Two Courier readers asked for clarification on the recycling process the city uses for its green and food wastes following our November 3 story, “State law requires city to inspect (some) trash bins.”

“The City contracts with American Organics to compost our organic waste,” Community Services Manager Kristin Mikula wrote in an email. “They utilize a Covered Aerated Static Pile (CASP) composting system, which I understand is known to be quite efficient.”

The Courier received a detailed description of covered aerated static pile composting in an email from Richard Haskell, physics professor emeritus at Harvey Mudd College.

He explained the “aerated” part of the name means that they are forcing air through the compost pile, where air is 21% oxygen and 78% nitrogen.

“So the aerated compost pile is aerobic,” Haskell wrote.

Usually compost piles are turned frequently because in the process of turning the pile, oxygen in the air has a chance to diffuse into the compost. But there is less labor involved if a company forgoes turning the piles, so they just blow air through the compost, hence the word “static” in the name of the composting technique.

“The main point here is that the food waste decomposes in an aerobic environment which minimizes the generation of methane,” Haskell wrote. “The covering of the pile simply catches the gases (air) emerging from the pile which enables them to check for methane generation or generation of carbon dioxide or anything else of interest, but the emerging gas is mostly just air.”