Hot Composting - The Importance of Aeration

While tending my HotBin compos bin over the Christmas holidays, I was reminded of the importance of compost aeration. I'll come to that in a moment but, first, I thought I should give a quick recap of the 'ingredients' needed for the successful hot composting of garden waste:

  1. A population of micro-organisms (fungal and bacterial) to breakdown the plant waste into a stable organic soil conditioner (finished compost) - garden waste will contain most, if not all, the necessary micro-organisms albeit at levels too low for rapid hot composting; hence the need for ingredients 2, 3, 4 and 5! Inoculating a new compost heap/pile/bin by recycling a sample from the previous compost heap/pile/bin will get you off to a fast start. I inoculate new and existing HotBin compost bins with course material sieved from my finished compost.
  2. A carbon-based energy source - the bulk of the plant waste provides this component, for example, in the form of polymeric (e.g. lignin) and complex (cellulose, hemicellulose) carbohydrates. Microorganisms oxidise carbohydrates to carbon dioxide (CO2), water (H2O) and energy for cell growth. Some of the energy is also released as heat and raises the temperature of the compost pile. Both brown and green garden waste are a 'food' source for the essential micro-organisms in your compost heap.
  3. A source of nutrients - predominantly nitrogen but also phosphorus, potassium and trace elements (sulfur, magnesium, manganese, calcium, cobalt, zinc, copper, molybdenum, etc). Nitrogen is the most significant nutrient being a major element in the biosynthesis of the amino acids and proteins needed to increase the activity and population of micro-organisms (see 1 above). Green waste is the main source of nitrogen. The general rule is to have a carbon/nitrogen ratio (C:N) somewhere between 20:1 and 40:1. Above 40:1, the lack of nitrogen limits bacterial and fungal growth. Below 20:1, too much nitrogen can lead to anaerobic conditions producing ammonia and putrid odours. The home composter learns by experience what a good green/brown balance is for their compost bin. You should err towards 20:1 when starting a new compost pile. In my experience, a well-functioning HotBin (say running at 60 ℃) can cope with a greater variety of C:N ratios.
  4. A regular supply of oxygen (aeration) - this is used by bacteria and fungi to oxidise the carbon-based energy source (see 2 above) and provide energy for bacterial and fungal growth; a byproduct of this oxidation is heat that raises the temperature of the compost pile and speeds up the composting process (positive feedback). Traditionally, the supply of oxygen is controlled by regular turning of the compost heap. The HotBin composter is engineered to enable this aeration process to occur automatically without any physical intervention.
  5. Moisture -  water is added to the compost pile as a constituent of the garden waste and is also produced as a part of the composting process (see 2 above). A moisture content of 40-65 per cent is considered ideal for composting. Too little water slows the composting process and too much can lead to anaerobic conditions (slow and smelly compost heaps). A simple 'squeeze test' such as this or this will make sure you are in the right ball park. In my experience, if the top active layer (10 cm) of your HotBin is running at 60+ ℃, then you don't need to worry about moisture content. Free water will naturally gravitate to the bottom of your HotBin. Generally, this is fine because composting worms prefer a higher moisture content (80-90%); however, it can be a problem if the compost becomes too compact and reduces the oxygen supply (see 4 above) to the upper active (i.e. hotter) layers.

OK! Back to the two incidents that reminded me of the importance of aeration.

Thanks to my neighbour's hedge cutting, there has been plenty of green and brown garden waste for hot composting this winter. December has been very mild after the cold snap at the beginning of the month. Approximately every 10 days, I will remove around 80 litres of partially-composted waste from the hatch of my 200 litre HotBin (Photo 1) and transfer it to one of my maturation bins where it will spend at least another 6 months before being used in the garden either before or after sieving.

Photo 1: Removing Part-Composted Waste from the HotBin Hatch

A few days ago, I noticed the top vent on the HotBin was fully closed (Photo 2) and the lid thermometer temperature had dropped to 37 ℃ from the 54 ℃ it had been on the previous day. The temperature of the top 10 cm of compost had also dropped from 61 ℃ to 50 ℃.

Photo 2: Closed Air Vent on HotBin

Whether the vent had closed by accident or, as in the past, because one of the local cats had chosen the top of the HotBin as a pleasantly warm place to rest, we will never know for sure. In any case, I needed to reinvigorate the bin and get it back up to hot composting temperatures.

First, I transferred 80 litres of partially composted material from the hatch of the HotBin (Photo 1) to one of my maturation bins where the composting process would continue under the supervision of local worm population. After refitting the hatch and resettling the contents, I added two hot water bottles and 24 litres of fresh garden waste before covering with a reflective mat and resetting the vent valve to its correct position (about a 2 - 3 mm gap). The lid thermometer now read 32 ℃ (Photo 2) but within a couple of hours it was reading 35 ℃ (Photo 3); a good indication the bin was back on the way to full health.

Photo 3: Correctly Positioned Lid Vent 

Unfortunately, the next day, while checking the HotBin during Storm Pia, I found the vent fully open (Photo 4). I'm assuming blown open by a gust of wind catching the edge of the adjustable valve.

Photo 4: Fully open HotBin vent valve

The lid thermometer read 37 ℃ and the upper compost layer was 53 ℃. It needed another two days of hot water bottles and fresh waste before the upper compost layer reached 60 ℃ and the lid thermometer read 50 ℃.

Figure 1 shows the HotBin compost temperatures at 30 cm and 10 cm before, during and after the problems with the aeration valve. The period when the valve was shut is shown in green and when it was fully open in yellow.

Figure 1: Compost Temperatures Before and After Aeration Issues

Shutting the valve reduced the natural aeration of the compost bin and had the largest effect with a > 10 ℃ drop in compost temperature over a 24 hour period. The fully open valve resulted in a slower heat-up rate due to the greater heat loss through the valve. Once the valve returned to its normal position (Day 6), the top 30 cm of compost quickly heated up to 55+ ℃ and, within a few days, was running at over 60 ℃. Combined with the use of a heat reflecting blanket, the compost was maintaining a temperature of 50 ℃ even at a depth of 50 cm.

To prevent possible accidental closure by neighbourhood cats or unexpected openings by wayward winds, some additional protection was offered for the aeration valve ...

Photo 5: Cat Deterrent and Wind Protector


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