Water may be flowing on Mars, but where we really need it is here on Earth. Over 75% of our planet's surface is covered in water, yet 2 billion people worldwide reside in water-stressed countries. With fresh water accounting for a meager 3% of Earth's total water supply (and the vast majority of it locked up in glaciers and icecaps), the demand for state-of-the-art desalination technologies that can purify ocean water has never been greater.
In a review published in this week's issue of Science, researchers William Phillip and Menachem Elimelech offer some insights on the future of our relationship with one of society's most valuable resources.
According to Philip and Elimelech, increases in population, contamination of fresh water sources, and climate change will likely contribute to a 66% increase in the percentage of people with limited access to water by 2025. But increasing the supply of available fresh water to a growing global population is shaping up to be a daunting task.
The first desalination plants (many of which remain in service to this day) heated seawater and condensed the evaporation to produce fresh water – a process known as "thermal desalination." Thermal desalination plants, while effective, consume vast amounts of energy in the form of electricity and heat, and produce large emissions of greenhouse gasses.
Since the early 1990's, however, the majority of desalination plants like the one modeled here have relied on reverse osmosis technology, wherein seawater is channeled through a semi-permeable membrane with properties that allow the water to pass through while the membrane holds on to salt and other impurities.
The last ten years or so have seen a marked spike in the rate of construction of desalination facilities worldwide. According to Philip and Elimelech, the global water produced via desalination plants in 2016 is projected to exceed 38 billion cubic meters per year — that's twice the rate of global water production by desalination in 2008. These newest large-scale seawater reverse osmosis (SWRO) desalination plants — like Israel's Hadera plant, the world's largest (pictured above) — have become the efficiency benchmarks upon which all future improvements are based.
What can be done tomorrow?
The efficiency of the membranes used in SWRO has been improving steadily for decades, but Phillip and Elimelech believe that membranes may be reaching an efficiency ceiling.
The energy saved by new technologies that employ aligned carbon nanotubes (cylindrically shaped carbon molecules) and aquaporins (tube-like protein channels typically found in cell membranes that regulate the flow of water into and out of a cell, see image) would likely be very small, leading the study's authors to conclude that solutions to improving energy efficiency lie elsewhere:
The overall energy consumption of new SWRO plants is three to four times higher than the theoretical minimum energy due to the need for extensive pretreatment and posttreatment steps. Because thermodynamics set the limit on the energy demand for the desalination step, we argue that future research to improve the energy efficiency of desalination should focus on the pretreatment and posttreatment stages of the SWRO plant.
This plan would require the development of so-called "defouling-resistant membranes," which is a fancy way of describing reverse-osmosis membranes that are resistant to degradation and can maintain their performance over time.
Philip and Elimelech lament the fact that while the molecular simulation tools that are required to develop such membranes are widely used in fields like drug development, their availability for the development of more resilient purification membranes is, disappointingly, lacking.
Desalination of the world's extensive supplies of salt water will certainly continue to be costly from an energy standpoint, but according to Philip and Elimelech, these technologies "may serve as the only viable means to provide the water supply necessary to sustain agriculture, support population, and promote economic development."
The authors continue:
Hopefully our paper helps provide some of the information needed to inform the decisions of policy makers, water resource planers, scientists, and engineers on the suitability of desalination as a means to meet the increasing demands for water.
The authors make no mention of searching for sources of water off planet, but given the shortages we appear to be facing in the coming decades, one can't help but wonder if we may one day know the map of our solar system's water sources with the same degree of familiarity that we know the oceans of our world today.