Hot Composting vs Vermicomposting
What’s the difference and which one is better?
As I mentioned in a recent article, I tend to lump hot (thermophilic) composting, vermicomposting, and (small-scale) ‘backyard’ composting together under the general term ‘composting’ (even though it is a term often referring to the thermophilic process). This of course is not to imply that they are all the same thing! They are in fact quite different.
In this article I will focus specifically on the difference between thermophilic composting and vermicomposting (composting with worms).
Let’s start by looking at the similarities.
Both of these processes involves the bio-oxidation of organic wastes – typically with human assistance – resulting in a rich, humus-like material. As ‘bio-oxidation’ implies, they both require oxygen for optimal performance.
The ‘carbon to nitrogen ratio’ (C:N ratio) is important for both, and plays an important role in determining the rate at which decomposition occurs – although it is definitely more important for hot composting.
That is essentially where the similarities end. Let’s now look at each process in more detail.
Hot Composting
As the name implies, thermophilic composting involves an important heating stage. This heat is caused by microbial metabolism and is dependent on the size of the heap, C:N ratio of the materials, moisture content and aeration. During this heating stage, temperatures will ideally be in the 140 F (60 C) range, but will often be higher or lower.
This type of composting typically follows the ‘batch’ model – that is to say all the materials for the heap are piled up at one time and no more is added. In order to establish a sustained heating phase a ‘critical mass’ of materials is required. Assuming adequate C:N ratio, a pile needs to be at least 1 cubic meter in size (somewhat larger is probably better though).
Hot composting can be achieved when materials in the pile have a C:N of between 20:1 and 40:1, but ideally it should be between 25:1 and 30:1.
Pros
1) Enables processing of larger quantities of materials in a smaller area
2) Can proceed relatively quickly under ideal conditions
3) Can kill weed seeds and pathogens
4) On a large scale can proceed easily in cold weather
Cons
1) Can be labor-intensive (piling wastes, turning pile etc) and require more attention
2) Heat can kill off many beneficial microbes
3) May require some stock-piling until sufficient materials available for ‘batch’
4) Heating can lead to considerable nitrogen loss
Vermicomposting
As mentioned, vermicomposting is somewhat similar to hot composting in that it involves the breakdown of organic wastes, but one of the major differences of course is that it involves the joint action of earthworms and microorganisms (whereas the other process relies solely on microbes). I should mention that the term ‘earthworm’ is actually somewhat misleading, since the worms needed for worm composting are specialized for life in rich organic materials (not soil).
Worm composting is also a much cooler process – working best at 59-86 F (15-30 C). It typically follows the ‘continuous’ composting model – that is to say materials are added continuously (usually in smaller amounts).
Again, C:N ratio while fairly important, is not quite so critical for optimal performance. Materials with high C:N ratios (such as paper sludge) have been processed quite readily via vermicomposting.
Pros
1) Tends to be somewhat less labor-intensive – no turning/aerating necessary (worm activity helps to mix, fragment and aerate materials)
2) Cooler temperatures help to conserve nitrogen
3) Higher moisture contents not an issue (and actually preferred)
4) Materials can be constantly added (no need to stock pile in preparation for next ‘batch’)
5) Size of system unimportant – ideally suited for both indoors and outdoors
6) Considerable academic evidence to indicate that vermicomposts have beneficial properties not found in hot composts
7) Under ideal conditions, wastes can be processed very quickly
Cons
1) Won’t kill seeds (and although there is a fair amount of evidence to indicate pathogen destruction, more research is required)
2) More space required to process similar amounts as hot composting – need to be careful with amount added (since excess heat will kill worms)
3) Outdoor systems much more limited by cold weather
4) Worms need to be separated from compost
5) Worms (although quite resilient) do require some attention and proper care.
As far as which of these two processes is “better”, it totally depends on the situation. I personally think an optimal approach (when possible) is to combine BOTH of these two methods. By ‘pre-composting’ then vermicomposting waste materials you can get the best of both worlds. Pathogen and weed seed destruction of hot composting (without too much nitrogen loss), followed by fast processing by worms and production of high quality vermicompost.
Needless to say, I’ll be talking about both these processes (along with other related composting methods) in much greater detail in coming weeks and months, so do stay tuned!
[tags]compost, composting, thermophilic, compost bin, worm composting, vermicomposting, red worms, red wigglers, organic wastes[/tags]
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Written by Compost Guy on September 28th, 2007 with
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#1. November 16th, 2009, at 6:42 PM.
You state that:
Hot composting can be achieved when materials in the pile have a C:N of between 20:1 and 40:1, but ideally it should be between 25:1 and 30:1.
Okay, so how does one acheive this ratio?