Desalination Demystified: Distillation vs. Reverse Osmosis and the Energy Cost of Making Fresh Water

Overview

Grady from Practical Engineering explains why desalination is challenging and demonstrates two do-it-yourself demos to illustrate the main methods used to remove salt from seawater. The video opens by describing the Carlsbad desalination plant near San Diego that supplies about 10% of the region’s water, then compares two approaches: simple distillation and a home-scale reverse osmosis setup using a pressure washer. Through heat, steam, brine discharge, and permeate measurements, he highlights the energy intensity, scale buildup, and environmental issues of desalination, and discusses why large plants use sophisticated pre-treatment, energy recovery, and post-treatment to deliver usable water. The video also touches economic considerations and the role of private companies in desalination projects.

Introduction and context

Desalination is presented as a potential solution to water scarcity, but one that is energy intensive. The host notes the global spread of desalination plants and their uneven contribution to water supply, setting up a discussion about the Carlsbad plant near San Diego which provides roughly 10 percent of the region's fresh water and draws seawater directly from the ocean.

Two main desalination approaches

The core technologies are distilled water separation and membrane processes. Distillation relies on heating seawater to generate vapor, which is then condensed to produce freshwater. Membrane approaches, dominated by reverse osmosis (RO), use high-pressure pumps to force water through a semipermeable membrane that blocks dissolved salts. The video explains the basic physics behind osmosis and its reversal, and emphasizes that modern plants optimize energy use and water quality through multiple design elements.

DIY demonstrations and energy accounting

Two garage-scale demonstrations are used to compare concepts. The first is a crude distillation setup using a hot plate, a simple condenser, and a thermocouple-like heat-tracking device to produce distilled water while recording energy consumption. The second uses a pressure washer as a makeshift RO system with a membrane housing, high-pressure input, brine discharge, and permeate output. The host tracks energy in both experiments and discusses what the results imply for real plants.

Salt, brine, and water quality

The video highlights that seawater desalination must remove around 98 percent of the salt to produce potable water. It discusses how brine concentrates and the environmental challenges of disposing concentrated salt solutions, including dilution strategies and possible environmental impacts on the sea floor. Pre-treatment to remove contaminants and post-treatment to re-mineralize and disinfect water are explained as essential steps in commercial plants.

Scale, cost, and real-world viability

The host compares the efficiency of crude DIY methods with industrial practice. He notes that modern distillation plants use heat recovery, staged evaporation, and energy reuse, while RO plants rely on high-efficiency pumps, energy recovery devices, and large membrane facilities. The economics are traced to the energy price, local electricity supply, and the broader context of droughts and private financing models that can influence project viability.

Energy limits and the future of desalination

The video touches on the theoretical minimum energy required to separate salt from water and discusses ongoing research in membranes, energy recycling, and alternative water sources such as brackish water and estuaries. It also mentions the role of private-public partnerships and how some large desalination projects are financed and rated by financial agencies to convey market confidence in the technology.