What to do with plastic waste
Composting is not a feasible way to remove ubiquitous plastic from waste biomass. Hydrothermal carbonization and hydrochar sequestration can help.
In recent years, many attempts have been made to create closed cycles in waste management.
For example, plastic collection systems have been set up to collect the plastic again and then recycle it. This has not always worked well, largely due to quality deficiencies of the raw materials — but it was at least a start. Similar projects were attempted in the area of biological waste.
The biodegradable fractions and such as plant waste, leaves from the streets and especially organic waste from households were turned into compost in large composting plants. In itself, it is a very good idea to close this cycle and to use the compost obtained as a substitute for peat or to improve the soil.
The problem with compost
Unfortunately, however, there is a problem with the above-mentioned raw material. Very often the organic waste is mixed with large amounts of plastic packaging (some of which is labelled as compostable), meaning it was not possible to produce truly plastic-free compost, regardless of which screening and separation methods were employed.
The compost from composting plants inevitably contains some percent of plastic — especially microplastic due to the shredding of the raw materials — so that this microplastic has ended up on agricultural fields and in gardens where the compost is used.
Anyone can take a look at such compost under a microscope and see how many microplastic particles are in it. And that is certainly a problem both for biological agriculture and environmental protection.
Hydrothermal carbonization (HTC) makes it possible to convert these contaminated biogenic raw materials into hydrochar in the way we describe before.
Some of the plastic remains (polyolefins) and some is broken down, such as PET and polylactic acid. All in all, they do not interfere with the hydrothermal carbonization process. If the intention is to sequester the carbon obtained anyway, i.e. to store it, then the plastic does not interfere.
The most important thing to take into account is that part of the carbon comes from fossil sources (the part that comes from the plastic) and should not be taken into account when creating carbon-negative certificates.
Thus, it would be possible to use low-grade waste streams that are neither entirely bio-waste nor entirely plastic waste. And it would be ensured that these plastics do not enter the environment in an uncontrolled way and pollute it with microplastics.
The same applies to waste streams like the ones you have near the ocean.
Seaweed and seawater-borne plastic particles accumulate in large quantities, and composting is not an option for the same reasons we mentioned above. Instead, the application of hydrothermal carbonization is more effective to produce carbon that can be sequestered, even if it is mixed with small amounts of plastic.
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