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

Free Air Space (FAS) is defined as the gaps around and between particles (in this instance waste) 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 composting, such as the HOTBIN process relies on aeration via buoyancy air flow which in turn is reliant on retaining around 20-30% free air space.

It has been argued (Haug, et el) that turning compost  to aerate the waste 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 required. To maintain free air space in the HOTBIN we advise adding Bulking Agent (partially composted woodchip) each time waste is added. This provides the self supporting particles allowing the air to flow through the waste.