From Sea to Tap: Desalination Explained

More than 70 percent of the Earth’s surface is covered by water, but only 2.5 percent of it is freshwater. This underscores the need for a technology to address a great environmental and technological challenge facing parts of the globe: access to a constant source of safe, clean, drinkable water.

With changes in climate affecting drought patterns, heatwaves, and natural disasters, current water sources in many places are not enough. This is where the technology of desalination can play a role. 

Desalination is the process of removing salt and other minerals from seawater and converting it into fresh water for people and agriculture. This technology is vital for many countries and states, supplying them with freshwater in areas with limited access to water resources. 

A common method is reverse osmosis, which uses pressure to force water through a semipermeable membrane, separating salts from water and removing other impurities in the process to produce fresh drinkable water. Another method is distillation, which utilizes evaporation and condensation to separate salt from water. This involves heating saltwater to produce steam, then condensing it into drinkable water. There are also technologies Electrodialysis and Freezing, but these are seldom used compared to previous methods. 

Currently, desalination projects exist in many states and countries. In Florida, the Tampa Bay Seawater Desalination Plant uses reverse osmosis to produce freshwater from saltwater.  According to Tampa Bay Water, the plant captures up to 44 million gallons per day (MGD) of seawater, separating it into freshwater that can be used by people and concentrated seawater, which is then released back into the bay. This project is also drought-proof and is able to supply the Tampa area with 25 MGD (about 94,600 m³/day) of fresh water. 

Putting this in perspective, the average American Household uses 300 gallons of water per day. Used for showers, drinking, cooking, and hygiene. Breaking it down further, flushing the toilet once uses 3 to 4 gallons of water per flush, an 8-minute shower 20 gallons, a full bath 36 gallons, dishwashers 6-16 gallons, and one load of laundry 25-40 gallons. 

In context, a desalination plant that produces 1 MGD can serve roughly 10,000 people, based on the average U.S. Household water usage (300 gallons/day according to the EPA, based on the average household of 3 people (100 gallons/ per person)). With this, the Tampa facility can supply roughly 250,000 people.

Pumped water goes through a rigorous pre-treatment process, which uses screens to sort out debris and waste. It undergoes two treatment processes: coagulation, which changes debris from a solid state to a semi-solid state, and flocculation, which clumps small particles together, removing suspended solid particles from the water.  Chemicals are added to help the removal of debris. The remaining water is passed through filters, a process known as sand filtration. Finally, cartridge filters act as the last barrier, ensuring that most unwanted particles are no longer present. After the pre-treatment process is complete, reverse osmosis comes into play. 

For the post-treatment process, additional chemicals are added to ensure the desalinated water is stabilized. The water is then transported to regional water facilities, where it is blended with drinking water and delivered to homes.

In California, the City of Camarillo North Pleasant Valley Desalter project, completed and expected to produce 3,800 acre feet (approximately 3.4 MGD or 12,800 m³/day). This project also sought to break dependence on imported water supplies. 

The Doheny Ocean Desalination Plant will include reverse osmosis via slant wells.  Producing 5 MGD (about 18,900 m³/day) of drinkable water. These projects will expand production, targeting 28,000 total acre-feet/year (25 million gallons/day or 94,600 m³/day) of groundwater desalination by 2030. 

Cost can often pose a barrier. Financial realities such as construction, operation costs, and overrun costs pose challenges. Understanding this and ways in which it can be mitigated is necessary for this solution to be a long-term, viable infrastructure investment. 

Capital investment costs can amount to between $1,000 and $ 2,500 per m³/day ($3.78 to $9.46 per gallon/day) of seawater with a reverse osmosis plant capacity. 

Total Plant estimates: 5 MGD plant capacity (Million gallons per day)

Total Plant estimates: 25 MGD plant capacity (Million gallons per day)

Reverse osmosis operating costs range from $ 0.50 to $ 1.00 per cubic meter, according to the World Bank. Key cost drivers include energy, chemicals, labor, maintenance, and membrane replacement every 3-5 years. There are also cost overruns, which average between 15 percent and 25 percent due to delays in construction or regulatory changes. There are, however, ways to reduce costs. For economies of scale like the United States, larger plants (25 MGD) reduce the per-unit cost by roughly 25-40 percent, and the use of onsite renewable energies (wind/solar) can lower costs by 15-30 percent when compared to using the electrical grid. Private-public partnerships and international loans can also ease challenges. 

Opponents of this method cite high expenditures often incurred in financing these projects, as they could be passed to ratepayers, and an increased environmental impact. There are also concerns about water quality, high energy consumption, and often high operating costs. 

Proponents with these concerns in mind still believe these plants provide for an increased water supply, allow for diverse applications (i.e., agricultural purposes), and could also mitigate water scarcity and drought issues, providing for a heightened independence of the water supply from various seasonal factors. 

While these plants are not perfect, desalination is a promising long-term solution. With innovation (low salinity RO, counter-flow RO, various energy recovery systems), it can become more efficient, cost-effective, and less environmentally harmful. Overall, this technology is worth investing in as it has yielded positive results across the globe, and with improvement, can become a long-term strategy addressing water scarcity and drought risk. 

Written by Trevor Mathia, Public Policy Intern