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John Edel is turning a former meatpacking plant on the edge of Chicago’s old Union Stockyards into an indoor farm.

In the basement, microorganisms are eating tilapia waste, converting it into fertilizer for the lettuce, kale and wheatgrass growing in a shallow pool of water nearby.

This process is called aquaponic farming. It minimizes water use while allowing year-round harvests, and it’s just the beginning of Edel’s vision for a futuristic, urban farm he has called “The Plant.”

“The idea is that nothing leaves the facility but food — period,” said Edel, 41. Half of The Plant will be rented to startup food companies, including a commercial brewery.

The heart of America’s urban agriculture movement is Milwaukee, where Will Allen, a sharecropper’s son and former professional basketball player, has turned a dilapidated plant nursery into a cutting-edge urban farm, raising plants and vegetables, plus the thousands of tilapia needed to fertilize them.

Allen’s disciples are converting far less logical sites into aquaponic farms. In Chicago’s Bridgeport neighborhood, for example, his daughter, Erika Allen, is building the Iron Street Farm in a former truck depot.

Edel will be among the first to expand this concept across multiple floors of a single building: A shuttered 93,500-square-foot former pork-packing plant.

We’re running out land to feed the world’s exploding population. (New York City’s approximately 8 million inhabitants, for instance, eat an amount of food that requires a land mass the size of Virginia to grow, according to Dickson Despommier, a Columbia University professor and prominent vertical farms advocate.)