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Desal - Reverse Osmosis Process

Note: This project page will be continually updated as there is more information to share.

Marin Water has explored several options for supplementing our water supply during times of water shortages, including seawater desalination.


Desalination is the process of removing salts from bay or ocean water to create freshwater suitable for drinking through the use of specialized treatment systems/facilities. Treatment of the saltwater uses a process called reverse osmosis where the water is pushed through a membrane (a barrier with tiny holes) with the salts and other minerals left in the brine solution. 

the desalination process overview. Water travesl from Bay Water to an intake system to pre treatment to reverse osmosis to our distribution system


Temporary (Emergency) Desalination Systems vs. Permanent (Long-Term) Desalination Facilities – What’s the Difference?

Throughout the summer of 2021, Marin Water explored a number of desalination project options to address both near and long-term water shortages. This included temporary desalination systems as well as permanent desalination facilities.

  • Temporary desalination systems may be purchased or leased and are containerized systems that can be brought online in a more expedited timeframe to address a more immediate water shortage. The capacity of the containerized systems are limited to what is readily available in the market. At the end of the water shortage, the temporary desalination facility could be dismantled.
  • Permanent desalination facilities are designed to address future, long-term water supply needs, are permanent structures, and are expected to take a much more involved and extended timeline to develop and implement.

By July 2021, despite community conservation efforts of 20 to 25-percent reduction in water, the District’s reservoirs were projected to drop to around 30 percent to below 10 percent of total storage capacity by the summer of 2022. With these projections in mind, Marin Water narrowed in focus on further investigating the potential of an Emergency Temporary Desalination facility to address the near-term water storage threat.


Current Project Status

Temporary Emergency Desalination, a proposed emergency measure to address near-term water shortage, is no longer being pursued as an option.  In October 2021, after carefully analyzing this water supply alternative, the District concluded that it was not the right project to pursue for its water supply needs due to a number of project challenges when other more promising water supply options were also being considered. Primary challenges with temporary emergency desalination included the timeline to bring the system online to address a looming water shortage, available capacity limitations with providing the necessary volume of needed water, and the cost of the temporary project per acre foot. 

Additionally, as of early January 2022, thanks to incredible winter rainfall, Marin Water’s reservoir storage levels have risen enough to ensure there is adequate water supply for the next two years. Therefore, the focus has shifted to long-term drought resilience efforts.

The District is continuing to investigate a permanent desalination facility as a future supplemental water supply option. Various potential permanent desalination facility opportunities are being explored and include:

  1. A facility serving just Marin Water
  2. A North Bay regional facility
  3. A regional east bay facility that the District could potentially access if its proposed intertie pipeline project were constructed providing a connection.

Because the District’s supplemental water needs are only in times of drought, an issue that will need to be investigated is how to operate a permanent desalination facility to protect the filter membranes. Extreme care is required to preserve reverse osmosis membranes when desalination production is slowed or stopped during normal rain years.

Previous District Documents/Studies on Deasalination