New work from a Xavier University professor and his colleagues proves that even the dirtiest of dirty water can be salvaged for reuse.
Think back to the last time you had your car washed. You probably noticed the gleam of the tires, the sparkle of the chrome. But did you look down? Did you see the grungy runoff water as it snaked toward the drain? By some estimates, the US produces more than 3.2 billion gallons of car-wash effluent each year, and most of it is too toxic to discharge into the ground water. Bryan Bilyeu, a chemist at Xavier University in New Orleans, has an idea of what we should be doing with it.
Ordinary waste water can be treated with chemicals known as coagulants. Those coagulants are like microscopic sheepdogs: They cause small oily particles dispersed in the waste water to flock together into big blobs. The bigger the blob, the easier the oil is to identify and remove; if a blob becomes big enough, it will float to the surface where it can be simply skimmed away.
By contrast, car-wash runoff—which is lased with a toxic mix of dirt, detergent, oil, grease, and organic pollutants—is stubbornly resistant to chemical coagulants. Bilyeu and his collaborators at the Autonomous University of Mexico State in Toluca, Mexico, decided to try cleaning it with electricity instead. The process, known as electrocoagulation, works much like chemical coagulation, except ions generated at metal electrodes play the role of the sheepdogs: By subtly altering electrostatic interactions in the liquid, they coax particles of oil and grime to clump together in a sludge, which can then be filtered away. The approach isn’t new—it’s been around for decades—but it hadn’t been widely explored as a way to treat car-wash effluent.
To put electrocoagulation to a real-world test, Bilyeu and his collaborators took samples of runoff water from a Toluca car wash. After optimizing for pH, current levels, and electrode material—aluminum worked better than iron—the researchers were able to reduce the amount of oil and grease in the water by 92% with an hour-long electrocoagulation treatment. The same treatment reduced color and cloudiness by 96%. When the team followed up electrocoagulation with two hours of electrooxidation—a related treatment that uses diamond electrodes instead of metal ones—they were able to completely eliminate oil and grease and reduce color and cloudiness by 99% and 98%, respectively.
That said, you probably wouldn’t want to drink a glass of the electro-purified water; the treatment leaves behind some sulfates and floating solids, and it actually produces residual chlorine. But the team’s analysis suggests the treated water is perfectly safe to discharge into the groundwater supply. Or, even better, it can be recycled for that next car wash. Considering the average car wash consumes 40 gallons of water, the savings could add up quickly. California, are you paying attention?
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