Saturday, 7 November 2015

Is landfill best for plastics?

I used to think that landfill is the best way to dispose of most plastics on the grounds that when plastics go into landfill they stay there - it is a form of carbon sequestration. On the other hand when plastic made from fossil fuels rots in soil or is incinerated, the net result is the same as burning the fuel and contributes to climate change. Since the plastic has come from the ground the best way to finish with it is to put it back into the ground. The main problems with plastic are due to the stuff that doesn't get into landfill - the stuff that litters the environment, contaminates the oceans and kills wildlife. However, from reading a report from ENSO Plastics on plastics disposal [1] I have come to realise that plastic in landfill is unlikely to remain pristine for thousands of years. Nature is incredibly resourceful and we keep finding examples of bugs and microbes that have evolved to consume plastic that was not supposed to be biodegradable [2][3][4]. So what is the best way to dispose of plastic materials?

There are 4 potential disposal routes: recycling, composting, landfill or incineration and they all have their upsides and their downside.

Every time plastic is recycled the polymer chains get shorter.
Recycling plastic is not a final solution because it can't keep going forever. Every time the plastic is reprocessed the polymer chains get shorter. This problem is similar to paper recycling - each time around the paper fibres get shorter so in practice you can only recycle paper five or six times. How much of a problem this is rather depends on the application. You can imagine a carpet might last 20 years per cycle, but a plastic bottle could be recycled six times in just a couple of years.

Plastic products may be easy or difficult to recycle depending on shape, size, colour and additives. Also if they are made of different materials they have to be easy to separate.
However, recycling plastic is much more complex than recycling paper because there are so many different types and to start with you have to separate them into separate streams. For example if you get a small amount of PLA in with your PET the whole batch is useless because the PLA melts at a low temperature and will clog up the machinery processing the PET. If you can't separate them, effectively the plastic is not recyclable regardless of the material used. The Association of Post Consumer Plastic Recyclers has a guide to how to design products to be easily recycled. The first issues are to do with size and shape because most recycled materials are collected in a mixed waste stream which has to be separated by machinery at a materials recycling facility. Products that are flat tend to get mixed up with the paper. Products smaller than 10cm tend to get mixed up with the glass. After that there are lots more issues to do with the processing required. The guidelines for PET include:
  • Labels and caps need to detach easily and then they must be lighter than water so that they can be easily separated in a float tank. Otherwise they contaminate the stream.
  • Colour in the starting material will affect the new product so colourless transparent bottles are easiest to recycle. Translucent green or blue is also acceptable - other colours and opaque plastics are definitely not recyclable.
  • Additives that make the plastic more degradable will contaminate the new product and this may not be a good thing. Also additives can affect the final product sheen and colour.
  • Products made up of different kinds of plastic that cannot easily be separated cannot be recycled.
Packaging could follow guideline for easier recycling if there were regulation.
Recycling could be made easier if plastic products were to follow these guidelines and this ought to be possible at least for plastics used for packaging. However, this will limit scope for branding and marketing so it won't happen without firm regulation.

Saying plastic is 'biodegradable' without saying under what conditions can be misleading.
Composting is the next option. Biodegradable plastic can be broken down by bugs and microbes to organic compost, generating CO2 in the process. However, much so-called biodegradable plastic only rots down if it has first been degraded by sunlight or heat - this breaks up the polymer chains in to smaller pieces that are easier for the microbes to digest. Saying plastic is 'biodegradable' without saying under what conditions can be misleading. For example a lot of biodegradable plastic will not rot if buried in your garden compost because there isn't enough light or heat to get it going.

Bioplastics are not necessarily low impact and can even require more fossil fuel to make.
When plastic made from fossil fuels is composted the CO2 released contributes to climate change. However, if the plastic is made from renewable sources like corn starch or sugar cane then the process is overall carbon neutral. However there are still environmental impacts related to agriculture (such as use of fertilisers and herbicides, agricultural runoff affecting water courses) and to chemical processing. When all of these are considered, the bioplastics are not necessarily lower impact. In some cases manufactur consumes more fossil fuel rather than less [5].

