Hot Composting versus Cold Composting

Thursday, 19 July 2012  |  HOTBIN Composting

What is hot composting?

HotBin Compost Bin with Steam

Establishing the benefits of hot composting

Hot Composting is a great way to make lots of rich organic compost fast - typically 30-90 days. There are many different composting methods - to establish if hot composting for the domestic home gardener has any benefits over other composting methods such as cold composting or food waste digestion, you need to first establish your composting objectives.

Here are some examples:

  • Do you want to make lots of rich/great compost for your garden that will improve its fertility and lessen/reduce your use of fertiliser and maybe even peat?
  • Do you just want to keep the garden tidy?
  • Do you want to make a more positive contribution to the environment by recycling all your food waste so your local council no longer has to collect and transport it to landfill or a central AD/IVC reprocessing plant?
  • Are you just fed up with allocating more and more of your flower or vegetable patch to overflowing compost bins that never seem to do anything?
  • Do you only have seasonal waste, or do you also have regular weekly amounts of food waste all year round

Benefits of hot composting over cold composting:

(Table excludes Vermicompost/Worms/Bokashi/AD, etc)

Hot (40-65°C) Cold (0-40°C)
The right hot composting system will work all year round – which is relevant when composting food waste. Cold heaps 'stop' in winter (-5 to +5°c). Anything added piles up until spring when the sunshine warms up the heap. Piling up food waste is not an option – it is just a free rat take away.

Hot Composting can take a wider variety of food waste types without causing issues.

Also the higher temperature results in water removal – and hence removes a prime cause of mushy/anaerobic food waste).

70% all household food waste is not added to 'cold' compost bins as it is likely to cause issues with odour, rats and flies.

Cold compost bins tend to be constructed with open vents and hatches – so any odour not only attracts but also allows access to the food – creating the infamous 'swarm' of flies when the lid is taken off, or even worse finding a nest of rats when the heap is broken open.

Hot composting kills weed seeds faster. The higher the temperature the more seeds are destroyed.

Many seeds will survive in 'cold' composting heaps. The problem is made worse as the seeds are planted in nature's best growing medium - humus/compost.

It's not just weed seeds but seeds from melons and tomatoes plus weeds like couch grass and dandelion will also survive and grow.

Hot composting kills pathogens and unwelcome bacteria. Which and how many is a function of both time and temperature. Defra offers a robust analysis, but a neat summary is that 60C for 1 hour equals good sanitisation,

There are two groups of pathogens those dangerous to farm animals (e.g. foot & mouth) and those dangerous to humans (nasty strains of e-coli orsalmonella).

You need to leave cold compost a long time (12-18 months) to achieve the same level of sanitisation and bacteria to die off.

It is also worth noting, whichever method you use, the main infection route back into the human gut is via dirty hands to cooking surfaces and food. Always wear gloves and wash hands.

There is a lot of confusion and hype about various herbicides wrecking vegetable plots when treated plants have been added to compost heaps.

Herbicides and pesticides are broken down in composting. They are broken down 32 times faster at 60°c than at 10°c.

The risks of pesticide infection via use of domestic compost is low because the concentration and volumes used are low. However the extra security of fast destruction in hot composting is an extra level of security.
Speed is not important to everyone - gardeners on the whole are a patient group. However if you are short of space speed means less bins. We often hear about cold bins that are overflowing, requiring a second, third or 4th unit. If you want more vegetable plot and garden and less compost plots – then hot composting quickly is a benefit. A UK ‘outdoor’ heap averages the same temperature as UK ambient air temp - i.e. 10°c. At 10°c the heap it is 32 times slower than at 60°C. If a soft waste like grass takes 6 months in a cold hep it will take about 2 days to reach the same state in hot heap. In general; cold = 12-24 months and hot = 1-3 months (because it does not stay at 60°c for the whole period).

A hot heap rarely produces putrid odours. Hot heaps transfer water away from the heap as steam (as long as the waste has free air space which is normally provided by bulking agent).

All composting produces other odours - known as volatile organic chemicals. These often have an odour. This is more noticeable in hot composting as they are made faster and the heat vaporises them. Hot composting needs a bio-filter unless the bin is away from human noses. But access to your bin is important if you want to recycle more food waste.

In a cold heap, water is not evaporated; it has to drain to the ground. This happens slowly and poorly in compost, so it gets water logged and turns anaerobic and putrid. The main solution is not to add 'wet' food waste to cold heaps - most of our diet is foods with 80% water. Cold composting releases VOC more slowly and they do not vaporise as much.
A hot bin will kill fly eggs & larvae - so no swarms of flies or maggots in a hot compost bin. It's too hot at the top for ants, rats, and most things you don’t like to see in a heap. Flies will lay eggs and larvae (e.g. maggots) will be present in a cold heap, and often swarms of flies can come out when lid open.

All compost eventually ends up in the same place. However the quality of what you put on the garden has an effect.

Are you making rich humus, light friable wood/leaf, or loads of large chunks of partly decomposed stalks and twigs?

What you end up with is not a function of hot or cold - but we have noticed the ease and methods used in hot composting does result in rich compost.