How can I prevent my HOTBIN from becoming a smelly/soggy mess?

The technical term used to describe the smelly, soggy contents of your HOTBIN is anaerobic.

So what does anaerobic mean?

All composting is either aerobic (with oxygen) or anaerobic (without oxygen).

The HOTBIN is designed to work using aerobic bacteria i.e. bacteria that use oxygen to decompose the waste to carbon dioxide and water.  When your bin goes anaerobic, all the oxygen has been used up and is not being replaced so the anaerobic bacteria (those that do not need oxygen) take over.

Why would my HOTBIN turn anaerobic?

Simply - The issue will arise through an incorrect mix of waste being added to the bin. Generally the main cause is not enough moisture and aeration balancing materials (shredded paper and bulking agent) being added with wet waste. This prevents oxygen flowing through the waste and the aerobic bacteria decomposing it.

The Science - Air contains 22% oxygen. Aerobic bacteria use the oxygen surrounding the waste in the HOTBIN and the percentage concentration falls. When it reaches around 6% the bin is anaerobic. The air (oxygen) is replaced by pulling fresh air in from the outside using ‘buoyant airflow’ (i.e. hot air rises creating a ‘pull’ force to drag new air in) [Read more].

Airflow is restricted and/or completely stops when:

• Excess water blocks the air gaps around the waste.
• No bulking agent is added which means the waste collapses into a thick dense layer with no air gaps [Read more].
• Accidental blockage – e.g. when a lot of paper is added but not mixed in, it forms a thick wet mushy layer that restricts airflow.
• There is no temperature difference between waste inside and the air outside. This is usually only a consideration in winter – see using the kick start bottle.
• Internal base plate clogged with humus – rare, normally only happens if not emptied out for 9 months [Read more].
• Mesh plate blocked – this is very rare, but can be solved [Read more].

Is an anaerobic HOTBIN bad?

Yes. Avoid at all costs and fix sooner rather than later. Anaerobic bins smell putrid, leak lots of water and will not get into the hot zone (40-60°C).

Not only will the bin smell awful, if you do not sort it out early eventually the whole bin will become anaerobic. If this happens, you will be unable to ‘restart’ and get back to aerobic hot composting without a complete empty out and remix of all the waste. This is a very unpleasant job.

NOTE: This issue can arise with any composting activity, and is a waste mix issue. It can take time to adjust and get used to the HOTBIN recipe, but once you have got it right, it takes very little time or effort to ensure the waste added each week is the ‘right mix’ to avoid it going anaerobic.

How can I tell if my HOTBIN is turning anaerobic (early signs)?

There is a distinct change in odour. Aerobic bacteria working at 40-60°C produce a fruity boiled cabbage type odour. Anaerobic bacteria produce pungent odours that smell of rotten eggs, sick, putrid drains or rancid fat – they are stomach wrenching odours.

What preventative measures can I take?

It is better to have too much bulking agent and paper than not enough – so always add plenty when you are starting out.

• Keep your nose alert to a change in odour.
• Keep an eye out for lots of leachate around base of the bin – an espresso cup full a week is normal [Read more].
• Monitor temperature – has the temperature stopped rising above 40°C when new waste is added?

Do’s and Don'ts

• DO add shredded paper or corrugated cardboard when adding kitchen or cooked food waste [Read more].
• DO add bulking agent (dry wood chip) to ensure air can move upwards [Read more].
• DO mix in paper to avoid it getting soaked and forming a thick dense layer that will block airflow.
• DO overcompensate with dry materials until you have got into the routine of the right HOTBIN mix. It is much
  easier to solve a dry bin that a wet one.
• DON’T add any liquids into the HOTBIN.