Indoor aerobic composting

It is not necessary to have a garden to compost.   In the UK  Bokashi fermentation ( Bokashi Composting ) and indoor wormeries (Wormeries) are frequently used in conjunction with, or as an alternative to, outdoor composting but indoor aerobic composting is also  possible.  Although nor commonly used in the UK indoor aerobic composting  appears to be used more frequently  in the US and features regularly on composting pages of social media.    

Materials suitable for indoor aerobic composting



In theory any organic waste that can be composted outdoors can be composted indoors  e.g. fruit and vegetable peelings,  coffee grounds, tea bags, shredded paper. In practise items that compost slowly are best avoided, due to limited size  of indoor bins  and lower temperatures reached in  smaller bins.   Fruit and vegetables  with a high-water content, e.g. squash, and strongly smelling waste such as onion  are best avoided unless the composter has a filter fitted. As with outdoor composting if the waste material is cut to lengths of about 2”  or less, it will breakdown more quickly. Meat, dairy, or fats are not suitable for composting aerobically in a conventional  indoor or outdoor bin  but can be treated in a Bokashi bin to produce pre-compost.

Shredded paper or cardboard are good carbon rich browns for use in an indoor bin as they are  easy to store and will not smell. More browns should be added every time greens are added.

Frequent aeration is necessary as the containers are not designed to provide a high level of ventilation, mixing  also breaks up potentially anaerobic lumps and helps provide even moisture distribution. As indoor compost containers are relatively small it difficult to use most types of commercial compost aerators,  but  a long-handled  hand fork makes an effective mixing tool.


Selection of an Indoor Compost Bin

A wide range of waterproof containers may be used for indoor composting with plastic or  metal being the  most popular choice. It is best to have air holes drilled in the lid or around the top of the bin. If the  bins  are  to be stacked to save  storage space the air holes should be drilled in the sides  near the rim rather that in the lid. Drainage van be though holes in the bottom  or a drainage tap can be fitted near the at the base of the bin.  If drainage holes are used the bin will need to be sit on feet to allow drainage in a flowerpot saucer to catch the leachate. This should be emptied regularly to avoid odours. I also fit an inverted flowerpot saucer inside the bin with larger drainage holes to  prevent the compost blocking the drainage holes in the base. Small pebbles can also be used but these are less easily cleaned.

The most used containers are :

Stackable plastic storage bins. These containers have the advantage of being available in a wide range of sizes so it should be relatively easy to get a lidded  bin or a suitable size for the space available. It is better to have a bin that can be filled, sealed, and set aside fairly frequently to avoid smells. A 5 gallon or 20l container is commonly used but larger containers of up to about 40 litres and even dustbins can be used.  However, the larger the bin the heavier it will be to move.

Five-gallon bucket with lids: While any lidded bucket can be used.  “Nappy” buckets with flat lids  are recommended if the bins are to be stacked when full.

To prevent fruit flies   and other flies from escaping from, or getting into, the bin  through the   air holes, they should be kept small or cut close together, and  covered with a piece of nylon screen, fine mesh garden netting  or fine weld mesh. glued to the inside of the container.

I keep my indoor bin and the drainage tray in a small dustbin to contain any escaping critters be they worms or fruit flies. However, in over five years or keeping an indoor bin in the garage I have only had worms escaping once and that was when I failed to refit the lid correctly.

The container can be kept in any suitable room  preferably with an easily cleaned floor and in a position where it will not form a trip hazard.


Filling the bin

An indoor bin can be filled in the same way as an outdoor cool composting bin   but alternatives have been suggested  designed spacifically for indoor composting. 

The bin is either quarter filled with a layer of soil or  a 4” layer of soil or compost used as a base layer. This is covered by a layer of shredded paper or torn up newspaper. Newspaper is more absorbent but can be recycled via most council schemes while shredded  office/computer paper is suitable and cannot be recycled.   

Vegetable waste from the kitchen can be added as it becomes available or can be saved and added weekly or when there is enough to make a layer in the bin. While waiting to be added to the bin waste kitchen  food, and other compostable waste,  can be stored in asealable container such as a kitchen caddy. 

Kitchen waste (Greens)  should be mixed with fresh  browns, such as shredded paper, either before being added to the bin or when being added. A handful of soil or compost should be mixed into the composting material  weekly.  

A layer of shredded paper or finished compost/soil can be used to cover the composting material to reduce the number of fruit flies. The organic materials should be mixed to aerated wekly. Following aeration a layer ofthe contents should be covered with a layer of soil or compost.

It is important that the compost should be mixed and aerated (turned) weekly using the trowel or a scoop to mix the layers of compost. Top off the composting material after aeration with a layer of compost/soil.  

When full the bin the contents should be given a final mix and then left for at least a week to allow all the food  to be decomposed.  It can then be transferred to another container and left to mature until  it is ready to use.