There is nothing wrong with composting of garden and kitchen waste, but no plastics of any kind should be introduced into the process.
A “Grocer” magazine survey of more than 1,000 individuals in 2019 found that “consumers think that plant-based compostable plastics are the most environmentally friendly packaging materials,”
but most consumers don’t realise that “compostable” plastic does not convert into compost. It is required by ASTM D6400 and EN13432 to convert rapidly into CO2 gas, and the last thing the planet needs is more CO2.
Also, many consumers do not know that “compostable” plastic is tested to biodegrade in an industrial composting facility – not in the open environment. In November 2019 a Danish court ruled in Ellepot v Sungrow that “compostable” PLA plant pots must not be described as biodegradable – because they are not biodegradable except in the special conditions found in an industrial composting facility.
“Compostable” plastics are really an irrelevance, because the main problem facing governments today is plastic waste which has escaped into the open environment, from which it cannot realistically be collected and taken to a composting facility.
Plastics marketed as compostable (ie hydro-biodegradable plastics) are far too expensive for everyday use, and there are very few industrial composting facilities available. For this reason the German courts in Güthoff v Deutsche Umwelthilfe (2014) held that it is deceptive to market plastic as “compostable.”
These plastics are often marketed as renewable, but this ignores the fossil fuels used in the agricultural production process by the machines which clear the land, plough the land, bring the seeds to the farm and sow them, harrow the land, bring the fertilisers and pesticides to the farm and spread them, harvest the crop and transport it to the factory, and by the machines which polymerise the raw material.
It also ignores the land and water resources devoted to producing the raw materials, which could be used for growing food. EASAC (March 2020 report) says that “replacing PE by a bio-PE would require almost all (93.5%) of global wheat production.” This is completely unsustainable.
Although these plastics are marketed as “bio-based” they can contain up to 60% oil-based material, but this is hardly ever mentioned in the marketing material. Conversion of organic materials to CO2 at a rapid rate is not ”recovery.” Nature’s lignocellulosic wastes do not behave in this way, and if they did the products would have little value as soil improvers and fertilisers, having lost most of their substance and their carbon.
On 11th September 2003 a Report to the Australian Government by the Nolan-ITU Consultancy concluded that: “oxo-biodegradable plastics based on polyolefins contribute to the amount and nutritive value of the compost because much of the carbon from the plastic is in the form of intermediate oxidation products, humic material and cell biomass.
This is in contrast to plastics such as hydro-biodegradable polyesters (eg starch-based) that biodegrade at rates comparable to purified cellulose. At the end of the commercial composting process, all of the carbon from the latter has been converted to CO2 so there is a contribution to greenhouse gas levels but not to the value of the compost.”
The same Report concluded that “degradable polymers manufactured from renewable resources (e.g., crops) have greater impacts upon eutrophication due to the application of fertilizers to land.”
On 15th July 2020 a report appeared in “Waste Management” Vol. 113, Pages 312-318. The conclusions were:
In many cases, plastic bags are being replaced with compostable plastic bags.
Industrial composting processes do not completely remove film fragments.
Compost is thus a potential source of fragments from compostable plastic bags.
Compostable plastic fragments are then deteriorated in soil to microplastics.
Compostable microplastic results in an increase number of aflatoxigenic fungi.
Composters reject it
Even the industrial composters do not want ”compostable” plastics.
In January 2020, the industrial composters of Oregon gave 9 reasons why they did not want it
Many areas do not have industrial composting plants, and the Welsh Government has refused to invest in them. https://www.bbc.co.uk/news/uk-wales-47238220 Plant-based compostable plastics are going to landfill rather than recycling because so many local authorities are unable to deal with them.
“Compostable” resins are worse than conventional or oxo-biodegradable plastics when it comes to oxygen transmission-rate or moisture vapour transmission-rate. These resins are also water sensitive, and their physical, optical, mechanical, and chemical properties are inferior.
Not only does plastic not convert into compost, but home composting of plastic is dangerous and should not be encouraged,
A study for the French government at https://www.ademe.fr/sites/default/files/assets/documents/compostage-domestique-industriel-sacs-plastiques-papier_2019.pdf says that “composting management must be in line with good practices recommended by ADEME (weekly brews for one month and then every 1 to 2 months, humidity control), – the average ambient temperature over the first three months of composting must be close to that of the standard: outside temperature of 25oC – 5oC. It is unlikely that all of these conditions will be met by individuals.”
The study also shows that “plastic bags are poorly disintegrated and biodegraded if good domestic composting practices are not applied It also shows that, even when good practices are followed, there are still a few pieces of plastic bags of micrometric or even millimetre size in composts beyond the standard year of home composting.”
In addition, the study says “it appears that the biodegradation of plastic bags suitable for domestic composting makes little or no contribution to the formation of humus because, in accordance with the biodegradation tests of these materials according to the NF T 51-800 standard, at least 90% of the carbon organic dioxide is converted into carbon dioxide.”
Worse still, there is a danger that the plastic may have been contaminated by pathogens e.g. from putrifying food, and that the temperature in a home compost may not be high enough to kill those pathogens.
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