Though water is one of the most abundant substances on the planet, much of it is hard to use: too salty to drink, too contaminated for crops or frozen away in ice. People need to expend energy to make this water usable.
This is especially troublesome as water goes to work in hydraulic fracturing, in which millions of gallons of water flows underground to break apart rock formations, releasing oil and natural gas. Much of this injected water is recovered and disposed of, but plenty of water also percolates up throughout the life of a well, seeping in from the surrounding rock. In an aging well, a driller may draw 10 times as much water as fuel.
This water is laden with toxic chemicals from the surrounding formations and from compounds injected to help extract the fuel. It can also be five times saltier than seawater, creating a huge disposal problem for drillers.
Traditional desalination methods tend to be ineffective or too costly with such salty water; reverse osmosis membranes foul up too easily, and thermal distillation requires a lot more energy to boil and then condense water.
Now researchers say they have a cheap, low-energy method to clean this "produced water." Anurag Bajpayee, a mechanical engineering research associate at the Massachusetts Institute of Technology, explained that the process, called humidification dehumidification (HDH), uses a dry carrier gas to pick up water as it evaporates. The gas leaves the salt and other impurities behind as it then flows through cool water, where the newly pure water condenses.
Potential use for villages and cities
However, there is an energy-saving twist. "We operate at low temperatures and pressures," said Prakash Govindan, another MIT researcher who worked on this project. Using a carrier gas under lower pressure, the water evaporates at a much lower temperature. By doing this, researchers shrank the temperature difference between the evaporation phase and the condensation phase of the process, substantially cutting the energy needed to remove impurities.
"If you compare this to a pure evaporation process, you are basically imputing the energy of the latent heat of water," Bajpayee said. "Here is a process that operates like a multistage evaporation system, but in a single stage."
This simplified design means researchers can make the purification system smaller and cheaper than conventional desalination methods. Bajpayee added that there is no liquid waste, so the remaining impurities emerge as a solid and drillers have a much smaller volume to dispose of.
The HDH system isn't just limited to oil and gas drillers; Govindan said it can treat water in rural, energy-scarce regions. "The strength that we see here is that we can essentially make this work at small scales," he said.
Because it's modular, the system can scale down to slake the thirst of a small village, producing 2,400 liters of clean water daily, or it can ramp up to quench entire cities. Such demands may grow as climate change dries out certain parts of the world and inundates others.
At the moment, the researchers have a lab-scale demonstration unit up and running, producing 700 liters of pure water per day. Bajpayee said he expects to have pilot HDH systems in the field next year.