So, you’re wondering how compost bins work?
The first thing to realize is that there are many different types of compost bin, and they can work in very different ways from each other.
We’ll see if we can define a bin, and then we’ll take a look at some of the different types of bins and how different bins work.
What is a compost bin anyway?
A compost bin is essentially a container designed for composting, turning organic materials into rich soil.
Kitchen compost bins are usually designed to hold compost until it goes to the main compost heap or bin, while outdoor compost bins are designed to turn organic material into a brown, crumbly material you can add to your soil or pots.
You might think you can take any old bin and put waste in it. However, that’s not always a good idea for actual composting.
The most common form of composting is aerobic composting. Aerobic literally means “with air”. That’s because the microorganisms that turn organic material into compost quickly need air to be effective.
A good (aerobic) compost bin is designed to promote airflow, and it may also provide insulation. That’s why a trash bin, unless modified, is not good for aerobic composting.
However, as we’ll see not every form of compost bin needs air.
Why use a compost bin?
Compost bins have several advantages such as maintaining structure, insulation for faster decomposition, pest control, aesthetic appeal, convenience, and the ability to capture valuable nutrients. These unique features make compost bins highly attractive compared to other composting methods like piles or trenches. Let’s explore each point one by one.
- Maintain structure: Compost bins keep the organic material in one place, preventing it from spreading out and allowing the bacteria and fungus to work more effectively.
- Insulation: Some compost bins come with an insulating layer that keeps the heat in, speeding up the decomposition process. This is particularly useful for getting hot compost with smaller amounts of material.
- Pest control: Compost bins can deter pests, such as rodents and flies, from getting to the organic material.
- Aesthetics: Compost bins can be more visually appealing and neater than piles of decomposing organic material.
- Convenience: Compost bins can be easier to manage and maintain compared to other composting methods, such as composting in a trench.
- Leachate: Some compost bins help you to capture the nutrients that leach from the compost. This can then be used as a fertilizer.
While compost bins have their advantages, you don’t necessarily need one to make compost. Other methods, such as building a pile, composting in a trench, or using bags, can also be effective.
However, closed systems produce large amounts of methane, which can be bad for the environment. Meanwhile, compost piles are messy and tend to spread out (in my case, they spread out all over the place).
Still, if you have huge amounts of material, a compost heap might be the best alternative. Even the largest bins place limits on the amount you can compost.
I do mean huge, though. Because compost material sinks down, you can handle a lot of material with the larger compost bins – especially if you use a multi-bin system.
How different types of compost bins work
So let’s take a look at how several bin systems work. If you’re not familiar with different bin systems, I recommend you check out our guide to the different types of bins first.
Kitchen compost bins
Kitchen compost bins are usually designed to collect kitchen waste, and not to make compost.
They come with filters that contain activated charcoal. This traps odors, preventing any smells from getting out.
These filters are very effective, but they don’t last forever. (I find they usually come loose and get dropped somewhere before they stop working!) You can usually buy replacement filters.
However, if your climate is not hot, and you empty your bin regularly, you don’t usually need to worry about the smell.
If you do worry about odors, check out our guide to stopping kitchen bins from smelling.
You can also compost in the kitchen with bokashi bins, worm bins and electric composters, all of which we will cover later on in this post.
Closed composting bins (outdoors)
Closed composting bins work by allowing you to add material from the top and retrieve compost from the bottom. They promote airflow for effective aerobic composting, and a good bin design facilitates easier retrieval. Typically these bins have a lid at the top and an opening at the bottom.
To add material, you simply take the lid off the top and put your material in. As with all aerobic composting, it’s important to include a mixture of brown (high carbon) and green (high nitrogen) material.
You should also include some bulking agents, as these help promote airflow in the compost.
Some bins come with a stirrer which allows you to mix the top layer of materials. Beyond that, it’s usually simply a matter of allowing the compost to do its work.
When the compost is ready, you usually retrieve it from an opening in the bottom.
(When selecting a compost bin, it’s important to look at the size of the opening. Some openings are too small, which makes it difficult to retrieve the compost. A good bin will have a large opening, making it easier to get a spade or fork in.)
