Vanderbilt researchers are part of a team that has developed a cutting-edge method that seeks to make the removal of salt from hypersaline industrial wastewater far more energy-efficient and cost-effective.
While desalination through reverse osmosis has made tremendous strides – allowing for salt removal from seawater for less than a penny per gallon – it still falls short in eliminating saline in wastewater from industries like mining, oil and gas and power generation and in inland brackish water.
The industrial brines are currently injected into deep geological formations or transferred to evaporation ponds, and both disposal methods are facing more regulatory and environmental challenges.
Minimise brine volume
Zero liquid discharge and minimal liquid discharge, which use engineered treatment systems to eliminate brines or minimise brine volume, are already required in some countries for certain industries, and they are expected to become more widely adopted soon.
Current ZLD/MLD treatments typically involve a technology called mechanical vapour compression, which generates heat from electricity to evaporate brines until the salt is all that remains. Because of the high capital and operating costs of MVC, these processes are unaffordable to many users.
Associate professor of civil and environmental engineering and 2023 Chancellor Faculty Fellow Shihong Lin and his team, including researchers from Colorado State University, believe they have an answer to this dilemma.
In a paper featured on the cover of the June 2023 issue of the journal Nature Water, Lin and his colleagues describe a novel brine treatment technology called electrodialytic crystallisation that has the potential to reduce the energy consumption and cost of brine crystallisation.
The fundamental principle of EDC, according to the researchers, is like electrodialysis, a process that has been used in various industries for desalination and brine concentration.
In ED, an electric field is applied to pull ions through ion exchange membranes. By placing different types of IEMs in a certain way, ED can produce streams of deionised water and streams of concentrated brine.
Induce salt crystallisation
With some configuration changes to that process, the researchers say EDC keeps the brine within the integrated system and uses an electric field to induce salt crystallisation without using costly evaporation methods.
"The elimination of evaporation is the key to developing potentially energy efficient brine crystallisation processes," according to the paper.
One major technical challenge is that when certain ions transport through the IEMs, they drag too much water across and reduce the effectiveness of the process in concentrating the brine stream. This phenomenon, called electro-osmosis, prevents some salts from being crystallised out effectively.
The researchers said that better membrane design and optimised operation can potentially address this challenge and make EDC more universally applicable.
Conventional approaches of precipitating out highly soluble salts (for brine reduction or elimination) typically require phase change of water via evaporation or freezing, which is energy intensive. Xudong Zhang et al. report a process, namely electrodialytic crystallisation, to precipitate soluble salts via forcing the salt concentration beyond solubility with the mechanism of electrodialysis. The cover image shows sodium sulphate crystals forming in electrodialytic crystallisation. Image: Ruoyu Wang and Lesa Brown, Vanderbilt University.
Nevertheless, for the salts that EDC can handle, the team performed a preliminary analysis and showed that EDC coupled with reverse osmosis can potentially consume much less energy than MVC for brine crystallisation.