From kitchen scraps to garden gold

You collect your food waste and soiled paper, put them into your organics cart, and your hauler comes and takes it away. But what happens after that? How does your rotten food become nutrient-rich compost? (I've unfortunately still never been to the composting site, so all these photos are borrowed. Ohhh, look at the lovely blades of green grass!)

At the Hennepin County school organics meeting last month, Anne Ludvik of Specialized Environmental Technologies (SET), operator of the only composting facility in the Twin Cities metro area (there's also one in Hutchinson and one in Duluth), gave a presentation on the process of commercial composting. SET is a member of the U.S. Composting Council.

After organics are collected at your house, they're taken to the Hennepin County Transfer Station in Brooklyn Park for inspection. Here's the organics bunker:

Then the load goes off to the SET composting site in Rosemount. Organics are mixed within 24 hours of arriving at the site.

City of Minneapolis organics cart (currently only available in three areas of the city)

Organics from schools, businesses and homes arrive at the composting site.

Yard waste is stored, so during the winter when there's no yard waste collected, this stored material can be mixed with food waste to get the proper carbon:nitrogen ratio.

The red piece of equipment is the RotoMix VXT 745, which tears open compostable bags and mixes the organics but doesn't crush them. This is very important because you need to maintain the organics' bulk; if you only have small particles they fall to the bottom, and the piles go anaerobic (which means stinky).


The RotoMix's twin augers mix the organics.

The organics are formed into 1,000-cubic-yard windrows over aeration pipes with holes every 6 inches attached to blowers. When the organics are piled on top of the pipes, the blowers cycle on and off, blowing air through the pipes into the windrows, making the perfect conditions for bacteria to do their work. It takes about a week to get enough material to build a pile, which then sits for four to eight weeks, cooking. Temperatures are taken daily with 3-foot-long thermometers. The compost must reach 131 degrees for 7 consecutive days to kill the pathogens. The compost gets screened and cured three times.

Organics before they're screened

Finished compost goes through the trommel screener, which uses a half-inch screener.



Contaminants pile up at the back of the trommel screener


Curing compost

And 4 to 6 months later, it's finished! Garden gold!