In the previous instalment, we defined plastics and explained why it has become a global issue. In this article, we will cover how we can make a better use of plastics when they become waste. We tend to hear mostly about recycling, but it’s not the only way to deal with them as we will see.

III Recovering plastic waste

Material recovery: recycling

The name of the game. Or is it?

Not all plastics are recyclable. The five main recyclable plastic families are:
  • polyvinyl chloride (PVC);
  • polyethylene (PE) and its high density (HDPE) or low density (LDPE) variants;
  • polystyrene (PS);
  • polyethylene terephthalate (PET);
  • polypropylene (PP).
The plastic content is often indicated on the product (or packaging) using the pictograms below. But it is not mandatory, following European Commission Decision 97/129/EC.

Plastics are collected in most places as a mixture of recyclable waste. This waste is firstly sent to a sorting centre where recyclables are sorted, recyclable plastics are separated from the rest and then sorted by families. They are then sent to a recycling factory. There, these plastics are crushed into small pieces and washed, in order to obtain granules that can be moulded again to become new items.

The overall process is summarised in the figure below.

Although the amount of recycled plastics in Europe increased from 4.7 MT to 7.5 MT between 2006 and 2013 – at a rate of almost 8% per year – recycled plastic only represent about 13% of the 59 MT currently produced each year in Europe.

The rest of the plastics are either incinerated (energetic recovery), dumped in landfills, or ends up in the environment, mainly in oceans.

If we don’t change our habits, the global yearly plastic production should reach 1,124 MT by 2050; considering the current scope of the marine pollution, the Ellen MacArthur Foundation forecasts there will then be more plastic than fish in the ocean, in weight ratio.

Focus on packaging

In 2013, the global packaging production reached 78 MT. 14% of this amount is collected for recycling, of which:
  • 2% is recycled in closed-loop: recycled into a similar or same-value product;
  • 8% is recycled in cascade, that is in other plastics of lower value;
  • 4% is process loss and cannot be recovered.

Energetic recovery: incineration

An alternative to landfill for non-recyclable waste

Energetic recovery consists of generating energy from wastes by burning them. This is the method of choice to dispose of complex plastic mixtures, like Tetrapak containers in which multiple layers of different type of plastics alternating with metallic films are glued together. For energetic recovery, plastics are not separated from household wastes and are incinerated together. This concerns about 14% of all plastic waste.

Plastics are interesting sources of energy for incineration due to their high calorific power, that is the amount of heat that their combustion generates. The calorific power of a material is often expressed in form of its Lower Heating Value (LHV), which is the minimum amount of energy that released when a kilogram of material is burned. As you can see in the table below, the LHV of plastics are quite high, with the calorific power of LDPE and PP somewhat similar to that of petrol, and twice that of wood.

Comparative LHV of plastics and usual sources of energies (sources: Determining accurate heating value of non-recycled plastics, Demetra et al., 2016; Well-to-Wheel Studies, Heating Values, and the Energy Conservation Principle, Bossel, 2003).

The energetic recovery process is straightforward, as seen below: the waste is burnt in a furnace, producing smoke with temperatures ranging from 850°C to 1000°C. This smoke is used to heat water, turning it into steam, which is either:

In addition to generating energy, incineration significantly reduces the volume of waste and make them more compact, less likely to be lost and pollute the environment. This does not mean that there’s no leakage: during combustion, the polymers are broken into smaller molecules, forming gases and particles. A great variety of molecules are found in incinerators fumes, including very toxic ones like dioxins.  Using a filtering step is thus necessary to remove these toxic compounds from the fumes before they are let out.

The remains are ashes and mud, also potentially toxic and requiring a specific treatment before being used in asphalts, cements, concrete, etc. or being dumped in landfills.

Are incinerators overshadowing recycling?

Although energetic recovery enables us to produce energy that is sometimes considered as clean (even though not everybody agrees with this definition), it is legitimate to question the usage of this technique as it creates a competition with recycling. Indeed, recycling plastics cost less energy than incinerating them, as it removes the energetic costs of extracting new raw material from the plastics lifecycle.

Environmental agencies aiming a “zero waste” objective often take a stance against incinerators, considered as an obstacle to recycling. Cases of illegal diversion of refuse from recycling to incineration for economic reasons have recently made the news giving some weight to their arguments. However, it should be possible to use both solutions complementarity, as they don’t address the same issues.

A not-so-clean energy

This is not the only reason why people are debating energetic recovery. Even filtered, gas emissions from incinerators are problematic, and in particular CO2. Indeed, incinerating non-recyclable plastics generates more CO2 than if the same amount was put into landfill. The only way to obtain a negative CO2 balance during plastic incineration is to ensure a very high energetic yield, in which case the emissions would be lower than if we produced the same amount of energy with your typical fossil fuel. This requires expensive optimisations of incineration processes, which are more often than not, not performed.

CO2 is not the only residue emitted during plastic incineration. Studies have highlighted that heavy metals, PAH (Polycyclic Aromatic Hydrocarbons) and persistent free radicals – all known for their toxicity – were found in plastic fumes as well as in plastic ashes. While in theory they are filtered out, in practice the risks of soil and air contamination – with dire consequences on nearby inhabitants’ health – are still high in countries with weak environmental regulations, particularly the developing countries.

Chemical recovery: depolymerisation

A third method of recovery, still quite underrated, is chemical recovery. It consists in re-transforming polymers into monomers, which can then be used to generate new, clean plastics. The process is called depolymerisation, as illustrated below:

Depolymerisation is essentially breaking down the plastics by cutting chemical bonds between the different monomers composing the polymer(s), using the equivalent of “molecular scissors”. These scissors can be physico-chemical (temperature, pressure, presence of a catalyst, etc.) or biological (enzymes).
There is yet no industrial solution to chemically recover polymers in a way that answers the needs of the markets in terms of quantity or yield. One reason for this is that the conditions required for depolymerisation are usually really harsh: high temperature, high pressure, toxic catalyst, etc., generally making them not very environmentally-friendly. This is because a high energy input is required to break a chemical bond. Furthermore, each type of “molecular scissor” can be used only with one type of chemical bond, and thus with only one type of plastic: either PET, or PP, or PVC, etc.
Private and public research laboratories are showing a growing interest for developing new, better depolymerisation methods. In 2015 researchers from the French Nuclear Agency (CEA Saclay) have developed a method to depolymerise polyethers, polyesters and polycarbonates in mild conditions: at room temperature and atmospheric pressure, using non-metallic and non-toxic catalysts. However, the researchers have yet to declare any intention to scale up this method to industrial purposes. It is thus still very much an open problem.
Read more about it in your next article about enzymatic recycling.

This work is under CC-BY SA licence: