One individual's wastewater could be another's gold. A new Stanford University study paves the way to mining sewage for valuable elements that find use in fertilizers and batteries that could support smartphones and planes in the future.
The research demonstrates how to improve electrical processes for transforming sulfur pollution, and could lead to more inexpensive, renewable energy-powered wastewater treatment that produces drinking water. The study was recently published in the journal ACS ES&T Engineering.
We are always looking for ways to close the loop on chemical manufacturing processes. Sulfur is a key elemental cycle with room for improvements in efficiently converting sulfur pollutants into products like fertilizer and battery components.
Will Tarpeh, Study Senior Author and Assistant Professor, Chemical Engineering, Stanford University
A Better Solution
As freshwater supplies become scarce, particularly in arid areas, the focus has shifted to creating wastewater-to-drinkable-water technology. Membrane systems that filter wastewater in anaerobic or oxygen-free settings are particularly promising since they utilize very little energy.
However, these reactions produce sulfide, a poisonous, caustic and odorous chemical. Chemical oxidation or the use of particular chemicals to transform sulfur into separable particles, for example, can generate byproducts and induce chemical processes that damage pipes and make it more difficult to disinfect the water.
Converting sulfide to compounds utilized in fertilizer and cathode material for lithium-sulfur batteries is a tantalizing approach for dealing with anaerobic filtration’s sulfide output, but the methods for doing so are currently unknown. As a result, Tarpeh and his colleagues set out to develop a cost-effective method that produced no chemical waste.
The researchers concentrated on electrochemical sulfur oxidation, which takes little energy and allows for the fine-tuning of the final sulfur compounds (while some products, such as elemental sulfur, can deposit on electrodes and delay chemical reactions, others, such as sulfate, can be recovered and reused).
The procedure may be powered by renewable energy and customized to treat wastewater collected from individual buildings or entire cities if it proved to be effective.
The researchers assessed the speeds of each phase of electrochemical sulfur oxidation, as well as the types and amounts of products generated, using scanning electrochemical microscopy, a method that allows for microscopic pictures of electrode surfaces while reactors are running.
They discovered the key chemical hurdles to sulfur recovery, such as electrode fouling and the most difficult intermediates to convert. They discovered that changing operating factors like reactor voltage could help with low-energy sulfur recovery from wastewater, among other things.
Energy efficiency, sulfide removal, sulfate production and time were all clarified as a result of these and other discoveries. The researchers used them to develop a paradigm for future electrochemical sulfide oxidation procedures that balance energy input, pollution removal and resource recovery.
Sulfur recovery technology could be integrated with other approaches in the future, such as nitrogen recovery from wastewater to generate ammonium sulfate fertilizer. The Codiga Resource Recovery Center, a pilot-scale treatment facility on Stanford’s campus, will almost certainly play a key role in speeding up the development and implementation of these technologies in the future.
Hopefully, this study will help accelerate adoption of technology that mitigates pollution, recovers valuable resources, and creates potable water all at the same time.
Xiaohan Shao, Study Lead Author and PhD Student, Civil and Environmental Engineering, Stanford University
Tarpeh is also an assistant professor of civil and environmental engineering, a Stanford Woods Institute for the Environment center fellow, a Stanford Water, Health, and Development affiliated scholar, and a Stanford Bio-X member. At the time of the study, additional author Sydney Johnson was a chemical engineering undergraduate student at Stanford.
Stanford’s Department of Chemical Engineering, the National Science Foundation’s Engineering Research Center for Re-inventing the Nation’s Urban Water Infrastructure (ReNUWIt), and the Stanford Woods Institute for the Environment’s Environmental Venture Projects program all contributed to the research.
Stanford researchers turn wastewater into a valuable resource in an innovative treatment process
Video Credit: Stanford University.
Shao, X., et al. (2022) Quantifying and Characterizing Sulfide Oxidation to Inform Operation of Electrochemical Sulfur Recovery from Wastewater. ACS ES&T Engineering. doi.org/10.1021/acsestengg.1c00376.