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."