These bins are usually used as a continuous system – so you can add material at the top while retrieving compost from the bottom.
A good bin is also designed to promote airflow. This may be through vents, or the bin may have a slatted design that allows air to flow through the sides. There is some trade-off here, as more airflow means less insulation.
MicroorganismsThe key to decomposition in most bins is microorganisms.
When you first add organic material, mesophilic bacteria start to break down the material. They generate heat, and if the compost gets hot enough thermophilic bacteria take over.
As the compost cools down, fungus plays a larger role.
They are joined by physical decomposers like woodlice, springtails, ants and many more, which also help to break down organic material into compost
Learn more about the science of composting.
Insulated compost bins (aka Hot Bins)
Insulated bins are closed bins that use an insulating layer to take enhance composting speed. These specially designed bins allow bacteria to work more efficiently, resulting in faster, more even decomposition.
The addition of this insulating layer makes them astonishingly effective at speeding up the composting process.
To understand how this works, it’s worth revisiting the role of bacteria.
The bacteria that break down compost fastest require heat to be effective.
Bacteria that kick off the process generate heat, but in a small, open pile this heat is quickly lost.
Even in a large compost heap, heat is lost from the outer layers, which causes uneven decomposition.
By retaining heat, insulated bins allow you to generate heat – and therefore a faster composting process.
What’s more, the heat is spread throughout the compost, allowing you to achieve even decomposition throughout the composting process.
The impact can be quite remarkable.
I’ve achieved hot composting conditions in the middle of winter with my HotBin, while Mark over at Great Green Systems has done the same with the Green Johanna.
HotBin: I’ve been using the HotBin (see full review) for years now. A combination of excellent airflow and an insulating layer make it highly effective for hot composting, although it can get a bit battered after several years.
Green Johanna: This bin also has a very good reputation, but you do need to use it with a compost jacket to get the insulating effect.
Options seem to be more limited in the US, but the SqueezeMaster appears to have a similar design to the HotBin.
Open bins/Three bin system
Open bins have a larger capacity and can process a larger amount of organic waste. In large open bins the outer part of the compost forms an insulating layer, providing ideal composting conditions in the core of the compost.
The three-bin system adds extra capacity in the form of additional bins. It also makes turning easier, which speeds up the decomposition process.
Here’s how it works:
You put organic material into the first bin. When the first bin is full, turn its contents into the second bin, putting the inside into the outside and vice versa.
When the second one has cooled down, you:
- Turn the contents of the second bin into the third bin.
- Turn the contents of the first bin into the second bin.
- Fill the first bin again.
This ensures even decomposition over time. It also means you have three bins at different stages at all times.
Bin 1: Filled with fresh organic material.
Bin 2: Filled with hot or maturing compost.
Bin 3: Maturing or ready-to-use compost.
I generally let my compost usage determine my turning schedule. So when I have used up all the compost in bin 3, I start the turning and filling process all over again.
Don’t fancy all that turning?
An alternative method is to use an insulating method.
For example, by putting straw around the edge of the bin, you can create additional warmth. You can also add bulking agents to the contents to promote airflow. This reduces the need for turning.
See our Guide to the Three Bin Composting System for a detailed guide, including how to build your own, the best options if you prefer to buy one and detailed instructions on how to use it.
A compost tumbler is a composting device that has a rotating drum which makes the composting process more efficient and easier to manage. Most devices have a crank that allows you to turn the drum, while others can be rolled on the floor.
The rotation helps to mix the materials and introduce air into the organic materials inside.
The bacteria then have plenty of air, which allows them to break down compost material more quickly.
Compost tumblers come in two types: single chamber and dual chamber.
A single-chamber compost tumbler consists of a large, enclosed drum mounted on a stand or a frame. It has a single compartment where all the compostable materials are added.
Dual-chamber compost tumblers take the concept of single-chamber tumblers a step further.
They feature two separate compartments within the drum, allowing you to fill one chamber while the other one is actively composting.
This design ensures a continuous supply of compost, as you can add fresh materials without disturbing the composting process in the other chamber.
Compost tumblers can also be insulated, which helps achieve faster and better compost.
Independent tests show that most tumblers are not as quick as a regularly turned bin. However, they also use a lot less energy!
