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Challenges and opportunities of a circular mushroom economy

Collar City Mushrooms occupies a small building along the post-industrial waterfront of Troy, New York. Out back, baking in the winter sun between a shed and a yellow Volkswagen bus, sits a waist-high heap of what looks like dozens of giant Frosted Mini-Wheats, each roughly the size of a cinder block. The weathered caps of oyster mushrooms sprout defiantly from various points in the pile.

The lumpy blocks are spent substrate, the living material left over after growing mushrooms. Composed of sawdust and soy pellets woven through with mycelium — the thread-like aspect of the fungus from which mushrooms sprout — spent substrate is a unique kind of waste. It's also one with many potential uses; it can be used as compost, as a means of decontaminating soil, as biofuel, and simply for growing more mushrooms. And while each of those uses could provide revenue potential for mushroom farms, the expanding piles of spent substrate also represent a mounting logistical challenge.

"Right now, we have people picking it up almost as a favor for us, because otherwise what are we doing with it?" said Avery Stempel, Collar City's co-founder, as we gazed upon the pile. Stempel currently takes most of the material to a nearby compost facility, but local farms, gardeners, and florists also take a portion. So do individuals, whether for compost in their gardens or just to grow mushrooms at home. "People will come and buy a bucket for five bucks," Stempel said.

Before it's put to work growing mushrooms, substrate is carefully mixed and sterilized to maximize efficiency and prevent competition for the fungus. Protected inside breathable plastic bags, the sawdust and soy hulls are inoculated with an edible mushroom strain, then stacked on racks in climate-controlled rooms. The bags are sliced open when the mycelium is ready, and out sprouts the first "flush" of mushrooms. To make the best use of space, many farms will dispose of the blocks after a single flush, but each block is capable of several rounds of mushroom production. In this sense, the substrate isn't really "spent."


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