Composting in a porous fabric sack is becoming increasingly popular. They difference in this technique and the use of a plastic sack for anaerobic composting is that the aerobic version of sack composting uses a bag that contains a large number of micro-pores to enable air to circulate and excess water to drain away. Aeration is essential to aerobic composting   fabric sacks such as the Compost Sak™ and the Riverstone Portable Composting Sack System being porous provides a sufficient supply of air to support aerobic micro-organisms which distinguishes the process from anaerobic composting using plastic sacks

 The sacks made using a durable textile fabric. The sacks are reusable. The most common size would seem to be 100 US gal. (about 380litres) capacity producing up to 12 cu. ft. of compost.

A well-designed compost sack will include flaps which can form lid or a pull cord closure system to prevent the contents becoming water logged during wet periods and to help retain heat.  The bags can be turned to aid aeration and watered if the contents appear to be to dry.

Having considered a number of different ways of making compost it is necessary to mention a further method that is simple, requires the minimum of equipment and can be undertaken where there is a minimum of space. Making compost in a sealed plastic refuse or old compost bag is simple and relatively quick. However the process involved is anaerobic rather than aerobic composting and as it produces methane, a greenhouse gas,   I would not recommend it the main composting method where a significant amount of garden or food waste is produced.  Different reports on the internet recommend bag composting as a means of dealing with container grown plants and the spent compost in which they are grown where the grower does not have a garden where the spent compost could be used as mulch. or space for a compost bin or as an alternative to adding the plants and their root ball to an existing heap (which is argued will cool a hot heap). It is also suggested as a way of composting kitchen waste during the winter but a disadvantage of this is that it will this entail saving the waste in a bin or other leak and smell proof container until there is sufficient, when combined with the browns, to fill the bag in one go.

Some of the demonstrations of bag composting on You-Tube show conventional kitchen refuse bags being used to contain the compost but I would recommend thicker bags, such as old compost bags. Double bagging is recommended to avoid the full bag splitting when being handled. . I would suggest the one bag is put inside the other at this stage rather than have to lift a full bag to put it inside the other later. As it helps to turn the bags during the composting process it is important to choose a size that is relatively easy to handle.  30-33 US gallon bags (approx 110-125litres) are said to be a perfect size. but  I have used 70litre bags which are easier to handle quite successfully. 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 primary as it is an anaerobic methods and it is necessary to exclude air and to avoid smelly leachate leaking out during the composting process.

When "Composting in a Bag" the mix of the material being composted is the normal one part of “brown” material to one part “green” with the addition of compost from an active heap, soil or commercial activator to introduce the micro-organisms that will undertake the decomposition

I recommend adding a layer of active compost  to the bag first to give everything a boost and more importantly to help absorb the liquid which will collect at the bottom of the bag. Then add alternate layers of kitchen waste and browns.

A little of the active compost or commercial compost activator can be sprinkled over the first or every layer of the kitchen waste to ensure that the micro-organisms   are distributed throughout the mix.  If spent compost from pots or planters is to be used it is best added at this stage the dry spent compost should soak up some of the produced from the food waste.  Continue alternate layers of greens, activator, browns and spent compost till the bag is full. The  contents of the bag then be soaked with water  Surplus air should be squeezed out of the first bag which should  then be tied, making sure no air can enter  The second bag should then be secured .

If possible the bag should be turned every two weeks to mix the materials. And this process is made easier if they 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 in preferably in a heated or frost free shed of garage during the winter.

Depending on the mix and conditions the immature compost may be ready in as little as eight weeks, but may need up to six months or even a year.  

Sources

www.plasticplace.com/blog/how-to-make-compost-in-a-garbage-bag

http://www.askorganic.co.uk/composting/Composting%20in%20a%20Bag%20merged.pdf

http://crazyaboutcompost.com/tag/garbage-bag-composting/