News Feature: Microbes for better sewage treatment
M. Mitchell Waldrop; PNAS August 10, 2021 118 (32) e2112863118
Going beyond conventional approaches, researchers are using carefully cultured bacterial communities to improve sewage treatment—and create useful products in the process
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Figure: Modern wastewater treatment plants represent the first great triumph of microbial-community engineering. In aeration tanks (Lower Right), air bubbles up through the brown, organic-rich water. This gives microbes the oxygen they need to digest the dissolved solids into “activated sludge,” which falls to the bottom. The water is then sent to clarification tanks (Blue Circles), where any remaining solids fall out. Image credit: Shutterstock/chekart.
When it comes to the murky-looking water tanks in Kartik Chandran’s laboratory, fume hoods are essential. Not only do the tanks get regular top-ups of sewage sludge and food waste from a nearby cafeteria, but the microbial colonies inside can give off butyric acid—the distilled essence of sour milk and rancid butter. The tanks also emit an occasional whiff of hydrogen sulfide, says Chandran, which reeks of rotten eggs “till the senses are numbed.”
But then, says Chandran, an environmental engineer at Columbia University in New York City, the smell is just a sign that the microbes in the tanks are doing their job—namely, to revolutionize the way humans handle that inescapable stuff flowing into urban sewage systems, rural septic tanks, and countless lakes and rivers.
“It’s not waste water,” Chandran insists. But today’s water treatment plants typically handle it that way: Anything that’s not H2O gets turned into carbon dioxide that’s released into the atmosphere or a pathogen- and toxin-ridden sludge that mostly winds up in incinerators or landfills. From the perspective of his microbes, Chandran says, it’s actually “enriched water,” a mélange of carbon, nitrogen, and phosphorous compounds that the microbes are happily turning into biofuels, bioplastics, fertilizers, and a host of other useful products.
The chemical pathways that his microbes use to accomplish that feat are reminiscent of the industrial fermentation processes used for millennia to make wine, yogurt, sauerkraut, and other foodstuffs. The difference is that Chandran’s colonies aren’t monocultures. They are rich microbial ecosystems comprising a diverse array of bacteria, archaea, and protozoa taken from soil, ditches, and just about any other ecological niche that researchers can find—including, yes, sewage plants. That diversity, in turn, allows these communities to accomplish feats of chemical processing that no one organism can hope to do in isolation. “In theory at least,” says Chandran, “microbes harbor infinite potential.”
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