Why do Compost heaps need turning?

Saturday, 19 May 2012  |  HOTBIN Composting

Composting websites often refer to the need to turn compost. This normally means using a fork to dismantle, turn and re-build the heap. In the HOTBIN, there is no turning; waste is added into the top of the bin. It is desirable, but not essential, to occasionally stir/mix/rake new waste into the active upper layer (top 10-20 cm).

Why is the HOTBIN different to other heaps?


Requirement in tumblers and heaps needing turning


Many sites advise turning as a means of aerating the heap, ie adding air/oxygen back in to the waste. Turning has a temporary effect as the amount of oxygen re-introduced is very small and will be used typically within Hours (Haug et al).

The underlying benefit of turning comes from the disturbance of layered/matted material by re-introducing ‘Free Air Space (FAS) through which oxygen can circulate. However, if the turned material has no ‘self-supporting’ particulate matter, it will quickly re-settle, and if the heap has self-supporting particulate structure, it will not need turning in the first place! (see opposite)

A number of well-researched scientific studies prove "passive aeration" (i.e. buoyancy flow using rising hot air to pull cold air in) is sufficient to aerate heaps up to 3m high. But, the science is based on the heap maintaining what is known as ‘free  air space'.

In the HOTBIN, bulking agent is used to maintain FAS, and no turning is needed. (see bulking agent for more detail)

Redistribute water (water is produced where the waste is decomposing - which is usually in centre of the heap where bacteria are most active. Without high temperature water will not be driven off as steam it will stay or or drain to ground.

Turning moves wet waste (often too wet) to the outside and dry waste (often too dry) to the middle.

In the HOTBIN, high temperatures are maintained and water is expelled as steam via the valve.

Heavy heaps compress the compost below. This prevents aeration and builds up water. Forking through the heap removes compression.

Tumblers can increase compression and reduce aeration - ie the opposite effect of that promised. The weight of material circulating can create a ‘solid’ compressed slug or ball of waste that rises to top of turn then falls to base 'slamming' the waste together. The wetter the waste the more pronounced the problem.

Bulking agent is self-supporting.  Compost is continuously removed, excessive compression never really happens.

Turning of heaps is used to move the outer layers which do not heat up, back into the centre.

With the insulated walls, the active zone extends across all waste. There is no need to turn.

Re-aerate anaerobic slimy mess

As soon as the soft food and grass matter starts to decay, it loses water and compresses into a wet slimy mass and oxygen stops flowing.

It needs significant turning to re-aerate this and often this effort has little impact (see panel above).

All wet waste has the potential to go anaerobic. This can occur in the HOTBIN if not enough bulking agent is added.

The benefit with the HOTBIN is how easy it is to correct. Composting new waste takes place in the upper layer (active top 10-20°C cm). On detection of putrid odour, it is relatively easy to stir in additional bulking agent to correct. (Note: we believe it is fair to refer to this as raking/stirring as opposed to turning as the effort and tools used are different).