The activity of the Mespohiles results in the production of heat and a significant increase in the temperature of the heap to that which favours thermophilic organisms (40°C to 70°C). These are the most efficient
decomposing bacteria in a hot composting pile and continue the Degradation Phase under the DGM classification. These organisms start to dominate the heap at temperatures of about 40ºC in batch composting. Composting proceeds at a much faster rate
under thermophilic conditions. It is during this stage that most of the organic matter is converted into carbon dioxide and humus. There is also a growth in the number of microorganisms present.
These high temperatures
accelerate the breakdown of cellulose, hemicellulose and other complex carbohydrates, proteins and fats. The thermophilic population continues generating more heat by decomposing the remaining organic matter. As this food source is used, the activity
of the thermophilic microbes declines, unless the heap is regularly fed new material and turned to aerate it and move materials from the cool sides to the centre, and the compost temperature falls after three to twelve days (depending on the compost technique
used). This allows the return of Mesophilic organisms once again become the dominate group of bacteria in the compost heap. During this period, the Conversion phase beings. In a compost bin or heap this conversion will occur at the bottom
as the organic material starts to take on the appearance of compost.
As mentioned above the period of thermophilic activity can be extended turning/mixing the heap and aeration.
stages are easily monitored in batch composting as the temperature changes can be made to apply to the whole heap provided it is turned regularly. If the material is not turned the area at which the high temperature occurs may only only to within
4 to 8 inches of the surface.
In hot composting using a heap, the temperature can be monitored and the heap turned, outside to the centre, as it starts to cool so as to hold the material at a sufficiently high temperature
(see the page on Compost Temperatures). Where a tumbler bin is being used this mixing and aeration occurs daily when the tumbler is turned. Hot composting at temperatures 55°-60° C and over will destroy the
majority of plant and human pathogens and weed seeds present.
There are mixed veiws on the number of times the heap should be turned. A single through turning may be sufficient eliminate the pathogens
and parasites but for this to be effective all the outside material most be completely turned to the inside, most hot composters will turn material at least twice with many opting for three or five turnsto be on the" safe side". Alternatively
the turning can be undertaken in conjunction with measure the temperature of the heap and continuing turning the material until there is no marked change in temperature following the aeration.
The killing of pathogens is
an advantage offered by the hot composting techniques but it is not necessary unless there is a specific concern about killing disease organisms and seeds.
An alternative to killing pathogens is not to add diseased
materials that will survive cold composting to the bin in the first place.
Many “good bacteria” i.e. decomposers are inactivated or killed when temperatures of the heap rises above 60ºC (140ºF).
If the heap temperature exceeds 72ºC (160ºF), it can be cooled turning/aerating. If a pile does overheat, exceeding 72-76ºC most of the bacteria will be destroyed and the composting process will almost come to a stop. It has been found
that during the thermophillic stage in some bins 87% of the organism present were Bacillus species, which are spore forming, with one species Bacillus stearothermophilus predominant at temperatures of over 65°C. Bacterial
spores are formed by bacteria of the species Bacillus and Clostridium as a survival mechanism when the organism is under stress such as heat. Bacterial spores will survive to germinate when the heap cools to a suitable temperature although most Bacillus species
are unable to grow unable to grow above 70oC. Thermus species, originally found in geothermal sites, which while growing at 40-80oC
have an optimum temperature of between 65oC and 75oC. are active in compost at these higher temperatures
of thermpphiles that play a role in composting. Fungi such as Rhizomucor pusillus, are active as the temperature rises but is inactivated at peak heating while a second group of thermophilic fungi now play a role in the decomposition
of cellulose and hemicelluloses, including:
Chaetomium thermophile, Humicola insolens, Humicola lanuginosus, Thermoascus aurantiacus and Aspergillus
Eventually insufficient nutrients will be available to maintain the higher temperatures and the heap will return to the cooler levels of the mesophilic stage, even when turned i.e mixing by turning
and turning the compost fails to lead to any increase in the temperature of the compost.
In cold composting, while temperatures that favour thermophiles may be reached in particular parts of the heap after
new material is added most of the heap will not exceed 40ºC and it is it is mesophiles, not thermophiles, that will do most of the work.