To recycle or not to recycle: A big question

September 20, 2021

As mentioned elsewhere, not all plastic can be recycled into the same type of products that it originated from.  Thermoplastics, which melt when heated and can be re-formed when cooled, can be recycled although the recycled plastic needs to be mixed with some virgin material to maintain functionality. 

The diagram below shows the categorisation of different types of plastic which can to some extent be recycled and what they are most often used for.

PETpolyethylene terephthalateWater bottles, soft and fizzy drink bottles, pots, tubs, oven ready trays, jam jars
HDPEhigh-density polyethyleneChemical drums, jerrycans, carboys, toys, picnic ware, household and kitchenware, cable insulation, carrier bags, food wrapping material.
PVCpolyvinyl chlorideWindow frames, drainage pipe, water service pipe, medical devices, blood storage bags, cable and wire insulation, resilient flooring, roofing membranes, stationery, automotive interiors and seat coverings, fashion and footwear, packaging, cling film, credit cards, synthetic leather and other coated fabrics.
LDPElow density polyethyleneSqueeze bottles, toys, carrier bags, high frequency insulation, chemical tank linings, heavy duty sacks, general packaging, gas and water pipes.
PPpolypropyleneBuckets, crates, toys, medical components, washing machine drums, bottle caps, and battery cases.
PSpolystyreneToys and novelties, rigid packaging, refrigerator trays and boxes, cosmetic packs and costume jewellery.
Otherother types of plastics 

Plastic recycling by municipal authorities varies; sometime, but not consistently, it is collected separately from other waste such as metal, paper etc.  Once collected it may be separated into different types of plastic, but a large proportion still ends in landfill.  

Mixed plastic packaging (trays, tubs, pots) made of plastic such as PS and polyurethane cannot easily be recycled. Plastic collected for recycling is first sorted for polymer type, then shredded, washed, melted, and pelletized before being made into new products that could include refuse sacks, carrier bags, flower pots, wheel bins, drink bottles, food trays, and even polyester fabric for clothing.   Segregation of plastic types is necessary to maximise the functionality of any products made from the recycled plastic.   Ultimately any recycling process only delays the fate of the plastic in ending up in landfill or water systems.  

Mixed plastic materials which cannot be recycled may be diverted to incineration or production of hydrogen fuel.

Complex materials widely used in construction such as glass or fibre-reinforced plastic (FRP, GRP)  are not amenable to recycling as such, but options for waste materials include:

  • Grinding to form a fine filler.   
  • Cement kiln recycling:  composite materials are shredded and used in cement kilns or organic resin is burnt for energy.
  • Energy recovery:  the ash produced may go to landfill or be used for aggregate.

What happens to plastic that goes to landfill or into the ocean?

Fossil fuel-derived and much biomass-based plastic is not currently biodegradable (i.e. converted back to basic molecular components) in any timescale comparable to the rate at which it is produced and may stay in the environment for centuries. Large plastic whether as litter or landfill breaks down progressively to meso-plastics (small fragments of 5–40 mm), micro-plastics (MPs, 1–5000 μm) (Thompson et al., 2004) and nano-plastics (NPs, plastic particles in the range of 0.1 μm or less) (Gigault et al., 2018) before being completely decomposed.

In the marine environment

Leaving aside larger marine creatures which may eat or get trapped in plastic, there are unseen consequences of the large amounts of plastic reaching our oceans, and this is becoming more evident in growing numbers of research reports.  Filter feeders (shellfish, such as clams, mussels, scallops and oysters) feed by drawing in seawater and filtering out the particles.  A group of marine scientists in Monterey Bay in California found plastic particles in the water column from the surface to 1000 metres depth, and found microplastics in both filter feeders and pelagic crabs (which are omnivorous).  The type of plastics.

found in the water column were predominantly PET (packaging, single use plastic) and polyamide (textiles and automotive applications), while polypropylene, which is the main material used for fishing gear, was less common.   A lab experiment in N Ireland has shown  that plastic in a marine environment  impacts on the behaviour of hermit crabs, resulting in impaired ability to select new shells as they grow.

 Investigations in other world oceans have shown high concentrations of plastics from both terrestrial and marine litter:  for example a study in the Atlantic Ocean showed that the combined mass of just the three most-littered plastics (polyethylene, polypropylene, and polystyrene) of 32–651 µm size-class suspended in the top 200 m is 11.6–21.1 Million Tonnes. The Arctic and Antarctic are not immune from plastic either;  it has been suggested that at least some of the particles found in these remote areas are generated from the wear on tyres and brakes of vehicles in the more highly populated parts of the world. Tyre and brake wear annually is estimated to produce about 6 and 0.5 million tonnes of particles respectively worldwide (equivalent to approximately 2% of plastic production). 

A survey of waste in the Southern Ocean beteen 1989 and 2019, recovered 10,112 items of waste weighing in total more than 100kg from Bird Island off South Georgia, and 1,304 items weighing in all 268 kg from the remote shores of Signy Island in the South Orkney archipelago;  90% of this was plastic.  Plastic particles may eventually sink to the sea floor.  A study of the deep ocean 380km from Australia, using a robotic submarine at depths as much as 3000 metres, found large deposits of plastic.  The scientists from CSIRO estimated on this basis that there may already be 14 million tonnes of plastic on the sea floor.  The currents that carry these particles to their resting place mean that these

deposits are likely to end up in the areas of highest biodiversity.  However, some may be recirculated into the atmosphere by ocean turbulence and seaspray, which may result in an estimated 136,000 tonnes per year being blown back onshore.

The presence of plastic particles in the marine environment is not just a form of inert pervasive litter, as can be seen from the research findings which are beginning to reach publication.   It seems that microplastic particles ingested by clams (a typical filter feeder) may damage their gills, possibly due to the presence of toxic chemicals present from manufacture or adherence, and which can leach out into the tissues.  A similar effect is seen in marine photosynthetic algae:  plastic pollution has been shown to interfere with the growth, photosynthesis and oxygen production of Prochlorococcus, the most abundant photosynthetic marine bacteria group.  These tiny organisms are important in producing the oxygen that we all rely on in the atmosphere.

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