Water is a hot commodity in the Western states of America, with the two largest reservoirs fed by the Colorado River at record lows. This is causing panic for everyone downstream, leading to water cuts this year:
“Lake Mead provides water to roughly 25 million people in Arizona, Nevada, California and Mexico, according to the National Park Service. Under the complex priority system…Arizona will see an 18% reduction…Nevada will need to adhere to a 7% reduction in its Colorado River water supply in 2022”
In the backdrop of the world’s shrinking freshwater supply, this post explores the dominant water desalination technique for “making” clean water for humans and irrigating crops: reverse osmosis. The goal of this is to provide an overview of the process and the corresponding environmental impact.
Water desalination falls into two classes of technologies: membrane filtration processes and thermal filtration processes. Thermal filtration uses thermal energy to heat and evaporate, then subsequently condense water. We’ll spend our time on reverse osmosis, which is the dominant membrane filtration process.
Osmosis was documented in the 1700s, but it wasn’t until 1950 when two researchers from UCLA and the University of Toronto discovered a commercially viable membrane for a large reverse osmosis system. Since then, we’ve steadily grown the capacity of reverse osmosis desalination plants across the world. Today, reverse osmosis accounts for 70% of all water desalination plants in the world, with over 16,000 desalination plants with a capacity to generate 120 million cubic meters of water per day. Based on New York City’s 2020 water usage, the current capacity of worldwide water desalination could support 32 cities with similar water consumption.
Membrane filtration works like a coffee filter, but on different scales. In a coffee filter, you have ground beans and hot water on one side and you push all of that through a coffee filter of your choosing. The beans stay on one side, but the extracted caffeine goodness (or the decaf for you savages) along with water move to the other side.
Water molecules are tiny, the smallest non-gas molecule known to humans. Everything else, is a tad bigger — salt molecules, microbes, viruses, you name it. So, we created a membrane that is just a bit bigger than water molecules, but smaller than everything else. Then we apply a ton of force (paying in energy, more on this in a moment) and we force the saltwater through the membrane and only water molecules remain on the other end.
Energy is a critical input for both water desalination processes and while both utilize mountains of energy, reverse osmosis is a bit more energy efficient and one reason it owns market share in water desalination.
Reverse Osmosis comes with 3 major environmental impacts: energy use, water intake and outfall of brine water.
On the energy front, it costs about 10,000 gallons of oil per year to desalinate 1,000 cubic meters of water per day (source). How much is 1,000 cubic meters of water? For context, Palm Springs, home to over 100 golf courses, averages 3,800 cubic meters of water per day, per course.
This is a staggering amount of energy consumption via fossil fuels. However, there’s a clear path to making water desalination more sustainable and reduce reliance on fossil fuels. Ever decreasing costs for solar power systems and technological improvements in battery technology will aid this transition to more sustainable energy.
We impact the environment in another key way: water used for desalination is imported from the ocean. The biggest environmental concern is sucking in marine organisms from fish to larva and creating an uninhabitable zone for marine life.
The lowest environmental impact way to take water from the ocean is to do it from a seabed aquifer. These are locations close to open ocean, but often segmented by sand and rocks. By drilling underneath the seabed or drilling a beach well — called galleries — the system takes seawater filtered by rocks and sand instead of directly from the open ocean.
Unfortunately, this method does not meet the capacity needs of the largest systems. The largest desalination plants must take water from the open ocean. This creates a current that brings seawater and all marine life in proximity, such as fish and larvae, into the system. Mitigation techniques include: speed limiters, mechanical exit gates, and screens.
Lastly, there’s the outfall. After we desalinate water, we are left with a brine water solution. Two concerns about this brine water: it’s higher salinity and temperature than ambient seawater, creating an unnatural zone for sea life. It can also have chemicals from cleaning and treatment of the reverse osmosis membrane and corrosive materials from pipes.
Salt is heavier so it will tend to fall to the seafloor — the concern is salt aggregating on the seafloor at an unnatural level. Desalination plants address the issue by directing exit pipes upwards, shooting the water upwards and using diffusers to speed the mixture of salt into the ambient ocean water.
The higher temperature creates an unnatural temperature zone, affecting sea life. A common mitigation technique includes releasing the water at deeper depths where there is less marine life or using diffusers to increase mixing with the colder ocean water.
While impossible to remove the environmental impacts of reverse osmosis, this critical process for humanity has a number of methods for lessening the impact on our environment while ensuring our own livelihood.
As water has floated to the top of critical items facing humanity, more innovations have occurred in smaller scale water desalination, utilizing renewable energy and nature inspired processes to desalinate water, targeted at smaller consumers. These solutions are interesting because they are decentralized with lower power draw. We’ll explore these in a future post.