Why greasy pizza boxes can't be recycled

A question readers asked was: "Why not? Why can't used pizza boxes be recycled?"

Experts at Stanford University offer this explanation:

"Pizza boxes are made from corrugated cardboard, however the cardboard becomes soiled with grease, cheese, and other foods once the pizza has been placed in the box. Once soiled, the paper cannot be recycled because the paper fibers will not be able to be separated from the oils during the pulping process. Food is a major source of contamination in the various paper categories."

They go on to say that doughnut boxes are among the paper products that also should not be recycled, as well as any paper container "that has been soiled with food."

"Since the paper is mixed with water in a large churner, the oil eventually separates from the paper fibers. The oil does not dissolve in the water, instead it mixes in with the paper." The result is new paper with "oil splotches."

How about plastics, metal and glass? Should they be cleaned before being put in the recycling?

"Plastics, metal, and glass are recycled using a heat process so usually food or oil contamination is not much of an issue," the Stanford recycling site says. In Philadelphia, however, the city says all containers placed out for curbside collection should be emptied and rinsed, which is a good idea to prevent bugs and other critters being attracted to your recycling.

In other recycling tips, Philly311 took to Twitter on Tuesday to remind Philadelphia residents that plastic bags are not among the accepted items in the city. (Some supermarkets, however, have bins for recycling plastic bags. A map of locations that accept plastic bags is here.)

A complete guide on recycling in Philadelphia can be found here.