What is bulking agent?

The HOTBIN is an aerobic compost bin. Aerobic composting relies upon supplying bacteria with oxygen for them to digest the waste.

The need to assist airflow occurs in all compost heaps/bins. HOTBIN uses bulking agent to eliminate the need for turning waste. Bulking agents are particles of material able to create a self-supporting structure which maintains free air spaces and hence airflow within the waste.

When and why do I need to use a composting bulking agent?

Add bulking agent (purchased or home made) when composting soft/wet materials like food waste or grass. When soft (non woody) waste breaks down, the cell structure is lost and the waste collapses into a mushy layer. Water is released from the cells and this collects within the mushy layer. This layer quickly blocks airflow and the waste will turn anaerobic with a putrid/rancid odour.

A bulking agent maintains airflow through the mushy layers by forming a structure around which air can flow and prevents the waste turning anaerobic. Think of it like a pile of building blocks stacked irregularly on top of each other!

In the HOTBIN we seek to compost all food wastes - this if often very soft/wet - so it is essential to add bulking agent.

How do bulking agents work?

The structures which bulking agents create are called free air spaces (FAS). FAS are 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. 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.

Is turning my compost a subsitute for bulking agent?

Turning moves around the collapsed particles and creates a new FAS structure through which air can circulate. It has been argued (Haug, et el) that turning only introduces enough oxygen to support bacterial growth for short periods of time (often hours).

However, without self-supporting particles in the turned waste such as those created by bulking agent, the heap will quickly collapse and restrict airflow again.

Conversely, if the heap has self-supporting particles, no turning is needed. To maintain free air space and create self-supporting particles within the HOTBIN, bulking agent (partially composted wood chip) is essential.