Modeling and analysis reveals technological, environmental challenges to increasing water recovery from desalination

In a new analysis by a team led by Northwestern Engineering's Jennifer Dunn that uses a novel optimization model, researchers concluded that while incorporating ZLD into desalination plants is a valuable way to fight future water scarcity. The process, however, poses notable tradeoffs when it comes to energy use, disposal of water that has salt, and cost for low-income areas.

In desalination, seawater is filtered through a membrane that removes salts, leaving fresh water and a salty brine. ZLD can increase water recovery from this brine and reduce its volume, leading to more manageable desalination waste streams. While desalination facilities are abundant in countries like Israel, Australia, and Saudi Arabia where water scarcity is acute, the energy required to desalinate water at scale presents a significant environmental hurdle.

Due to the pressure needed to push water through membranes, high energy demand is a considerable obstacle to desalination and ZLD. This demand presents a perplexing cycle -- energy production often requires water, and water production from desalination now requires significant energy.

"The big challenge is that you need a lot of energy to desalinate water and increase water production using zero liquid discharge," Dunn said. "That energy comes at a high environmental cost, especially if fossil fuels are the primary energy source. Renewable energy is being investigated as a cleaner power source, but these options are still limited, depending on location and available infrastructure."

Dunn is a professor of chemical and biological engineering at the McCormick School of Engineering. She reported her findings in the paper "Analysis of Energy, Water, Land and Cost Implications of Zero and Minimal Liquid Discharge Desalination Technologies," published November 18 in the journal Nature Water. Dunn directs the Center for Engineering Sustainability and Resilience and is the associate director of the Northwestern-Argonne Institute of Science and Engineering.

In the paper, Dunn and her colleagues evaluated methods to make ZLD more efficient. They did this by using a new optimization model that aids in the design of desalination treatment trains (multiple technologies that work together), including seven different options for treatment trains. This required extensive research on each technology included in an overall process train (series of steps that result in zero liquid discharge). The model, WaterTap, is funded by the US Department of Energy and is led by the National Alliance for Water Innovation.

"ZLD and minimal liquid discharge processes give you more water, which can be crucial in water-scarce areas, but you're increasing the energy and costs," Dunn said. "In each plant, decisions need to be made based on the specific location and the resources available. It's all about trade-offs."

Brine disposal also poses an environmental issue. Coastal desalination plants often pump the brine back into the ocean. However, the long-term effects of that practice are not yet known. A concern is that brine has more saline than seawater, giving it the potential to disrupt marine life in sensitive areas.

Dunn emphasized that monitoring brine disposal will be essential as desalination becomes more widespread.

"There's not enough data on the effects of high-salinity brine on marine ecosystems," Dunn said. "In some areas, the damage may be minimal, but in others, it could be disruptive. We're working to fill those gaps."

Desalination is costly, presenting problems for low-income regions that have the biggest issues with water access. Desalination plants are expensive to build, operate, and maintain, and require large amounts of energy. Some countries do provide subsidies for desalinated water; unfortunately, they can be insufficient.

"Desalination can't be the only solution," Dunn said. "In some areas, it's essential, but it must be part of a broader water management strategy."

Dunn pointed out that several countries are taking a "multi-faceted" approach to address a lack of water by combining desalination with methods such as water recycling, rainwater harvesting, and conservation measures. That mix of techniques has obvious advantages, better preparing communities for unpredictable resources and increasing demand.

"Desalination is crucial in certain regions, but it can't be the only answer to water scarcity," Dunn said. "To make real progress, we need to look at it as one piece of a broader, more sustainable water management strategy that's adapted to the unique needs and constraints of each area."