Philadelphia has sand filtration. Los Angeles uses aeration discs. Waste-eating microorganisms are put to work in almost every city.

The Clean Water Act of 1972 mandated that every municipality in America must clean its sewage before discharging the water back into rivers, lakes, or the ocean. Today, there are around 16,000 publicly owned wastewater treatment plants in operation across the country, all of them responsible for transforming what we flush down the toilet into something useful—or at least something not actively harmful.

Chicago-based photographer Brad Temkin became interested in water treatment infrastructure while working on a previous project about rooftop gardens. One of the purposes of such gardens is to absorb rainwater, thus reducing runoff and street flooding. That got him wondering what happened with all the rainwater that wasn't captured, which led him to ask permission to photograph Chicago's network of underground water tunnels. The more Temkin learned about water, the more he wanted to know. "We take it for granted," he says. "It's the most valuable resource we have, outside of air, and we just assume that it's always going to be there."

Temkin's quest led him to photograph water treatment systems across the country including Atlanta, Houston, Phoenix, and Seattle. Each city faces a different set of challenges. In Houston, it's all about flood prevention, while for Phoenix, currently enduring its 19th straight year of drought, it's all about maintaining the water level in its aquifer. Because most water infrastructure is underground, few people ever see it; Temkin had to get special permission from each municipality to photograph its facilities. Taking pictures in sewers required its own set of precautions, including an oxygen meter and gas mask in case of emergencies. When he was near running water or an open sewer hole, he had to wear a harness.

The photographer found himself drawn to the unusual architecture of these underground tunnels and cisterns. "All my pictures are about making an interesting image first, and then hopefully saying something," he says. For their part, many water sanitation workers seemed gratified that someone was shining a spotlight on their work. "I had people thank me for showing this world that they see every day," Temkin adds. "They're excited about it."

Although a book of the photographs will be published in April by Radius, Temkin notes that the series is ongoing. He hopes to photograph water infrastructure in San Diego, in Israel, and possibly even further afield. A committed environmentalist, he's less interested in highlighting threats to the natural world than focusing on potential solutions. "For all the bad things that are happening, I think we're at our best when facing a problem," he says. "I'm more interested in how we fix things than in how we screw them up."

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