Some of you, if you live in the UK, may be familiar with a little green or brown bin that lives in your kitchen and is filled with food waste. If you have you may wonder what happens to this food waste. I visited Welsh Water's waste to energy plant to find out exactly how they turn food waste into electricity.

In the countries and counties where there are no food waste bins, usually people's old food just ends up in their general waste bin and makes it way to landfill (or maybe gets incinerated). In landfill it gets buried and decomposes without oxygen through a process called anaerobic digestion. This process creates lots of natural gases including copious amounts of methane, a very potent greenhouse gas with 21x the global warming potential of CO2. 40% of all the UK's methane comes from Landfill[1]

So at waste to energy plants, like this one in Cardiff, that process of anaerobic digestion is used industrially to create methane for electricity production. And I'm going to tell you how. There's also a video if you prefer to watch content over reading it.

Stage 1 - Transport and Separation
Food waste is collected from front doorsteps, taken to plants ready to be processed! Trucks are weighed on the way in, dump their waste and go on their merry way.

Food waste bags need to be separated from their contents so they don't clog up any pumps or vats. Bags are put in a matrix where they're ripped apart and then everything is put through an 8-12mm screen so only food waste is allowed through. Then comes a lot of very big vats.

Stage 2 - Hydrolysis
In the first of the big vats- the reception tank named Blodwyn- food waste is mixed with water  and undergoes the process of hydrolysis. Put simply this breaks down big chains of molecules in the food waste into smaller ones to make them easier to digest.

Ferric Acid is also added to to the tank depending on the levels of sulphur and hydrogen sulphide to keep the reaction balanced.

This hydrolysis process takes between 8-12 days before waste is pumped into tank 2.
Stage 3 - Fermentation
In tank 2 and tank 3, Shirley and Sheila, digestion occurs. In Shirley the temperature is at 41°C and bugs and microbes have a nice big party to digest a lot of the food waste. Then in Sheila the temperature is at a still barmy 27°C. Lots of natural gas, including methane, is produced in this process which gets syphoned off into tank 4, this can be identified by the characteristic dome on the top of it.

Stage 4 - Pasteurisation
After around 28-32 days of fermentation the liquid food waste is ready for pasteurisation. This process is similar to how milk is pasteurised, it's heated to 71.5°C to kill any nasty bugs in it and then it's ready to be collected and spread on local farmer's lives.

Stage 5 - Energy and Compost!
Natural gasses, including the methane I talked about earlier is taken from tank number 4 to be put into what is essentially a giant engine so it can be burnt to create electricity! The electricity that is produced by this one plant is the equivalent to what it would take to power 2000 homes, though it is not put into the national grid, it is sent over the road to the waste water treatment works to process sewage. This plant creates around 20% of the energy needed to process the sewage, while the waste water treatment works create 30% of their own energy through a similar process. Yep it takes a lot of energy to process poop.

And to give you an idea of the rate of energy production. One pumpkin could power a hair dryer for 13 minutes!
Back to the biodegradable bags from earlier these are shredded up with other organic matter that can't be put through anaerobic digestion and are made into compost, also used on land.

Of course like any process this isn't perfect, and it can't handle things like compostable straws, cups, and cutlery that are all the rage at the moment. The advice here, if you live in Cardiff and The Vale, is to put these in your own compost bin to be composted instead. If you don't have a home compost bin, put them in your general waste. But generally it's always better to use reusables rather than one use items, and I've talked about the issues with compostable and biodegradable plastics before.

So food waste is used to help grow more food and to help process human waste! Kind of poetic in a bit of a gross way... Definitely a much better use than letting it rot in landfill anyhow!

Though this plant uses Anaerobic digestion to make electricity to process sewage, there are now over 1 million homes in the UK powered by AD[2]. So if your local council doesn't have a food waste collection, ask them why! There's no point in your food waste going to waste in black bins when it can be used to fuel your house instead. And if you do have a food waste bin available to you, use it!

In Cardiff and the Vale you can do this for free! You'll need a kitchen caddy, a roll of caddy liners, and a kerbside caddy. They can be ordered online in Cardiff at and collected from stockists at the vale of glamorgan (find stockists at If you live elsewhere in the UK then contact your local council!

I hope you've found this post helpful and maybe learned from this, I have more post on waste and energy below and have more coming in the future so stay tuned for that. Thank you to welsh water for having me and for more you can go to greenbin2greenenergy for a simple interactive look at how it all works, or at Welsh Water Organic Energy's website for more information!

More on waste and energy production:

[1] Organic Resource Agency : Comparison of GHG Emissions for Landfill and AD - 201