Composters on the internet recommend bag or sack composting where the householder does not have a garden or space for a compost bin as the sacks can be stored in a small space or in a shed or garage. It is also
offered as a way of composting kitchen waste during the winter but a disadvantage of this is that it will entail saving the waste in a bin, or other leak and smell proof container, until there is enough to fill the bag in one go when combined with the browns.
It is also argued that anaerobic composting in a bag or sack require less work than conventional composting as the contents do not need to be aerated by turning at regular intervals, but it will smell.
The simplest method
is to put the kitchen waste in a refuse bag, soaking the material and sealing the bag closed. Some of the demonstrations of bag composting on You-Tube show conventional kitchen waste bags being used to contain the compost but I would recommend thicker
bags, such as old compost bags or builders refuse bags. As a safety measure double bagging is recommended to avoid the full bag splitting when being handled. I would suggest the one bag is put inside the other before filling with organic material rather
than have to lift a full bag to put it inside the other
Do not use Compostable or Biodegradable bags the Compostable bags may decompose before the compost and biodegradable bags are unlikely to be strong enough.
As it helps the process to turn the bags occasionally during the composting process it is important to choose a size that is relatively easy to handle. I00-150 litre bags are a good size, but a smaller 70 litre
bags are easier to handle and can be used if the larger bags are too heavy to turn. If only one or two bags of compost are being made I would suggest putting them in an old plastic dust bin which can be laid on its side and rolled to mix the decomposing
material. It is important to ensure that there are no holes in the bags to prevent air entering the mix as the objective is to achieve anaerobic conditions.
Mix for anaerobic bag composting
"Composting in a Bag" is usually treated as a batch composting system with the materials being set aside separately until the bag can be fully filled. As mentioned above nitrogen rich wet material is ideal and while it is necessary to include carbon
rich Browns to provide a roughly equal quantity of Greens and Browns chose the Browns to avoid adding too much very dry material such as such as leaves, sawdust and woodchip.
Some recommend the addition of commercial compost
activator, active compost from a compost heap, or garden soil to speed the start fermentation process by introducing microbes to the bag.
This can be added in two ways:
using the above method of alternative layers of Greens and Browns active compost, commercial compost activator or soil can be sprinkled over every layer of the Greens (or kitchen waste). ensure that composting micro-organisms are distributed
throughout the material in the bag.
- Rather than using a mix of equal parts Green and Browns use compost or soil as a third layer using 1/3 compost or soil, 1/3 Greens and 1/3 Browns.
With either method
I recommend adding a layer of active compost to the bottom of the bag to act as an activator and to help absorb the liquid which will drain to the bottom of the bag.
The dry spent compost from pots or planters, can be used
if available, and is best added at an early stage to help soak up some of the liquid produced from the decomposing food waste.
Continue with alternate layers of greens, activator, browns and spent compost till the
bag is full. When the bag is full the contents are soaked with water and the surplus air squeezed out of the bag. The bag should be tied, or taped, to prevent the access of air.
Ideally the bag should be turned every
couple of weeks to mix the materials. Turning is made easier if the bags are put into a plastic dust bin which can be put on its side and rolled with minimum effort. The bags should be left in a sunny spot during the summer and preferably in a heated or frost-free
shed of garage during the winter. In colder spots the fermentation will take longer.
The time to produce compost varies with the mix and conditions in a sunny spot immature in eight weeks to six months in colder positions
the fermentation will take longer. When the process is complete the material should look and smell like normal compost however it should be stored under aerobic conditions for a least a month before use on plants to allow it to lose its acidity.