When organic materials including plastic rot in landfill they produce methane, a useful source of energy but also a powerful greenhouse gas.
When plastic or organic material rots in landfill there isn't enough oxygen to produce CO2 - the microbes generate methane (CH4) instead.  This is much worse than CO2 as a greenhouse gas but it is also a useful fuel so modern landfill sites collect it and use it, usually to generate electricity. ENSO regards this as a zero-waste disposal option because instead of the plastic being wasted it is used to produce energy. In fact the main purpose of their report seems to be to promote their product called ENSO Restore that can be used to treat plastic to make it biodegrade in landfill. 

In practice, some methane will leak - typically 1% to 10%
In practice not all the methane produced is captured and used - gas production in most sites peaks in about 10 years and tails off slowly as shown in the chart. As the rate decreases there comes a point where it isn't economic to generate power so the gas is flared off instead so that it releases CO2 rather than the more damaging methane. But after 50 years there isn't even enough to support a flare. The gas can't be contained however, as that would lead to a serious risk of underground explosions. It would also be possible to use a biofilm on the surface to oxidise the methane as it is released but this is expensive to maintain because biofilms need to be kept moist [6].  The ENSO treatment is intended to ensure the plastic rots in less than 50 years. There will be some leakage though, if there are any defects in the landfill containment system. In practice modern landfill gas management allows 1% to 10% of the gas to leak [7]. Even at the 1% level, that adds 25% to the greenhouse impact because methane is 25 times as powerful as CO2 over 100 years.

Rate of methane production from landfill [6].

Plastics can be incinerated fairly cleanly, though burning mixed waste leaves toxins behind.
The final option is incineration, preferably with energy recovery so the heat is used to make electricity. Waste incineration plants are often unpopular, partly because of air pollution, and relatively expensive because of the air cleaning equipment needed to manage this. However, incineration means all the plastic is converted to the less damaging CO2 rather than methane.  Plastic burns fairly cleanly but incineration plants typically take mixed waste that is much less clean. About 25% of the mass remains as ash, including toxic metals. This is regarded as hazardous waste and needs special handling in landfill.

Despite all these problems, for many applications plastic is the lowest impact option
None of these disposal options are truly sustainable, but on the other hand it isn't sensible to simply stop using plastic because for many applications plastic is the lowest-impact option. For example it takes less energy to make a plastic bottle than to make it from recycled glass, and that is before you take into account the extra transport emissions because glass is heavier [8].

Suggested guidelines for minimising plastic impact.
I suggest the following:

  • Where possible, all packaging should be made from renewable, compostable materials (such as paper, card and organic waxes).
  • Alternatively, packaging should be designed for re-use. For example glass bottles and jars should be transparent and of a standard shape that can easily be washed.
  • When plastic is used for packaging it must be designed for recycling (e.g. single materials, no colour, easily removed labels).
  • When plastic is used other than for packaging and cannot practically be made recyclable, it should be made from material that composts in landfill.
  • When plastic is sent for final disposal, it should be incinerated if there is a facility nearby, but if not it should be sent to landfill.


I do agree that it is better for plastic to biodegrade quickly in landfill than slowly so that the leaks cannot be controlled. But it would be much better for it not to get into landfill in the first place.


[1] Plastics: Establishing The Path To Zero Waste . Teresa Clark (Enso Plastics) 2015
[2] Evidence of Polyethylene Biodegradation by Bacterial Strains from the Guts of Plastic-Eating Waxworms. Jun Yang, Yu Yang, Wei-Min Wu‡, Jiao Zhao, and Lei Jiang (ACS Publications) 2014
[3] Plastic eating microbes help marine debris sink (Discovery News) 2014
[4] Biodegradation of polyethylene by the thermophilic bacterium Brevibacillus borstelensis. D. Hadad, S. Geresh and A. Sivan (Journal of Applied Microbiology) 2005
[5] Pitt Researchers: Plant-Based Plastics Not Necessarily Greener Than Oil-Based Relatives (University of Pittsburgh) 2010
[6] Control of landfill gas containing low concentrations of methane (Environment Agency) 2009
[7] Methane emissions from different landfill categories (Environment Agency)
[8] Energy and carbon emissions: the way we live today. Nicola Terry (UIT Cambridge) 2011

1 comment:

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