Would you believe it if someone said Hawaii was considered “water insecure”?
Freshwater scarcity is common not just in arid areas, but islands surrounded by seawater and cities with limited access to clean resources. Whereas Hawaii reuses wastewater to protect the state’s drinking water, other regions use desalination — the process of removing salt from liquid to create a fresh product suitable for cleaning or consuming. And many of these operations are rooted in a technology called reverse osmosis (RO).
With today's standards for sustainability, it’s important for desalination to keep up. Here are three components of a strong RO process that work to make desalination a safe and efficient solution for power stations, public taps and the like.
Reverse osmosis (RO) is a cornerstone of desalination solutions today. Used in seawater, brackish (or brine) water and related ecosystems, it pushes water through a semi-permeable membrane opposite to the direction its molecules would normally move — filtering contaminants and large particles as a result. See more on its use in seawater below:
Historically, desalination is known for being energy-intensive, but RO can make the technology more efficient when coupled with thermal-based desalination. By reusing waste steam from local power operations, one can evaporate some of the saline water and distill it into freshwater, leaving a more manageable volume to enter RO. Solutions like these can bring the technology’s power usage down to as little as $1 per cubic meter (it typically costs around $3/m3). And with 15 new desalination plants proposed in California alone, this reduction could pay enormous dividends for the state as it currently invests millions in energy to protect its water amid an ongoing drought.
When desalination uses ocean water, it’s important to ensure these energy efficiencies don’t just reduce utility costs, but protect the marine environment as well.
Energy recovery devices (ERDs) can ensure the high-pressure process of RO doesn’t harm the aquatic ecosystem. Facilities like the one in Sharqiyah, Oman employ ERDs to reuse power from pre-treatment to depressurize the water that touches an RO membrane, before osmosis occurs. This allows the final product to reenter habitats safely if discharged into the environment. Multiple plants have managed to recapture energy in this way — and reuse water from other properties — to limit their impact and dependence on natural resources. It gives people the water they need while minimizing the expense to their surroundings.
Zero Liquid Waste
Efficient desalination services must also ensure RO doesn’t reintroduce content that would imbalance the original water source. This is important when dealing with public watersheds, some of which neighbor mines that contain saltwater and need to follow strict environmental regulations. These watersheds deliver groundwater to communities via aquifers that depend on natural freshwater.
Zero liquid waste (ZLW) is a three-step process with RO at the center:
- ZLW first softens the raw saltwater so it’s more easily filtered.
- It then puts it through RO to remove chloride and sulfate content. Plants that use thermal desalination then combine this RO-treated water with the thermal distillate.
- Lastly, it’s separated from these salts through crystallization. This stage brings these salts to a solid brine so only clean water remains. The water is then discharged back into the watershed so neither salt nor any new materials inhibit the flow of freshwater.
Better still, these salts can be repurposed, such as for road salt in cities that face snowy winters.
Communities shouldn’t have to waste one resource to protect another. And whether they’re looking to reduce their electric bill, conserve an ocean’s saltwater or remove it from a wetland’s groundwater, contemporary desalination presents clear opportunities for water and energy providers to become more efficient.