Odour Free Composting

14 October 2014  |  HOTBIN Composting

All composting produces odorous chemicals. The ability for the human nose to detect them depends on the concentration in which they are released. This in turn depends on how much waste is decaying and at what rate (i.e. cold or hot composting) and other factors such as temperature, humidity and wind speed.

What should my compost smell like?

It all depends on the age of the compost (fresh or mature), and whether it is working aerobically (with oxygen) or anaerobically (without oxygen). The different types of compost odours, what causes them and how to limit or reduce them are explained below:

The 4 Main Composting Odours

1. Cabbage/Fruity Odour
2. Ammonia/Urine Odour
3. Putrid/Rotten Odour
4. Musky/Earthy Odour

Recognising each of them is a relatively simple matter of learning and experience.

1. Cabbage / Fruity Odour | Normal in all composting

Explained: When lifting the HOTBIN lid you may smell a fruity boiled cabbage type odour; this is a sign of a highly efficient HOTBIN. This odour is due to bacteria breaking down complex sugars and cellulose into smaller chemical compounds rather than carbon dioxide. These hundreds of chemical compounds vaporise (become odorous gases) and are given the collective name of volatile organic compounds (VOC’s). At hot composting temperatures of 60°C, VOC’s are produced faster and vaporise quicker leaving the HOTBIN via the valve. The HOTBINs filter pack in the lid help reduce these odours below nuisance levels. However there will always be a short whiff each time the lid is opened. In cold composting, these chemicals are produced slowly and do not vaporise to the same degree, hence why you rarely notice them.

2. Ammonia / Urine Odour | Excess nitrogen in mix

Explained: This smell is created when excess nitrogen leaves the heap as ammonia gas. You are most likely to notice this when composting large quantities of 1-2 day old grass mowing’s. When you turn a pile of grass you will often notice the odour when you get indoors as it has clung to your clothes. Read more on composting grass.

3. Putrid / Rotten Odour | Lack of oxygen

Explained: These odours are associated with anaerobic heaps. There are a wide range of chemicals including valeric acid (sick), hydrogen sulphide (bad eggs), dimethyl sulphide & acetic (vinegar) that are described as sour, acidic and very unpleasant smells, which create a natural "gut wrench" reflex in humans. It is believed this is a defence mechanism to prevent us eating rotting dangerous food. This is a group of odours you definitely do not want. Not only are they unpleasant but they are a robust indication that the compost bin has turned anaerobic and you will eventually end up with a wet slimy mush rather than brown crumbly compost if this is not rectified.

4. Musky / Earthy Odour | Normal – compost ready

Explained: This musky, soil-like smell is generated when the compost is mature and ready to use. This is what your final product should smell like.

Tips for preventing odours

Odour Type   Prevention Tips
Cabbage/Fruity   Always present. The HOTBIN has an inbuilt bio-filter in the lid that helps to minimise these odours below nuisance level.
Ammonia/Urine   Mix in plenty of dry, easy to digest shredded corrugated cardboard/office paper and bulking agent.
Putrid/Rotten   Take action as soon as the HOTBIN begins to have a sour smell. Shredded corrugated cardboard or office paper (to absorb excess moisture) and bulking agent (to aerate waste) need to be added. Read more.
 Musky/Earthy   This is what your final compost should smell like.

Why do de want to avoid unplesent composting odours?

Not only are putrid odours unpleasant to the human nose, they also run the risk of attracting unwanted flies and vermin. They are sensitive to the odours and use them as a means of detecting rotting food which they will try and get to using any means.

Will Turning the compost remove odours?

No. In fact, turning will increase the odours given off during the process.