Sustainability | Agriculture | Ecotrope

Using Soldier Flies To Compost Food Scraps

Ecotrope | Aug. 13, 2012 2:51 a.m. | Updated: Feb. 19, 2013 1:30 p.m.

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Radu Popa, an associate professor at Portland State University, reaches into a black soldier fly composting operation in a lab. As the fly larvae eat the food waste in the bins, they produce a liquid leachate that can be sprayed on plants as fertilizer.

Radu Popa, an associate professor at Portland State University, reaches into a black soldier fly composting operation in a lab. As the fly larvae eat the food waste in the bins, they produce a liquid leachate that can be sprayed on plants as fertilizer.

Portland researchers Radu Popa and Terry Green have an idea they say will make food scrap composting easier, more sustainable and even less stinky.

Through experiments and now a pilot project at a local farm, they’re exploring how black soldier flies can help address the flip side of local food: Local food recycling.

Here’s how their system works: The black soldier flies will actually seed a backyard composting bin on their own and continue fueling the composting process for about  nine months out of the year in the Northwest. The flies’ eggs grow into larvae that will eat eight to 10 times their body volume in food before they mature.

Black soldier flies are indigenous all over the world, and they will continue to lay eggs in food scrap bins for nine months out of the year in the Northwest climate.

Black soldier flies are indigenous all over the world, and they will continue to lay eggs in food scrap bins for nine months out of the year in the Northwest climate.

The larvae process the nutrients in the food and leave behind a compost tea that can be sprayed directly on plants as a fertilizer. The larvae remove themselves from the compost bin once they’re mature, and the leachate left behind turns a dark black as new larvae continue to feed on it.

“It’s an alternative for dealing with food scraps,” said Green. “You get all kinds of benefits as well as the leachate. It’s good for the garden and you’re doing something better for the world as well.” 

When the larvae pupate, they leave behind a shell that can be used as chicken feed or used to make other products such as wound dressing or paint.

When the larvae pupate, they leave behind a shell that can be used as chicken feed or used to make other products such as wound dressing or paint.

Black soldier flies aren’t the buzzing houseflies you’re used to seeing around, said Green, but they are indigenous to the Northwest. They are longer and larger than houseflies, but they don’t hang around the house because, he said, they aren’t hungry or searching for food. They live for four or five days after emerging from their larval stage – long enough to lay some more eggs to continue populating the compost bin with hungry larvae.

Green, of Terry Green & Associates, and Popa, an associate professor at Portland State University, have started their own business called DipTerra to promote black soldier flies as food scrap composting helpers. They offer a long list of benefits of using black soldier flies instead of hot composting (the decomposition method used in Portland’s curbside composting program).**

For example, the empty larvae shells can be used as chicken feed or turned into wound dressing products, and the larvae themselves can be used to make biodiesel or as feed for farmed fish. (Side note here: Portland Purple Water is planning to use black soldier flies to make feed for the fish in its closed-loop aquaponics project in North Portland.)

Plus, they say, it doesn’t require the woody debris or aeration that hot composting. And it’s much easier to move the liquid compost tea than it is to haul huge piles of solid compost.

The liquid left behind by soldier fly larvae can be used as a compost tea in a garden or on a commercial farm.

The liquid left behind by soldier fly larvae can be used as a compost tea in a garden or on a commercial farm.

“Composting is actually hard work,” said Popa. “You have to aerate or you’ll have too much water, and then you’re going to have a problem because it starts fermenting.”

The soldier fly bins can be sized for a backyard or scaled up for a commercial operation, which Popa and Green are testing at New Earth Farm in North Plains.

“This is the kind of thing anybody can do. You don’t have to be a big institution to do it,” said Green. “We think this technology an bring waste disposal back into the local area, instead of trucking it into a far-off place.”

Farm owner Scott Olsen already composts about 3 tons of food scraps a week from the food service company Bon Appetit.

He’s currently using a Japanese composting method called bokashi that ferments the food in a closed container, where microbes process it. But it’s a slow process, he said, and he’s hoping the black soldier flies will compliment the bokashi method and speed things up.

“We’re really excited about bringing the black soldier fly in because we have a crop of chickens, and a byproduct is the chicken feed,” he said.

Radu Popa, left, and Terry Green are testing a commercial-scale black soldier fly composting system on the university campus and recently expanded their experiment to a farm in North Plains.

Radu Popa, left, and Terry Green are testing a commercial-scale black soldier fly composting system on the university campus and recently expanded their experiment to a farm in North Plains.

Olsen has a 10-acre farm and grows a variety of veggies on about two acres: beans, tomatoes, swiss chard, cabbage, lettuce, and zucchini, to name a few. He’s been using his own compost tea to fertilize his crops, and has found it works really well.

“The result has been not only helping products grow but helping with pest reduction as well,” he said. “And we haven’t had nearly as much deer pressure. It’s a triple win.”

He’s testing out black soldier fly composting in a small, superinsulated building on his property with several small bins for flies to lay eggs and larger bins for larvae and food scraps.

“You’ll have the complete life cycle of the fly in the building,” Olsen said. “There will be lots of flies and larvae munching on food in the bins. It will be warm and humid in there with lots of light because that’s what they like. … We’ll have to figure out how much energy we have to add to keep it going during the wintertime. We might be able to do that with solar or compost heat.”

In the future, Popa and Green envision people either having their own backyard bins or taking their food scraps to commercial composters like Olsen at the farmers’ market.

“I believe the day is going to come where haulers are going to pay to have this done,” said Popa. “There’s a lot of energy locked up in food scraps.”

**CORRECTION: The original version of this story misstated the affiliation of the two black soldier fly researchers. Terry Green works for Terry Green & Associates and Radu Popa is an associate professor at Portland State University. Together, they started DipTerra, which is a business based on black soldier fly food scrap composting. Their work has been primarily funded through Terry Green & Associates.

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