Hot and Cold Composting

 Find out about how and where home composting decays in hot, warm and other temperature ranges

Background

The temperature of a compost pile is influenced by several factors, including the size of the pile (and container/bin) , how moist it is, how well it's aerated,  the C:N ratio (carbon to nitrogen) and naturally  the outdoor temperature.

A higher temperature speeds up the overall microbial process and creates better compost (as well as killing undesirable elements such as many weeds or seeds). The maximum temperature  in the optimum range is around 65 ℃. 

Highlights

• The largest proportion (48%) of our compost decays in the active range range [40-55℃], with 19% in hot range [55-70℃] and 5% in cold range [0-25℃].

• The highest temperature attained is  66℃, typically in larger bins [A,B,E] with sizeable top-ups and a bin already over 30% full

• Examples and illustrations of compost thermometer in hot regimes and time-series chart on top-ups and temperatures

Home composting

One of the main differences between home and industrial (large scale composting) is hot vs cold composting. Because of smaller bin size and small quantities of waste available at time, home composting tends to be on the low temp side (cold composting). Below are images of temperature regions in composting and a typical (home) compost thermometer. In home composting, a proportion of the waste may decompose in different temperature ranges whereas in large scale composting, a significant proportion of the waste will decompose in hot conditions.

At MCL, the larger bins have attained the optimum temperature when relatively large loads (>60L) have been added.  We measure the temperature for all bins during the various (cycle) phases, in particular during the Active phase, a few days after waste top-ups. The pie chart below shows the distribution of the Input Waste volumes that were decomposed under different heating regimes -  during their first two weeks of the Active phase (meso & thermophilic stages).  On a long term basis, we can see that 19% of this waste  was processed in hot conditions ]55-70℃] and the largest portion  (48%) in the active temperature region ]40-55℃]. Nevertheless 5% of the waste volume is in  cold composting territory. 

Hot bin examples

The multitude of microorganisms in the bin  feed on organic matter  and they  create heat as part of the decomposition process. It is part of their growth and reproduction.

The photo below shows an example of a bin in hot conditions. A compost thermometer inserted  in  Bin E (cycle 41), temperature reads 66℃ on 04/05/20. 117L of mixed waste (brown & green) were added on 02/05/20  (on top of an existing 318L volume), and the temperature reached  66℃  2 days later.

 Another instance below of when a bin heats up (photo below). We have what I call the doughnut effect. You can see the ring around the inside pile as the waste has shrunk. The centre of the pile heats up and decomposes more quickly than the outer edges. The increased microbial activity means that oxygen at the centre of the pile depletes quickly.  That is why mixing and stirring a bin is important.

Active phase illustration

By way of illustration, the chart below depicts the Active phase of cycle 38 (bin A, 300L Dalek). The chart shows the top-ups and the resulting temperature  a few days later (when the thermophilic stage triggers). The temperatures spike each time there is a large drop (top-up). Temperatures tend to get higher and keep higher if there is an existing volume already in the Active phase. The highest temperature in this cycle is 59 ℃ after a sequence of high top-ups.  Note that here a significant portion of the 300L top-up is from external sources. The 30L drop at day 65 does not register a significant temperature spike given it is a low volume. To contrast this, The average (highs/lows) temperature in Leeds in August and September is  21/13℃ and 18/10℃ respectively. After the last drop (30L), the cycle#38 enters the Cooling phase, where it starts to follow more or less the ambient outdoor temperature. Roughly day 80 in the chart (first week of November), the high/low outdoor temperatures in Leeds drop 10/4℃

I normally do not disturb a bin in hot conditions because microbial activities are still in operation. As soon as the temperature drops to a warm level; then stirring/mixing and perhaps adjustments are made (e.g. adding water if too dry).  

We often see some persistent seeds in cold composting; tomato seeds are a typical example. In our experience even weeds don’t make it through the final compost but tomato seeds often do. So much so that when we compost kitchen waste, one of the elements we remove are tomato seeds before putting the waste in the kitchen bin.