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Is hot composting worth it? What are the benefits of hot composting?


What a brilliant turn out HOTBIN had in The Potting Shed at the Edible Garden show last weekend. There is definitely an appetite to learn more about the benefits of hot composting over cold composting and how it can contribute to reducing what each household sends to landfill.

If you didn’t get there you can read a PDF  transcript of Tony’s presentation by clicking on the graphic below.

It essence he challenged the audience to consider during his talk whether Hot Composting was worthwhile or or just a lot of hot air!

He wanted them to reflect on these 5  key things:

  • Time – is hot composting faster all year-round?
  • Quantity –  do you get any more compost if you hot compost?
  • Endeavor – does hot composting take less effort?
  • Quality – is the compost from hot composting any better?
  • Planet – is hot composting good for the environment?

I think over the 3 presentation sessions that the only question mark remained was on whether it was more or less effort to hot compost! Well only you can decide whether the effort is worth it to achieve all the other benefits.

Our view is this…yes hot composting in a HOTBIN requires some active participation. This includes making sure you keep the bacteria happy with smaller pieces with a larger surface area and making sure you feed it approximately 1 caddy 2 times a week. But really the benefits of composting all food waste, more quickly, all year round to get a good quality compost whilst cutting your contribution to landfill is surely worth it!

You can read the full transcript of  Tony’s talk below and  let us know what you think.

HOTBIN at the Edible Garden Show

Thank you to The Edible Garden Show for the opportunity to present and to Potty Innovations for the photography, we had a lovely time!


Remember you can also downlaod teh PDF form teh link above…

Hot Composting – Worth it or just a lot of hot air?

By Tony Callaghan, transcript of the Talk at the Edible Garden show 15-17th March 2013

Good morning, thank you for attending – when you volunteer for do talk, your worst nightmare is just 1 person in the audience, we have a full house and that is great!

My name is Tony Callaghan, I am the Inventor of a product called the HOTBIN.   Today’s talk is about Hot Composting – is it worth it or just a lot of hot air.

I’d like to start at the end by asking, how are we going to judge if it is worth while? On the handout sheets you will see I have suggested 5 things we should ask and answer during this session:

  • Time – is it faster, all year-round?
  • Do we get any more compost?
  • Does it take less effort?
  • Is the compost any better?
  • Is it good for the environment?

Keep these items in mind when we get to the summary!

I started my hot composting journey about 4 years ago.  We had been in our new house 2 years, the black dalex cone had been filled with garden clippings and vegetable peeling from the kitchen, I had nailed together pallets to create a second bin and that was also full – it had carrot peelings over flowing from the top. One day the inevitable happened – the old rat popped its head out. I was politely informed to sort out the compost bin or stop composting!

Now all I really wanted was to find a compost bin online that worked and move on.  I was looking for a 5-star review on Amazon etc that said this product works and customer really recommend it. I could not find it – I found instead was “it arrived on time / did not arrive on time, it was easy or hard to assemble – 1-5 stars, often with “I’ll let you know if it works in 12 months”. Of course no-one comes back to fill it the rating later. The more I looked the harder it became to find vendors that actually had case studies or evidence that the products worked. I got scared off and decided I would make my own. I had loads of attempts, kept reading website advice, adjusting, adding insulation, turning etc. They all work initially but then failed quite quickly. I soon released that something was amiss. Eventually out of frustration I bought a compost engineering book – 500 pages. Having read it cover to cover (I do have a science background!), I began to understand – some advice was out of context, other bits were just plain ‘myth’.

So let’s look at what hot composting is.


Could I ask for a show of hands – how many compost? (approx. 100/120 of  audience); how many think they hot compost? (3-6 from 120); how many ever use a thermometer to check? (zero from 120)

In the UK, winter is around zero to 5C, spring 5-15C, summer 15-25C. The average over the year is 10C. Most UK compost heaps run at 10C. If you shower in the morning, that is normally around 35-40C. If you put your hand on a very hot radiator – that is about 60C. So when we say 40-60C is the hot composting range, it is very hot.


We will be looking at a number of benefits, but I want to focus on heat and speed first. I do not want to get too scientific, but there is a rule of nature called the Q10 equation. Very simply for every increase in temperature, the speed of reaction doubles. So if we take out UK compost heap as 10C, and say that is speed 1, then 20C = twice as fast, 30C 4 x times, 40C 8 x times, 50C 16 x times and 60C 32 x times faster. So a rough rule of thumb – if a material takes 12 months in a cold (10C) heap, it will take 12 days at 60C. Please do not go away thinking everything will compost in 12 days – it won’t – some things are faster than others.


