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Which compost bulking agents can I use in the HotBin?

Monday, 3 September 2012  | 

What is a composting bulking agent?

Bulking agents are particles of material able to create a self-supporting structure which maintains air spaces and hence airflow within the heap.

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

Add bulking agent when composting 'soft' 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 layer by forming a structure around which air can flow -think of it like a pile of building blocks stacked irregularly on top of each other!).

The need to assist airflow occurs in all compost heaps. However when adding woody garden materials the problem rarely arises. In the HOTBIN we seek to compost all food waste - this if often very soft - so it is essential to add bulking agent.

What is the best (recommended) bulking agent?

Our recommended bulking agent is ‘Composted Wood Chip’.

What others are available?

The table below contains a list of other possible bulking agents and an assessment of their relative merits. Note: scored 1-5, 5 being highest and good, except on cost, where reversed 1 = high cost, undesirable)

Bulking Agent Performance Cost Availability Total Score
Composted wood chip (dry, not damp) 5 4 5 14
Fresh wood chip 4 4 5 13
Shredded garden pruning 4 5 4 13
Crushed charcoal (BBQ wood – new, unused) 5 3 4 12
Wood bark (dry not damp) 4 2 4 10
Wood shavings (dry) 2 3 4 9

Can I use Biochar / crushed charcoal as a bulking agent?

Yes - but it is a very expensive way to create air spaces. There is a growing community adding biochar to the soil. However the benefits of finely ground biochar powder versus 'chips' for air flow are different objectives.

How do bulking agents work?

Bulking agents work by creating 'Free air spaces’ (FAS). 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).

Bulking Agent Composition