The compostable green bags are fine, but a typical load of organic waste collected by the city contains lots of inorganic waste that can't be composted. This load included beer cans, plastic spoons and plastic bags.

That moldy cucumber in the back of the crisper drawer is OK. So are bones, houseplants and coffee grounds.

But participants in Madison’s pilot program to compost organic waste need to do a better job of separating their trash from their biodegradables, city recycling coordinator Bryan Johnson said Monday.

In an email to residents who volunteered for the program, Johnson clicked off a list of no-nos, including no more waxy paper, potting soil, wine corks or crustacean shells in the organic bin.

The biggest offender: plastic bags.

“We still have too many people using plastic bags,” Johnson said. “We need to stop using plastic bags in the organics container.”

Drivers pay attention to bags or other contaminants they see as they dump the black organic waste carts into their trucks, Johnson said. If a cart has banned materials, “I will send a letter instructing the homeowner to change their habits and to ask questions if they need help in figuring out if something is appropriate for organics or not,” Johnson said in his email. “Otherwise, if the contamination continues, we will be forced to remove that home from the program.”

According to Blue Ribbon Organics, the compost company that accepts Madison’s organic waste, capital city composters barely meet the grade. On a scale of 1 to 10, with 1 being the worst and 10 the best, the city’s last load of food waste was a 6, Johnson said.

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“This represents an improvement,” Johnson said. “It was described as ‘by far the cleanest’ we’ve delivered to them, which is fantastic, but a 6 is still a 6. We can do better.”

While the city provides participants with small in-home containers to hold the organic material until it’s ready to be taken out, it no longer provides compostable bags. Those can be bought at some Ace Hardware stores, Bed Bath & Beyond, Willy Street Co-op and Woodman’s Food Markets, according to the city’s organics website, www.cityofmadison.com/streets/organics.

The city cautions that biodegradable bags are not the same as compostable, although regular paper bags are also accepted.

As for waxed paper and potting soil, the wax doesn’t break down fast enough for composting, and potting soil contains more rock than soil, so it’s not good for organic waste.

And crab shells and lobster shells? You’d think they should be compostable, but the current method used by the city doesn’t keep the material heated long enough to break them down, Johnson said. Wine corks, too, will compost, but only if they’re 100 percent cork. Many contain some plastic, so it’s best to just toss them all out, Johnson said.

The organics pilot project started in 2011 and serves selected parts of the city. By the end of 2017, the city hopes to have 2,000 residents and 60 businesses in the program.

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