"Compostable" bags are those that can be decomposed by aerobic bacteria at a compost site until it cannot be distinguished from the other compostable material. This process should be completed at the same rate as the
other compostable materials.
Officially as the product of the decomposition of compostable plastic bags will contain inorganic compounds it is not classified as pure compost or humus. There are two main types of compostable
bag based on polyethylenethat meet the European standard (EN13432) for composting. These should degrade at least 60% within 180 days.
In most cases the term “compostable” bags applies to industrial compost facilities as they have been designed not for home composting systems and temperatures. They are designed toy degrade during an industrial composting process which exceeds
a minimum temperature of 55°C the Composting Association Standards for Composts.
The symbol for bags suitable for home composting has the words Home Composting clearly displayed and should not be confused with those designed for
Only bags carrying b the words Home Composting are suitable to be home composted as they.can compost at the lower temperatures common to backyard composting environments.
A Power Point presentation of signage may be downloaded from this page
The plastic bags of choice for home composting are Starch or Biobased (Hydrodegradable) made using corn (maize), potatoes or wheat. These have
a limited shelf life but if stored in cool, dark, dry conditions should have a shelf life of approximately 18 months. Provided the right gauge of bag is used for the type and volume of compostable material with which it is to be filled the likelihood
of the bag splitting is now small due to improvements in the strength and degradability of compostable bags for household use.
However, there are bags that are designed to meet the European home compost standards.
These should completely degrade in 90 days in a compost bin that maintains a minimum temperature of 45°C. Some are claimed to degrade in as little as 10-45 days depending the composting system used. The bags carrying the home composting label have
been tested at ambient temperatures (20 -30) for up to 26 weeks mimicing the temperatures in a home cold composting bin.
Please note that in the results in your home composting bin may be different and the bags may
not breakdown easily. A Garden Organic members experiment, reported in The Organic Way 217 Spring summer 2017, showed that "home compostable" compost caddy bags made from plant based film showed a high degree of varibility when home composted with only
26% completetly degraded after a year in the compost heap.
It is unfortuately the case that if these bags are used in a cold system, you can expect to find them stuck on the prongs of your fork or the end of our compost
aerator for some considerable time. No clear colleration was found between the type of compost heap or the heap temperature and degraration.
A correctly operated hot composting system, where the temperature is maintained
at 45C or above should be able to deal with the bags that are made to meet this standard.
Compostable bags will decompose in a home hot composting system, indeed Hotbin (http://hotbincompostingblog.com/tag/compostable-bags/)
found that at 60 C they broke down within 30 days. The bags are breathable bags and the the contents e.g. waste food may have composted before the bag is degraded. But there is still a problem when using them as the plastic bags reduces the airflow
and oxygen available to the microorganism within the bag resulting in a risk the food being decomposed anaerobically until the bag itself has disintegrated. This takes about 10 days in a Hotbin. If a high proportion of the food waste, is added
to the compost bin in such bags the anaerobic contents may create an impervious layer similar to that which would occur if a quantity of grass were added without mixing an adequate supply of “browns”. This will result in a cooling of the bin, anaerobic
contents and the job of emptying and remixing the contents.
It is recommended in the Garden Organic Book of Compost (New Holland Publishers (UK) 2011 that the contents of the plastic liner are emptied into the bin
and the now empty compostable bag is added separately.
The bag will now decompose without affecting aeration of the whole bin. However, while you have a clean caddy as a result of using the bag, it will mean
that you probably have the waste all over your hands