People sometimes ask do we add heating rods and electric etc. The answer is no. The heat comes from the bacteria. As they work and reproduce they create heat. Think of it like going for a run – the faster you go, the more heat you make. There are two groups of bacteria we focus on the mesophilic who work up to 40C, and the thermophilic who work from 40-70C. Now the thing about the thermophilic bacteria is that other things do not survive above 40C. So flies get fried, worms make an exodus and come back in later as things cool down.


We need to keep the bacteria happy. I’m going to look at 5 things. Think of them as linked circles – if they are all working, we get a virtuous circle and everything spirals up and we have success. If one of the links is broken, then we get a vicious circle that spirals down.

And we all know where it leads – the stinking pungent heap that upsets the neighbours, attracts more vermin and flies – and sorting it out is a very unpleasant task.

As I talk through the 5 items, I will try and compare it to other advice, and try to help you with the context.

Number 1 – Retain the heat

So we know heat increases the compost speed and we get compost faster. To hot compost, you also need to retain the heat.

We all know hot moves to cold. If we place a compost heap in the outdoors, the heat will rapidly move to the cooler air. It is a fact of life, a law of science. You cannot prevent heat moving out and the heap cooling – that is why most heaps in the UK run “cold”.

To retain heat we need to insulate and reduce the rate of heat loss.  There are two ways of doing this: use compost to create an insulation layer – a very large heap (at least 1XX1m, preferably 2m3) – the outer one metre of waste acts as insulation around the central hot core. This is why when you read books about hot composting they almost always say you need a big heap with lots of material. (They might not know why – but the experience shows it works!). Alternatively we can use a modern insulation material like EPP (expanded polypropylene. 50mm of EPP has the same insulation as 1 m of compost. Think of it like insulting your loft, you can leave it losing heat, add 200mm of fibre glass wool, or add a super 50mm insulation board.

Number 2 – small bits please!

Every time you chop things up you double the surface area. Think of it like this – you can fit one million bacteria on a pin head (apologies during the presentation I think I might have said 500 million!). Now add a whole potato into the heap. The bacteria have to eat through the peel (designed to protect it) and then all the potato. If you chop it they can get to more potato faster. Give the bacteria a break and chop things up.

In the kitchen this really is not that hard. We tend to chop vegetables and even the broccoli stalk does not take much effort to run the knife through. But if you are lucky enough to have a large garden, creating wheel barrows of waste, it takes a bit more effort with secateurs. If you have a hedge trimmer you can use it to chop waste, a rotary mower is good for leaves, and if you do not have a shredder you might be able to share one with a neighbour. Chopping and shredding makes a big difference to all composting.

Number 4 Oxygen / aeration

All the audience has heard about greens and browns and Carbon/Nitrogen ration of 30:1. I do not want to focus on these…they are important but are not high priority. After all we can compost grass (10:1) and compost wood (200:1).

I want to focus on how easy things are to digest. So I would like you to think about your diet! If you had consumed a bottle of coke (other fizzy drinks are available!), then you would be running around for an hour or so on a sugar high. You can digest glucose very easily. Now if you had porridge oats for breakfast, the theory is the more complex carbohydrates are harder to digest and energy is released more slowly – so you have less of a high, but keep going to lunch time. Bacteria are very similar – there are things they can digest easily and things that are hard. Sugars and carbohydrates are easy, cellulose (the main parts of plants) are in the middle – relatively easy and things like wood that contain lignin are very hard for bacteria to digest.

If you want to get your heap hot – you need to add some easy to digest materials! Grass and food waste are both very easy for bacteria to digest.

How many people add newspaper to the compost bin (a fair few) And how many add shredded office paper (less). Has anyone added newspaper (especially balled up) only to find in 6 months it is a small blob of non composted mush?  Newspaper is “bits of wood” (hard to digest), but office paper goes through another step and is de-lignified – it is cellulose and much easier for bacteria to digest – Try them both and see who much faster one is…

Number 4 Oxygen / aeration

You all know we need oxygen/air to have aerobic composting. If we have no air we end up with anaerobic conditions, and all the odour issues.

