Using zero liquid discharge (ZLD) to enhance water recovery from desalination is a growing strategy to tackle water shortages. However, this approach is costly, energy-demanding, and can pose environmental challenges. Researchers have worked on optimizing ZLD technologies to minimize costs, energy use, and land requirements.
Water scarcity is becoming increasingly problematic due to climate change. A potential solution to this issue is desalination technology, which allows us to utilize seawater. While it holds promise, desalination also raises concerns about its environmental footprint, affordability, and accessibility. ZLD technology aims to maximize water extraction from desalination brine, thereby addressing both water shortage and waste management from desalination plants. However, it comes with higher costs and possible environmental consequences.
In a recent study led by Jennifer Dunn from Northwestern Engineering, researchers used an innovative optimization model to analyze ZLD’s role in desalination plants as a means to address future water scarcity. They discovered that while ZLD can be beneficial, it involves considerable trade-offs regarding energy usage, saline water disposal, and affordability for low-income populations.
During desalination, seawater is processed through a membrane that filters out salts, resulting in fresh water and concentrated brine. ZLD has the potential to enhance water recovery from this brine and diminish its volume, making disposal less challenging. Although desalination facilities are common in nations like Israel, Australia, and Saudi Arabia—where water shortages are critical—the significant energy requirements for large-scale desalination pose a major environmental challenge.
The substantial energy needed to push water through membranes represents a significant barrier to both desalination and ZLD. This situation creates a complex cycle—producing energy generally requires water, while generating water through desalination demands a large amount of energy.
“The main difficulty is that a significant amount of energy is necessary to desalinate water and utilize zero liquid discharge effectively,” Dunn remarked. “If fossil fuels serve as the primary energy source, this can lead to considerable environmental repercussions. While clean renewable energy is being explored, its availability is still limited depending on the location and the existing infrastructure.”
Dunn, a professor in the McCormick School of Engineering’s chemical and biological engineering department, published her findings in the paper titled “Analysis of Energy, Water, Land and Cost Implications of Zero and Minimal Liquid Discharge Desalination Technologies” in the journal Nature Water on November 18. She also heads the Center for Engineering Sustainability and Resilience and is the associate director of the Northwestern-Argonne Institute of Science and Engineering.
In their research, Dunn and her team sought ways to improve the efficiency of ZLD. They created a new optimization model designed to facilitate the design of desalination treatment processes (a series of technologies that work together) that included seven distinct treatment options. This involved extensive research on each technology used in the overall process, which is referred to as a treatment train (a series of steps leading to zero liquid discharge). The model, called WaterTap, is supported by the US Department of Energy and led by the National Alliance for Water Innovation.
“ZLD and minimal liquid discharge processes generate more water, which can be critical in areas experiencing water shortages, but they also raise energy consumption and costs,” Dunn explained. “Each facility needs to make informed decisions based on its specific situation and available resources. It’s all about finding the right balance.”
The disposal of brine also raises environmental concerns. Many coastal desalination plants return brine to the ocean; however, the long-term consequences of this practice are still not fully understood. There are worries that brine, which has a higher salt concentration than seawater, could disrupt marine ecosystems in vulnerable areas.
Dunn stressed the importance of monitoring brine disposal as desalination becomes more common.
“Currently, there isn’t enough information about how high-salinity brine affects marine environments,” Dunn stated. “In some regions, the impact might be minimal, but in others, it could be quite harmful. We are dedicated to addressing these knowledge gaps.”
Desalination can be prohibitively expensive, especially for low-income areas most affected by water shortages. The costs involved in constructing, operating, and maintaining desalination plants are significant, alongside the heavy energy demands. Even when subsidies are offered for desalinated water in some nations, they often fall short.
“Desalination shouldn’t be viewed as the sole solution,” Dunn noted. “It is essential in some regions, but it must complement a wider water management strategy.”
Dunn highlighted that several countries are adopting a “multi-faceted” approach to address water scarcity by combining desalination with techniques like water recycling, rainwater collection, and conservation efforts. This variety of strategies provides distinct advantages, equipping communities to better handle fluctuating resources and increased demand.
“While desalination is vital in specific areas, it shouldn’t be the exclusive strategy for addressing water shortages,” Dunn concluded. “For significant advancements, we need to consider it as just one part of a more comprehensive, sustainable water management plan that caters to the unique challenges and resources of each region.”
