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How long does it take for waste materials to decompose?

How long does it take a plastic bag to break down or a glass bottle to decompose? What about a milk carton or a Styrofoam cup?

Sources for rates of decomposition of litter (trash) on the web give you different rates. Once you've done quite a few of these searches, you realise that it boils down to about three different lists, all repeatedly quoted (but not always mentioned as the source):

1. The New York Times (Nemve E. Metropolitan Diary, October 1, 2001):
Paper- 2.5 months; Orange Peel- 6 months; Milk Carton- 5 years; Cigarette Butt- 10-12 years; Plastic bag- 10-20 years; Disposable diaper- 75 years; Tin can- 100 years; Beer can- 200-500 years; Styrofoam- never (immortal)

2. Penn State University*: Paper-2-4 Weeks; Leaves-1-3 Months; Orange Peel- 6 Months; Milk Carton- 5 years; Plastic Bag- 10-20 Years; Plastic Container- 50-80 Years; Aluminium Can- 80 Years; Tin Can- 100 Years; Plastic Soda Bottle- 450 Years; Glass Bottle-500 Years; Styrofoam-Never.
*This list is widely quoted, but I could never actually find the original source.

3. "Pocket Guide to Marine Debris," The Ocean Conservancy, 2004*
Paper towel - 2-4 weeks; Orange or banana peel- 2-5 weeks; Newspaper- 6 weeks; Apple core- 2 months ; Waxed milk carton- 3 months; Plywood- 1-3 years; Wool sock- 1-5 years; Cigarette filter- 1-50 years; Plastic Bag- 10-20 years; Plastic film canister- 20-30 years ; Nylon Fabric- 30-40 years; Leather- 50 years; Tin can- 50 years; Foamed plastic cup- 50 years; Rubber boat sole- 50-80 years; Foamed plastic buoy- 80 years; Aluminium can- 80-200 year ; Disposable diapers- 450 years; Plastic beverage bottles- 450 year; Plastic beverage bottles- 450 year; Monofilament fishing line- 600 years; Glass Bottle- 1,000,000 years.
* Quoted in U.S National Park Service; Mote Marine Lab, FL and "Garbage In, Garbage Out," Audobon Magazine, Spt/Oct 1998.

So what does all this mean, and how do we explain differences above?

Lets separate the first two lists from the third. People seem to have missed the word "ocean" in the source, and it stands to reason that degradation at sea for some materials would be different to that on dry land.

Then there's some clear overlap between the first two lists, so it is likely the NY Times article was using the Penn State info to some degree, and topping it up from other sources.

Then a couple of further observations:

1. Plastic bags: Although all three lists above say it takes 10-20 years for a plastic bag to degrade, there are quite a few references on the Net saying that plastic bags actually take hundreds of years to degrade. So where does this discrepancy come from? Well, it seems that scientists don't actually know the answer to that one, although the time it takes a plastic bag to degrade is obviously a lot longer than on the lists popularly quoted on the Internet.

2. Different rates of breakdown: It turns out that materials decompose differently depending on a lot of factors, including temperature, oxygen levels and many others. One important factor is the presence of water. Many landfill sites are hermetically sealed with plastic (and covered at night), so water doesn't seep into the waste. Ohio State University has shown that adding water to waste sites increases their rate of decomposition. And of course material degrades differently in the ocean.

Context: Where does most household garbage eventually end up once it leaves the home? Clearly in landfills. That's where the next set of stats should come from.

Happy recycling!

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