colin williamson explains his plea to stop using biodegradable plastic &lsquoplease don&rsquot use them, don&rsquot recycle them and don&rsquot tell me how wonderful they are.&rsquoin the 21st century drive for sustainability, the concept of biodegradable plastics seems fantastic. the iconic manmade product reverting to nature seems too good to be true. and, of course, like anything that seems too good to be true, it probably is.biodegradable plastics do exist, of course, made from agricultural materials or made by modifying conventional petrochemical based materials. now, for the sake of simplicity i have ignored the degradable plastics based on oil, as they use more energy than normal products as well as exhibiting the other problems mentioned below. but, rest assured, any claims of biodegradable plastic ecoefficiency are based more on politics and economics than mathematics. so let&39s look at the big picture.most scientists accept global warming is the biggest environmental threat to human life. we measure this by the carbon footprint over the lifecycle of a product or system. so, let&39s consider the carbon footprint of some plastics products by comparing the lifecycle of a biodegradable plastic bag or bottle with one made from &39traditional&39 oil based plastics.oil is pumped from the ground, refined into plastic and made into a product. it uses energy to make the transformation of course and this can be added to the energy embedded in the oil itself. alternatively, corn is grown by a farmer who uses energy to drive his tractors and chemicals to spray the plants. afterharvesting the corn is converted to a plastic product by an industrial process, which itself uses more energy.we can calculate the total amount of energy expended in making a bottle or bag. if the packaging is oil based it probably weighs less than the biodegradable alternative so an allowance has to be made for this.few independent ecoaudits have been conducted on biodegradable plastic products though, and we still have yet to discoverthe true ecofootprint made by agrosourced plastics. some biosourced plastics are based on a waste product from the agricultural industries, such as bagasse, and claim a zero carbon footprint, others are made from foodstuffs.when the packaging has fulfilled its primary function it becomes waste, and that&39s where it gets interesting. most waste in the uk ends up in landfill site, so let&39s consider what happens once the stuff gets buried. oil based plastics may take centuries to degrade but until then they stay inert, just like a lump of rock or stainless steel. in other words they have no further effect on the environment.the biodegradable bag or bottle on the other hand starts to degrade relatively quickly although nowhere near as quickly as the manufacturers claim &ndash just try it if you don&39t believe me. it biodegrades, not just to carbon dioxide co2 and water as there is little oxygen in a landfill site, but to other chemicals that escape as complex molecules and gases, normally methane.methane is one of the powerful &39global warming&39 gases, about 24 times more damaging than co2. recent eu directives relating to landfill sites acknowledge this by limiting and restricting the amount of biodegradables especially garden refuse going to landfill. there are other significant issues with biodegradables in landfills including land instability and leachates into the water table.so which is better, a bottle in a landfill site that has no further influence on the environment or one that biodegrades to a harmful global warming gaswhat about recyclingand as we recycle more and more waste, including plastics bottles and bags, one of the wellestablished uses of old polyethylene bags and film is to be recycled in to black builders&39 film to be used as dampproofing. imagine what would happen if biodegradable bags get mixed into this recycling stream.the recycler can&39t differentiate between the biodegradable bag and the standard one so he makes and sells the sheets that then gets used under a floor in a new building. this is the ideal situation for degradation to start and the film develops a hole and no longer is a water barrier, the house gets a damp patch and noone knows why. so the presence of biodegradable plastic carrier bags in the recycling waste stream is seriously impacting the recycling industry.in some countries where biodegradable bottles have already been introduced, major problems are being encountered by the recyclers who have already taken billions of bottles out of the waste stream for recycling."hangon", i hear you say, "these biodegradable bottles can be put in the green waste collection bins to be composted" well, yes, they can, but the guys doing the composting remove any plastic and discard it for landfill as they cannot differentiate biodegradable from traditional plastic.biodegradable plastics sound wonderful, but are largely a brilliant marketing concept. if landfilled they contribute greatly to global warming, if recycled they are a major hindrance to existing recycling schemes we have battled so hard to encourage.biodegradable plastics will have their uses, of course, but only when their end uses are clearly identified. one is as the bags for local authorities to collect garden refuse for composting. ordinary polythene bags are normally used, but as they don&39t biodegrade, they have to be emptied of their contents, either by the collectors or at the composting facility. biodegradable bags would be excellent for this application, able to be properly composted and unlikely to enter the recycling stream.if you can think of any other suitable uses, please let me know, but until then please don&39t use them, don&39t recycle them and don&39t tell me how wonderful they are.about the author colin williamson is technical consultant at smile plastics, a dedicated plastics recycler.authorjustin cunninghamthis material is protected by findlay media copyrightsee terms and conditions.oneoff usage is permitted but bulk copying is not.for multiple copies contact the sales team. 
