(Photo by Mark Brodie-KJZZ)

Probably not what you have in your freezer.

So on the fourth floor of a building on ASU’s Tempe campus, in a kind of back hallway, there’s a bank of roughly a dozen freezers – some commercial-grade, but many like the one in your kitchen.

These freezers don’t hold employees’ lunches, or even frozen organs to study.

"We have here at Arizona State University the largest collection of sewage sludges, probably for the United States, representing some 200 cities," said Rolf Halden, the director of the Center for Environmental Security at the Biodesign Institute at ASU

To answer the obvious question— no— it doesn’t smell. It’s all frozen.

And, to answer the next obvious question – why - Halden said the samples can give lots of clues about cities, and the people who live in them.

"So rather than looking at physical materials that go into the city, we now measure the signatures of human activities in the city," Halden said. "And that’s very exciting, because by taking these samples, we essentially get real-time information — what people eat, what they drink, what they smoke, what they inhale — all this type of information is now accessible and very inexpensively so using this instrumentation."

Halden said there are automated samplers at wastewater treatment plants across the country. Those samples are constantly sent to the lab where they’re analyzed, usually within a day.

The samples are anonymous, though Halden said it can be extremely helpful to researchers.

Halden said this is also a more cost-effective way to get this kind of information, since researchers can measure the exposure of hundreds of thousands of people in a single sample.

Halden said this field, called urban metabolism, aims to create a human health observatory which could help researchers, city leaders and residents.