Part of Colin Thompson’s job at Batch Bakehouse is hauling garbage and recycling to the curb. For Thompson, this involves an extra layer of sorting.

Each week, Thompson also puts out eight bins full of the bakery’s food waste. “It reduces waste on the whole and increases the overall sustainability of our business and lessens our environmental impact,” Thompson says. “If local gardeners can end up using it, it builds the community.”

Batch is part of a group of 40 businesses and 1,100 residents participating in a pilot composting project that began in 2011. Although the project doesn’t take much effort, it requires vigilance.

Every once in a while, Thompson catches someone tossing compost into a garbage bag. Or, worse, throwing plastic into one of the composting bins. “I usually catch it but I can see the potential for slip-ups like that,” Thompson says.

Those slip-ups are putting a wrench in Madison’s ambitions to join San Francisco, Seattle, Portland and other cities that have a curbside composting collection. Contamination in the pilot project has left the city with no place to send its composting waste, says Madison recycling coordinator Bryan Johnson.

“The most persistent contamination has been plastic, like plastic bags, plastic bottles, and other plastic containers like food service boxes or salad containers,” Johnson says. “On the surface of it, you would think it would be easy to only put food scraps or compostables into a container, but everyone makes mistakes from time to time and a plastic bottle or a plastic utensil winds up in the wrong container.”

Johnson says a small amount of plastic can contaminate a lot of composting. “Organics are routinely shredded prior to composting, so the plastic bottles or plastic bags would also be shredded and spread throughout the material being processed,” Johnson says. “There are screens to get some of the debris out, but tiny flecks of plastic, metal, or glass are very difficult to remove with just screens.”

Initially, the city sent composting to a digester at UW-Oshkosh. After that site stopped accepting the material, the city turned to digesters owned by Gundersen-Lutheran in Middleton and Blue Ribbon Organics in Caledonia — both have since stopped doing business with the city due to persistent contamination.

The companies have to be picky because they’re trying to sell a product to home gardeners. “Marketable compost does not have room for error when it comes to plastic or other debris in the product,” Johnson says.

Although the city’s composting currently has no where to go, Johnson is asking residents in the pilot to keep participating. “We are hanging on to the organics until we find a place to go with them,” he says. But, he adds, “In instances where the pile is too foul to keep holding, we have no choice but to send it to the landfill.”

The city could create its own composting site or biodigester, which would be more forgiving since it would not be trying to sell the end product to home gardeners. Instead, its composting could be used for agriculture or construction, which allows for higher levels of contamination.

For years, the city has proposed building its own biodigester, but the project keeps getting postponed because of high costs and tight budgets. A biodigester would cost $12-$18 million to build and require more trucks and workers to collect and process the composting. However, the project would slowly pay for itself, by reducing the amount of waste the city pays to dump at the landfill.

Ald. Marsha Rummel, Common Council president, says several alders support the project, but ultimately the mayor sets the budget. “Recently we were faced with a new police station, fire station and other projects so the biodigester got pushed back by the mayor,” Rummel says. “The council supports the pilot and I believe alders see the benefits of a biodigester.”

The city is looking for possible partners — such as the Madison Metropolitan Sewer District — to help pay for it, Johnson says.

“Exploring partnerships is a way to help share the costs, but it is still difficult to predict when the economics of the situation will align to allow for the continued investment,” he says. “However, when we can, we will.”

Despite the challenges, Johnson sees curbside composting in Madison as inevitable. “The food we toss out is a valuable resource when we manage and process it correctly. I have no doubts we will be there.”

Thompson is hoping the city figures it out soon.

“I’d like to see it expanded and brought into my neighborhood,” he says. “Even on an individual basis it cuts down on a lot of waste.”