Desalination Demystified: Distillation vs Reverse Osmosis at Carlsbad Plant

Overview

In this video, Grady from Practical Engineering compares two mainstream methods to desalinate seawater using accessible garage demos. It situates the Carlsbad desalination plant near San Diego, which provides about 10% of the region's fresh water, and explains why desalination is energy-intensive and complex. The host contrasts distillation and reverse osmosis, and tracks energy use and other challenges as he demonstrates both approaches, highlighting scale issues, brine management, and post-treatment considerations. The video also discusses when desalination makes sense and how private-public partnerships shape large projects.

Introduction and Context

Desalination is a potential tool to address water scarcity, but it comes with high energy demands and environmental considerations. The video frames the Carlsbad plant outside San Diego as a real-world example, noting that it produces roughly 10% of the region's fresh water and draws seawater directly rather than treating river or lake input. The host emphasizes that desalination has global presence but remains energy-intensive and costly, making it one option among many in a water portfolio that must consider drought risk, climate variability, and electricity prices.

Two Main Desalination Approaches

The video explains that there are two fundamental technologies for desalination. Distillation relies on phase change to separate water from salt, while reverse osmosis uses high pressure to force water through a semipermeable membrane, leaving salts behind. The demonstration setups are deliberately simple to illustrate core principles, but both processes in full-scale plants require substantial energy, pretreatment, and post-treatment steps.

Distillation Demo: Energy and Limitations

The distillation demonstration places seawater in a heated flask, with water vapor condensed to yield nearly salt-free water. The host tracks energy use with a kilowatt meter. In the example, about 200 mL of distilled water takes roughly two hours and about one kilowatt-hour of electricity. Extrapolating to typical US household water needs would require scaling the system by hundreds of times, resulting in several thousand kilowatt-hours per day and costs on the order of hundreds of dollars per day. Scale deposits and salt buildup on heating surfaces create maintenance challenges, and while distillation can be integrated with waste-heat sources, it remains energetically expensive at small scales.

Reverse Osmosis Demo: Pressure, Permeate, and Brine

The RO demonstration uses a low-cost high-pressure setup to push seawater through a membrane wrapped in a spiral for high surface area. At roughly 600 psi, the system produces a small amount of permeate water while concentrating the brine. The host shows how adjusting feed pressure shifts the balance between brine and permeate flow and notes practical considerations such as membrane fouling and the need to maintain some flow to carry salt away. The crude setup yields water with about 1–2 parts per 1000 dissolved solids after tens of minutes, far from drinking water quality but clearly drinkable in a pinch. The energy cost of this scale is still significant, with the demonstration consuming about 1.2 kW and producing a liter in roughly 5 minutes, implying daily energy use far higher than distillation for comparable outputs.

Pre-Treatment, Brine, and Post-Treatment

Both methods require pretreatment to remove dirt, algae, and organics that could foul membranes or evaporators. Post-treatment adds minerals to permeate water for taste and disinfection to ensure safety for distribution. Brine, the concentrated waste stream, poses environmental challenges and is typically discharged with diffusers or blended with other streams to dilute before release, or in some cases injected into the ground. Modern plants aim to recover about half of the intake water, but the brine is concentrated and must be managed to minimize ecological impact.

Economics, Energy, and the Private Sector

The video highlights how desalination prices are tied to electricity costs, making drought periods when hydropower is scarce especially expensive. It discusses large-scale deployments in the Middle East and in drought-prone areas like California, Texas, and Florida as part of diversified resource portfolios. Private sector partnerships are common, transferring financing and operational risk to utilities or specialized firms through water purchase agreements. The Carlsbad project, including a new stand-alone seawater intake, underwent financing and rating assessments that reflect confidence in desalination technology despite economic and regulatory hurdles.

Outlook

Desalination offers a reliable, climate-resilient water supply in regions with high demand and limited freshwater sources, but it is not universally cost-effective. When combined with brackish water sources or other non-ocean feeds, and when paired with energy recovery and integrated with other water-supply strategies, desalination becomes a more viable option. The video concludes that the practical challenges—energy intensity, environmental impacts, pretreatment and maintenance, and financing—mean desalination remains one tool among a broader, diversified approach to water security.