Below we explain how to Compost ALL domestic food waste
(i.e. at home, in a garden, or via backyard composting).
Why do we need this blog?
Surely food waste is just like other waste for composting – we just add it to the compost heap and it breaks down?
What’s the big problem? Well to a degree this is true, food waste is carbon/organic and will compost. The problem is NOT that food waste does not compost, it’s just that more often than not, it creates a putrid stinky mush that attracts rats and flies.
What are the specific issues with food waste composting
Here at HOTBIN Composting we talk to a lot of people who compost – from Master Composters, Council Recycling Officers, expert gardeners to complete novices and we have also read hundreds of composting forums and blogs: the advice is near universal: ‘do not add meat, fish, cooked food waste, mouldy bread, left over bones, cakes, bits pizza, chip boxes, dairy products, gone off fruit, out of date fridge contents to your compost heap. If you do, they will rot, produce putrid odour which in turn will attract vermin and flies. Only add kitchen peelings and tea bags,coffee grinds’.
Kitchen peelings,tea bags, coffee grinds only account for 40% of domestic food waste.
The other 60% falls into the “do not” add to a compost heap.
[We know this because very detailed waste analysis of what goes in our disposal bins and onto municipal refuse collection and landfill was undertaken by the Love Food Hate Waste campaign in 2011 (recycle Now, WRAP) . Teams of people sifted and weighed the contents of 1000’s of household waste bins over many months (nice job!). So we know on average, a UK household produces 250Kg/year of food waste. Every item wasted was listed and weighed (what they call a composition analysis), so it is easy to look at the list and reclassify per the compost “in/out” list.]
So even those with a compost bin, will most likely still be sending food waste to landfill.
They might still have stinky food waste sitting in a kitchen or wheelie bin for two weeks. (We accept there is a growing number of councils rolling out kerbside collection to be used in AD/EfW recovery plants – but this is a small part of the total).
Millions of home composters want to add ALL food waste to their compost bins.
This is what why the HOTBIN was invented: ‘to create a compost bin and method that enables millions of existing home composters to compost ALL food waste without the inherent problems of odour, vermin and flies’
Why does food waste rot and go putrid
As food waste starts to breakdown it forms a thick slimy mush. This soft mush prevents airflow and so the waste quickly turns anaerobic. As soon as anaerobic bacteria take over, the waste releases putrid gut wrenching odours. From here it is all downhill – the smell attracts vermin and flies and everything becomes unpleasant. As food is composted, the structure/fibre of the food weakens and water is released. There are three places the water can go – it can drain to the ground, it can be driven off as steam (water vapour) or it can remain in the waste. Under most composting conditions the water stays in the waste until it becomes saturated and then it will start to drain into the ground. Composting soft food waste naturally produces a mushy slime.
The alternative is to use heat produced by the bacteria to drive of water as steam vapour. This happens when we hot compost.
The golden rules for successfully composting ALL food waste are:
- Remove excess water
- Keep the waste aerated
What is composting?
The biggest part of the composting process relies on bacteria and if you refer to our post So What is Composting? you will note their requirements are not dissimilar to what humans need to survive and grow!
So how do we get rid of excess water and keep the waste aerated?
We take a lead from industrial composting and apply the science and engineering they use to a domestic compost bin.
(If you want to look up the science and engineering, have a read through Haug – Practical Handbook of Compost Engineering) on Amazon book review.
Let’s cut to the chase
- To remove excess water
You need lots of heat i.e. you need to be ‘HOT composting’
- To aerate you need to keep adding lots of oxygen/air
Unless you have the means to force airflow (e.g. a pump / blower), or you can constantly turn/tumble (yes we mean constantly), then you are reliant on “buoyant airflow’ or the chimney principle of hot air rising creating a pressure drop that pulls cold air through from below. You only get buoyant airflow if there is a temperature gradient – i.e. you need heat.
- To maintain buoyant airflow (even with heat)
You need to a heap structure that maintains buoyant airflow. To get a structure that stops food waste collapsing into a mush you need to add what we refer to as a bulking agent (typically this is wood chip).
In summary to compost food waste, you need to get the waste hot, aerate it (via buoyancy airflow) and ensure it stays aerated by adding a bulking agent. It sounds technical and it would be easy to achieve where it not for nature’s laws on heat production and heat loss!
Nature has a law on how much heat is produced
Bacteria release heat as a by-product when they ‘eat’ the waste– just like humans release heat when we eat and exercise. The amount of heat is capped by the calorific value – the more calories the more energy and potential heat. The rate of heat released varies by food type.
Think of it like this human analogy – eat coke and sweets (sugar!) for breakfast and you’ll be on a sugar high for a few hours, then hungry again. East oats/muesli and it will be digested more slowly but over a longer period – you’ll make it to lunchtime.
Bacteria are the same – they digest and release heat from sugars and carbohydrate food very fast, from cellulose (plant material) slower, and from lignin (wood) even slower. The amount and rate of heat generated is determined by what goes in and how much goes in. For most households – it is a challenge to create a hot compost heap, you need about X10 more food waste than most households create.
Nature has a law on how much heat is lost
Heat transfers from a hot place to a cold place until they both reach equilibrium, i.e. the same temperature. This law is scientifically defined by Newton’s law of cooling. Let’s simplify it for a compost heap – even in summer (25C), a compost heap will not stay hot (40-60C) for long as the heat rapidly moves to the cool air. If you want to keep your waste hot, you need to reduce the rate of heat loss ie you need to insulate it. There are two ways of doing this – by having a large heap so the outer metre of waste acts as insulation or use a specialist insulation material.
Oh, one last thing – odour, vermin and flies
Lets assume we are now hot composting away. All composting creates odour and food odour attracts rats and flies. Unless you want a heap infested with rats and flies then you are going to need to control odour so it does not attract vermin and control access the compost heap just in case. In other words, compost in a container that won’t attract rats or let flies in.
Successful food waste composting needs heat (for water removal), oxygen (to ensure aerobic bacteria active), which in turn needs a heat gradient, which needs bulking agent to allow airflow. We need to balance the heat produced with the rate of heat loss (insulation, protection from wind) to keep the bacteria warm and composting fast and then protect the whole operation from infestation with rats and flies.
Achieving the above without a specialist bin is very hard.