Frequently Asked Questions: Contamination

Q: What items are considered contaminants in recycling?
: 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?
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.

Q: What should I do with slightly used paper towels, paper plates, paper napkins, or paper cups?
Although these items are paper, they should be placed in the garbage. Often times these items are soiled with food or other contaminant. Many paper products have a thin plastic lining to give strength to the product and prevent leaking. This plastic lining is consider a contaminant in the pulping process. Please throw paper plates, napkins, and cups in the garbage.

Q: Should I recycle materials contaminated with food? For example, should I recycle cardboard pizza boxes with food residuals attached to the cardboard? Or should I recycle plastic frozen dinner trays with a small amount of food residuals?
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 will 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.

With the food tray, it isn't so much that the food is a contaminant or a problem during the process. The issue is cleanliness and sanitary conditions. The plastic tray could sit in the recycling bin for a week. Once we pick up the commingled containers we bring them back to our yard to sort them. This could take another week. Finally, once all the materials are sorted, they are baled or placed in a large bin until enough material has accumulated to send it to a market. In the case of plastics we only sell them once a month for PETE and every 1.5 - 2 months for HDPE and plastics 3-7. So what this boils down to is that your tray with a little bit of food will likely sit around for a least a month. In that time, the food residue will likely grow mold. Also the food will slide off from the plastic tray onto other materials which then creates a mess. We have four employees who sort the recyclable material and are likely to come into contact with the plastic tray. The best way to handle the food tray is to quickly rinse off the food residue before putting it in the bin.

Q: I am confused about paper food containers. What is recyclable?
Any food container that has been soiled with food is not recyclable and should be put in the garbage. For example, pizza boxes and other fast food or to-go containers. 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 mixed paper bin. Milk and juice carton and soy milk containers should be placed in the bottle and can bin on campus and in the aseptic container bin at the Stanford Recycling Drop-Off Center. If you are unsure if an item is recyclable, ask one of our employees or call us at PSSI.

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