Where do we need the air/oxygen? Well remember our tiny little bacteria trying to eat the waste. They need air on a microscopic level – everywhere in the heap. Getting air to all the heap is a big task. Often you will read the books and they state drill holes in the side for aeration, or that turning is essential. Others will describe complex ducting and airflow tubes. We researched loads of different designs. It turns out the compost engineers really know their subject. Turning does not really help – unless you want to turn it every second of the day! Most turning takes the outer none composted waste in to create a new central hot core of waste. Aeration holes in side of bins do not work – air has a particular way of flowing – it follows the path of least resistance – so through the wall and up and out. Tubes and central aeration is even worse the hot air rises up the pipe and out. After much failed experimentation we started to do what the industrial composters do – we added bits of wood chip. Think of this like building blocks – the bits stack on top of each other, they do not collapse and the air flows up and over. So add cold air in at the base, draw it up, keep it flowing up to all parts using wood chip. Because the structure does not collapse into a slushy mess – you will find you no longer need to turn. We have found no aeration system that gets even close to the success of using “buoyant airflow’ with wood chip.

Number 5 – water – We know as composters that if the waste is dry nothing happen. The bacteria need water to develop. But excess water is also a problem. It blocks all the holes around the food and wood chip and no air can flow. Food waste has a lot of water. So if you see advice to wet your material until wringing like a sponge, that might be ok for a cold heap where the water can drain away, but it is a ‘no-no’ for hot composting. We normally have to add dry materials (like shredded paper) to balance to excess water in food waste.

So the 5 things to keep bacteria happy: 1) Retain heat (insulation); 2) Small bits (shred/chop); 3) Waste mix (some easy to digest); 4) Oxygen to all bits (wood chip – bulking agent); 5) Water (need, but not too much). Get all five right and we have the virtuous circle. If one is wrong then a vicious spiral down.


Manage residual odour – all composting produces odour! We want to avoid the putrid horrible anaerobic odours. But aerobic still produces odour – if it is released slowly you probably do not smell them. When you are hot composting and they are released quickly you do. We call it the boiled cabbage odour. It is preferable to filter hot compost odours before they leave the heap. We tend to call this a bio filter – and they are normally made form materials like charcoal and compost.

Protect from vermin and flies – you need to enclose the heap in a protective enclosure that that will prevent odour leaking out. If the odour is released then vermin and flies will be attracted. So you have to enclose the heap, and yet keep the aeration flowing through!

At the start we asked can we add more. Well if it is breaking down faster the bin will compost more. But there is another aspect – hot composting in a bin allows us to compost all food waste. Let me describe what we mean by “all” A couple of years ago, WRAP under the Love food, hate waste’ banner, did a huge study, It was something like 10,000 homes; they collected the black bag (general waste with food in) and took it away. A brave group of workers unloaded, took out, weighed and measured every piece of food waste to create a big long list. They found a huge 7.2 million tonnes of food waste was sent to landfill – about 250 Kgs for every household in the UK. Most of this was unnecessary waste (poor planning on sell by dates etc). Within the list of items we then went through and identified if it was recommend to add to compost heap (cold). About 65% of the items are on the “do not add” list – think about these items –  meat, fish, bones, rice, pasta, bread, cakes – a vast amount of the food we waste is on the out list. With a correct hot composting bin, you can compost a whole lot more.

Now let us think about the garden waste. Has anyone dug up all the weeds, composted them used the compost only to find a lovely patch of weeds where the compost has been used? A few of you. Weeds seeds (and a few others seeds) tend not to be broken down in cold composting – so you replant them in natures best growing medium. In hot composting seeds breakdown – se we can add more weeds even difficult stuff like couch grass – as long as we get the high temperatures.

What about grass treated with weed killer herbicides? OK good to see not many using weed killer. If you read a lot of advice it says do not compost grass. People take it down the recycling centre – guess what they compost it! Herbicides break down in composting – faster in hot composting. So if the instructions say 12 months before using compost, you are on the safe side if grass composted at 60C and it has been in for 90 days.

The last item on our list was – is it better for the environment? Well if we have anaerobic digestion we get methane and that is 24 times worse than CO2 for the environment. It is great if we have large scale AD where the plant collects all the methane and uses it for energy, but not if we have anaerobic piles or landfill piles.

If we can compost all food waste we no longer have to collect – so we save on fuel and transport and landfill.


It depends…

It is not easy to create via DIY techniques a domestic hot composting system that controls all 5 virtuous items and stays hot with relatively small amount of food waste through winter. If you have lots of garden waste (not food waste) you can create big hot heaps, but it takes a lot of effort to turn it.

Most food waste comes in small amounts each week all through the year. We think you need a specialist hot composting bin to handle this.  The HOTBIN was specifically designed to achieve HOT composting to allow ALL Food Waste to be composted.

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