?
The desalination industry has made fantastic progress in reducing energy consumption, but has paid relatively less attention to brine discharge so far. Management of brines not only reduces the impact on marine ecosystems, but also creates the chance to build a whole new industry around ‘mining’ the numerous elements contained in seawater. This opportunity has not escaped the attention of research funding agencies around the world. For example, the EU-funded sea4value project aims to recover nine elements from seawater, including magnesium, lithium and rubidium. Other recent initiatives include the Sandooq Al Watan ‘Think Brine Challenge’ and the King Adbullah University of Science and Technology ‘Brains for Brine’ competition.
The final stages of brine concentration and mineral recovery typically remain the territory of thermal technologies, because RO is unable to handle the extreme salinities encountered. However, the thermal technologies have less work to do (and spend less energy) if supplied with a feed that is already preconcentrated somewhat. This idea is currently driving several developments in high-recovery RO that range from stages of technological readiness, from the purely conceptual to operational industrial plant. For example, Yale University has proposed ‘Low Salt Rejection Reverse Osmosis’ (LSRRO) whereby RO membranes should be tuned to have imperfect salt rejection. A modest amount of salt leakage lowers the differential osmotic pressure across the membrane, which might otherwise exceed 200 atmospheres (enough to crush even the most robust membranes available). Use of several such membranes in series is theoretically predicted to provide near 100% rejection with an energy consumption just a fraction of traditional brine concentration methods. A more mature technology is that of counter flow RO, in which the feed is diluted by water transferred from brine exiting the system, to the point where it can be handled by a conventional RO device. As in LSRRO, the trick is to avoid a large concentration difference across the RO membrane while the absolute concentrations remain high, thus avoiding the destructive pressures otherwise associated with high salinity RO. Counter flow RO is currently being implemented in western Saudi Arabia.
Seawater and brine can be concentrated by solar evaporation as is the case in traditional salt works – the main downside being large land utilisation and maintenance costs of the salt works. But wind-driven evaporation can also be used with considerable land and cost saving. In fact, a wind-driven evaporator can provide benefits of cooling as demonstrated by Seawater Greenhouse technology in the Horn of Africa, thus reducing the cost below zero. An optimised system combining seawater desalination, brine concentration, greenhouse cultivation and mineral recovery, could provide a winning solution at the water-energy-food nexus.
Saving energy at high recovery
Higher salinity in RO systems tends to result in higher pressures, but how this translates into energy consumption also depends on whether you’re talking about average or peak pressure. Techniques of batch and semi-batch RO are designed to work at an average pressure well below the peak pressure. Used together with positive displacement pumps that can happily handle pressure variations, this can save a lot of energy. Savings are small at low recovery of less than 50%. However, at higher recovery of say 90%, the savings rocket. Batch RO is the most energy efficient version, superior to semi-batch RO, but requires additional pressure vessels that can drive up capital cost. Recent work from the University of Birmingham promises to overcome this drawback by combining the batch and semi-batch processes. The hybrid version remains compact and has near ideal efficiency while achieving >90% recovery. This means that 90% of the feed water is converted to freshwater while the remaining brine is reduced to one tenth the initial volume.
This is just the beginning for desalination
Whereas desalination technology has reached a state of maturity, it is also on the verge of a step change. To date, desalinated water has been almost exclusively for municipal use, but it is now reaching the threshold of affordability for irrigation as well. Given that irrigation accounts for 90% of water usage across the Middle East, this is likely to bring about a new growth phase to help meet food demands of the growing regional population. This is expected to attract further investments into the industry, with bright prospects to accelerate the many emerging technologies that will make desalination more affordable and sustainable.
About the author
Professor Philip Davies, Professor of Water Technology at the University of Birmingham’s School of Engineering.
The aim of his current research is to achieve sustainable treatment and utilisation of water resources in arid regions. His research areas include: desalination and water re-use, solar-powered cooling using seawater, seawater greenhouse technology, and negative emissions technologies. These areas contribute to the achievement of Sustainable Development Goals.
Philip participates in several international collaborations in regions including North Africa, the Middle East and the Indian sub-continent.