What is meant by ‘Free Air Space’ (FAS)?

Tuesday, 4 February 2014  |  HOTBIN Composting

'Free Air Space’ (FAS) is the sum of all the gaps around and between particles though which air can circulate. When particles are able to create a ‘self-supporting structure’, the gaps and spaces are maintained for long periods.

Aerobic compost heaps (eg the HOTBIN and almost every commercial domestic compost bin) rely on aeration via buoyancy air flow which in turn is reliant on retaining around 20-30% 'free air space’ (FAS). It has been argued (Haug, et el) that turning only introduces enough oxygen to support bacterial growth for short periods of time (hours). Turning moves around the collapsed particles and creates a new FAS structure through which air can circulate.

However, if there are no 'self-supporting' particles in the turned waste, the heap will quickly collapse and restrict airflow again. Conversely, if the heap has self-supporting particles, no turning is needed. To maintain FAS, the HOTBIN uses a 'Bulking Agent’ (in our case wood chip particles).