Worm bins, also known as vermicomposting systems, use worms to break down organic material into nutrient-rich compost.
These bins typically consist of multiple stacked trays with ventilation holes, where worms consume the organic waste and produce worm castings.
The bottom tray collects excess liquid, referred to as worm tea, which can be used as a liquid fertilizer.
A layer of bedding (such as shredded paper or coconut coir) and a small amount of soil is added to the bottom tray, and then composting worms such as red wiggler are introduced.
Kitchen scraps, such as fruit and vegetable peels, are fed to the worms.
As the worms consume the scraps and produce castings, they will move upward to the next tray, leaving worm castings in the bottom tray.
Do note that strictly speaking worms produce castings rather than compost. It’s still good for the soil!
Bokashi bins are anaerobic composting systems that work by utilizing effective microorganisms (EM) in the form of Bokashi bran to ferment organic waste.
The bin is a sealed container, usually with a spigot at the bottom to drain liquid produced during fermentation.
Food waste is combined with Bokashi bran which contains the organisms. The food is compressed to remove air pockets, and the lid is closed tightly to create an anaerobic (without air) environment.
After approximately two weeks, the fermented waste can be buried in the soil, where it will break down further and enrich the soil. Alternatively, it can be used in the compost heap.
The liquid produced during fermentation can be diluted and used as a liquid fertilizer.
Do note that Bokashi Bins do not produce finished compost, although the results are sometimes called pre-compost.
These bins are good for dealing with the waste you might not want to put in a cool compost heap (such as meat and cooked food scraps), but otherwise I I prefer to use a system that produces actual compost!
Digesters, also known as anaerobic composting systems or biodigesters, break down organic material in the absence of oxygen, producing biogas and a liquid digestate.
These systems use microorganisms to convert waste into methane and carbon dioxide. Some devices enable you to capture these gases and utilize them as renewable energy sources.
Digesters come in various sizes, from small-scale home units to large industrial systems.
To use a digester, simply add organic waste to the system and allow the microorganisms to break down the material anaerobically.
Do note that digesters can release a lot of methane, which is harmful to the environment.
A good example of a digester is the Green Cone. This is dug into the soil, and uses the sun to heat up and speed up decomposition.
Electric Composters (AKA Compost Grinders)
Above: The bran inside an electric composter I am currently reviewing.
Electric composters are indoor closed devices that are marketed as being able to turn organic food waste into compost.
Some electric composters that claim to make actual compost do no such thing. Instead, they use heat and grinding to reduce the size of the food and dehydrate it.
However, newer versions do actually make compost. For example, the Reencle contains bacteria in bran, and the bacteria quickly break down organic food into actual compost.
Food is added to the composter, and heat is supplied by the composter. Air is supplied via a filter and activated charcoal, and a paddle constantly stirs the mixture.
The bacteria then break down the food waste into compost.
Do note that some electric composters state that the compost can be used in days or weeks after it has been made.
However, one recent study found that compost from electric composters needs to be left to mature for longer than the time recommended by manufacturers.
Electric composters can be expensive for the amount of space you have, so if you have a large garden you may be better off with one of the other solutions we have explored here.
However, they are ideal for people who don’t have a large garden, don’t like the idea of an indoor worm bin and want to:
- Reduce waste
- Deal with odors from food
- Make compost in the kitchen
Just remember to allow your compost to mature before you use it 😉
Which compost bins to use?
That’s a lot of different systems, so to wrap this article up here’s a few factors to help you choose the right one for your needs.
If you have a garden and a small to medium amount of material: Choose a closed compost bin or a tumbler.
If you need compost fast: Choose an insulated compost bin or insulated tumbler.
If you have a small amount of food waste: Choose a worm bin.
If you need to compost inside: Choose a worm bin or electric composter. A bokashi bin can also work, but bear in mind it doesn’t make finished compost.
If you have a lot of garden waste to compost: Choose an open bin or three bin system. Or, if you have a very large quantity, construct a compost heap.
If you have space, lots of material and are on a tight budget: Construct a pallet bin.
Now you know how a compost bin works, let’s take a look at how to use a compost bin…