It is not necessary to have a garden to compost. In the UK Bokashi fermentation and indoor 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.
There is more information at carryoncomposting.com