Can I start composting in the winter?

Monday, 9 January 2012  |  HOTBIN Composting

Getting your HOTBIN started in autumn and winter.

You can kick-start your HOTBIN to 60°c in the autumn and winter by carefully adding boiling water into the HDPE plastic bottle provided with the HOTBIN.

(Please DO NOT attempt to use glass (these can shatter and release boiling water and glass over you), plastic fizzy drinks bottle (made from PET - they will melt at 100°c) or the plastic milk cartons (they are so thin they deform).

How to use the kick-start water bottle

Carefully fill your bottle with hot water. Use gloves to twist on the cap and carry the container to your HOTBIN (100°c heat will burn you through plastic very fast). Bury the bottle in the waste, covering it with 10-20 cm of new easy to compost waste, close the HOTBIN lid and leave 4-6 hours before re-checking. 

The science on how and why the kick-start bottle works

Once you know this, it is easy to identify when it will and will not work.

  • Heat in a compost heap is produced from bacterial activity (ie. growth).
  • The more bacterial activity = the more heat produced.
  • Bacteria do not grow quickly below 5°c and not at all at 0°c.
  • When the waste is cold, only a tiny amount of heat is created by the bacteria and this is quickly lost to the cold air - this is why cold composting heaps lie almost dormant through winter. 
  • When you add the bottle with hot water into the waste, this heat (1 litre of boiling water contains 4200 J energy as heat) moves to the colder waste. 
  • The HOTBIN's insulated walls help retain this heat - just like a thermos flask. The waste around the bottle heats up to 30-40°c for an hour or so
  • During this short period, bacterial are warm enough to become active. They generate heat, which makes then even more active which quickly heats the waste.
  • BUT bacteria only generate heat when they have food to eat.

This winter kick-starting technique only works if there is enough easy to compost food waste. We liken it to a human diet - if you eat a high fibre breakfast you get steady energy all day, but drink coke and eat sweets and you'll be on sugar high for an hour! Since the heat from the bottle only last an hour or so, the bacteria need easily compostable food during this hour. If your bin is full of woody stuff that they find hard to compost, then no new heat will be generated. The bottle works  when bacteria have a diet of fast food; ie shredded white paper and cardboard, food waste and soft plant material (grass nettles, comfrey).

You should be able to establish from the above that you do not need to use the bottle in summer. The air temperature will be 20°c (if we are lucky!) and the bacteria will be eating fast enough to develop and generate heat on their own.

A simple test - add grass to the HOTBIN in summer - to date everyone has observed the same - the HOTBIN gets to 60-70°c within 4-8 hours.

Please note: Do not pour boiling water directly into the heap - the water will cool and there will be no energy to dry the extra water (see energy/drying).

(At time of posting, the cost of this ‘kick start’ is approx 10p/use. 1 Kg water = 1 litre, to raise 1 litre to 100°C requires 4,100 KJ)