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Frequently Asked Questions: Contamination

Q: What items are considered contaminants in recycling?
A: Many items can be considered contaminants. Contamination happens when non-recyclable items are mixed in with recyclables items or when recyclable items are placed in the wrong recycling bins. Innocent looking paper smeared with food or grease cannot be processed with clean paper and can ruin a newly made product if it is not caught before it goes to the factory. Too much contamination is the reason manufacturers reject tons of recyclable paper each year. For each category, the sign on the bins tells you what not to put in the bin. Please read these signs carefully!

Q: Why can't pizza boxes be recycled?
A: 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 paper recycling.

Q: What should I do with slightly used paper towels and paper napkins?

A: Although these items are paper, these items are not recyclable. However, you can place paper towels and napkins in the compost bin on campus.

Q: What should I do with paper plates and paper cups?

A: Although these items are paper, these items are not recyclable nor compostable, unless they are certified compostable. Often times these items are soiled with food or other contaminants which causes problems in the paper making process. In addition, many paper products have a thin petroleum plastic lining to give strength to the product and prevent leaking. This petroleum plastic lining is consider a contaminant in the composting process so they cannot be composted. Please throw paper plates and cups in the landfill bins unless they are certified compostable.  If they are certified compostable (look for the term "compostable" on the item), the product has a corn-based PLA liner.  These products will compost in a commercial compost facility and therefore can be placed in the compost bins on campus.

Q: Should I recycle materials contaminated with food? For example, should I recycle cardboard pizza boxes with food residuals attached to the cardboard?
A: Food or oil contaminated paper is considered a contaminant in the paper recycling bins. Best examples of this are pizza boxes and donut boxes. 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 eventually result is new paper with oil splotches. The mill we take our paper to asks us specifically to not include pizza boxes.

Plastics, metal, and glass are recycled using a heat process so usually food or oil contamination is not much of an issue. Sometimes with plastics, depending at what temperature the plastics melts, other types of material including other plastics can be a contaminant. In order to keep your recycling bin clean, it is a good idea to give these items a quick rinse, not a sanitarily deep cleaning.

Q: I am confused about paper food containers. What is recyclable?
A: Any paper food container that has been soiled with food is not recyclable. It may be compostable, if it is lined with a compostable plastic liner (PLA). Pizza boxes are compostable as are other packaging that are labeled "compostable". This term, "compostable", is legally defined and only allowed on products that have been determined to break down at a commercial compost facility.  Paperboard food containers such as cereal boxes, paper egg cartons, and cake mix boxes that are unsoiled are recyclable. Please be sure to remove the plastic liner and shake out extra food crumbs. Frozen food boxes should be placed in the paper recycling bin. Milk and juice carton and soy milk containers should be placed in the plastic, metal, and glass recycling bin on campus and in the paper bin at the Stanford Recycling Drop-Off Center.

Q: Can I recycle old photographs and negatives?
A: No, we cannot recycle these materials.