Compostable plastic bags: Economics work for municipalities

A lot of progress has occured in the production of plastics from renewable materials, the goal generally being to reduce the consumption of hydrocarbons and to make a “greener” product. A secondary goal is to produce plastics that can decompose after use, thus avoiding a disposal problem (e.g. in landfills). When the decomposition can be done by naturally occurring organisms, leaving no toxic materials, the plastic is termed “compostable”.  However, the composting conditions must be relatively severe and temperatures must reach around 150 degrees Fahrenheit for at least ten days for 90% of the plastic to decompose mostly into carbon dioxide and water, as desired. Plastics that meet this criterion must pass an ASTM test which allows the manufacturer to characterize it with a special logo.

Polylactic acid (PLA) made by Cargill Dow is the most common plastic that fits the purpose, with chemical bonds that are readily hydrolized under proper composting conditions. Polyalkanoates, and polybutylene succinate also work as does a BASF product called Ecoflex. Interestingly, these aliphatic polyesters are or can be made from petroleum (non-renewable) feedstocks, and are most likely less expensive than PLA. All of these plastics are considerably more costly than polyethylene, the most common plastic used for trash bags.

These biodegradable plastics will not decompose in your typical backyard compost heap. But municipalities can set up large, high temperature composting facilities and have started to do this because it makes economic sense even if the trash bags the city supplies for collecting leaves are more expensive. This is because a city like Houston can now divert organic waste away from landfills and put his material into compost piles, thus saving the $ 25-per-ton “tipping fee” for landfill collection. (See cover story in C&EN March 19, 2012). The bags are transparent to keep residents from putting their regular trash into the bags. Many cities, including San Francisco, Ann Arbor and Madison, Wis, are also involved in municipal composting. Portland, Or recently started a program and San Antonio is considering it. These programs do not necessarily include the use of compostable plastics, as far as I can determine, but they may.

Europe is well ahead of the U.S. in this area, as well as in incineration of garbage and trash. Stands to reason: There is much less land available for landfills and Europe is also much “greener”.

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4 Responses to Compostable plastic bags: Economics work for municipalities

  1. Joe Pilaro says:

    If the result of composting “degradeable” plastics at “sever” conditions” is as stated,” However, the composting conditions must be relatively severe and temperatures must reach around 150 degrees Fahrenheit for at least ten days for 90% of the plastic to decompose mostly into carbon dioxide and water, as desired.” then why not combust the plastics into the same carbon dioxide and water and recover the heat for power generation. Or in blunter words, why bother with the expensive “degradeable” plastics technology?

  2. Peter Spitz says:

    Two reasons, most likely. First, incineration is not very popular in the US for “nimby” reasons. Secondly, composting collected leaves is a “green” way of disposing of them rather than dumping into landfill, but then you need compostable plastic bags to collect them.

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