Book Review: ‘Flush,’ by Bryn Nelson

FLUSH: The Remarkable Science of an Unlikely Treasure, by Bryn Nelson

Human waste may trigger automatic disgust, but fun facts about turds are another matter. Who wouldn’t be intrigued to learn that those produced in low-income countries weigh fully twice as much, on average, as those produced in wealthier nations? (The short explanation: fiber.) That’s the kind of tidbit that Bryn Nelson, a writer with a Ph.D. in microbiology, seamlessly weaves into his wide-ranging, science-based explorations of the surprisingly complicated world of waste.

“Flush” begins with a gulp, meanders through the chemical and mechanical byways of the gut, and lingers to marvel at our gastrointestinal fauna. Our microbiome has evolved with us for millenniums, but some species, Nelson reports with an obvious pang, are in danger of extinction thanks to our misuse of antibiotics and sanitizers. Fortunately, human waste is something of a miracle product, and fecal microbial transplants — which “reseed the GI tract with an approximation of its normal inhabitants” — can treat a range of intestinal ills, from irritable bowel syndrome to ulcerative colitis.

Nelson eventually moves on to social and global concerns — at which point things get considerably more interesting. We learn how wastewater can be used to measure community health, as poop sleuths sample their way upstream to find sources of heavy viral loads, allowing cities to concentrate resources in neighborhoods where they are most needed.

With the help of methane-producing microbes, giant tanks and dedicated engineers, feces can also be employed to generate heat, electricity and energy, and to produce fertilizer. Spreading processed sewage on food crops isn’t uncontroversial. Nelson interviews an expert who claims these “biosolids” bind with nutrients as well as heavy metals, making them unavailable to plants. But he doesn’t address the accumulation of potential contaminants: What happens with repeated applications of biosolids over time?

Irrepressibly curious, prone to punning and incapable of embarrassment, Nelson examined his stool daily for a year, using three apps that tracked frequency, speed of transit, quantity, consistency and color. He spent $169 to have his gut flora sequenced, then began experimenting with yogurt and fiber supplements. Is this T.M.I.? If you’re nodding yes, you may be a Republican. “Studies have suggested that people with high levels of disgust sensitivity overall are more likely to be politically conservative,” he writes. (For the record, this reviewer didn’t find these bits gross, just rather boring.)

Nelson’s overarching message is that humans must become comfortable with what’s inside them in order to accept its massive potential to do some planetary good. Our waste can be reused to create potable water, or to capture biogas from our guts to use as carbon-neutral transportation fuel. We can also heat sludge into biochar that becomes cooking fuel, or use it as a feed supplement that reduces methane emissions from cattle. By locking up lead in contaminated soils, biosolids can help restore degraded land, Nelson continues. Spread on gardens and farms, they can help increase yields and reduce the use of synthetic fertilizers.

One admires Nelson’s devotion to his subject, and the scope of his research. But under the burden of all this information, “Flush” can feel overwhelming, and at times the author comes off as boosterish and lacking in skepticism. He rarely discusses scalability, returns on energy investment, our planet’s capacity to absorb a growing population’s waste.

Still, for anyone interested in consumption, or anyone who wonders what happens after flushing, this book has value, and Nelson’s warnings can’t be repeated often enough. We are not separate from the rest of nature; our actions have consequences; and there is no such thing as waste.

Elizabeth Royte is the author of “Garbage Land,” “The Tapir’s Morning Bath: Solving the Mysteries of the Tropical Rain Forest” and “Bottlemania: How Water Went on Sale and Why We Bought It.”

FLUSH: The Remarkable Science of an Unlikely Treasure | By Bryn Nelson | 421 pp. | Grand Central Publishing | $29