How a giant cow’s stomach can boil your kettle


Green energy can come from all sorts of weird and wonderful sources. Take anaerobic digestion, which makes gas and electricity from waste in a sort of huge cow stomach. Here’s how it works...


Right now, the energy you’ll use to watch a future episode of Game of Thrones could be sitting inside a mouldy carrot. And the power you’ll need to boil the kettle afterwards could be swishing around in the bowels of a pig, or being chewed by a large Friesian cow.

Fermented vegetables, animal manure, garden waste…it’s all biomass: organic material containing stored energy from the sun. Plants absorb the sun’s energy (via photosynthesis), and if we’re clever, we can tap into it by converting it into something we can work with, like electricity.

Now humans are very good at converting energy from one form to another to do work – it’s more or less how we built civilisation.

The problem is that most of the spectacularly effective methods we’ve discovered of doing so involve burning stuff. In particular, burning ancient, non-renewable forms of organic matter like coal, oil and gas. And these have turned out to release CO2 into the atmosphere and warm the planet.

However, although you can burn biomass (e.g. wood fires), burning isn’t the only way to harvest energy from it.

You can do it with renewable sources and without the carbon emissions. It’s called anaerobic digestion, and the trick lies in understanding and copying nature.

Which is where the cow’s stomach comes in...


‘Feeding the rumen bugs’

Cows’ stomachs are surprisingly interesting. Contrary to popular belief, cows do not have four stomachs. They actually have a single stomach with four distinct components.

In Texas there’s a veterinary professor whose job includes explaining to ranchers what makes a good diet for their cattle. His method is to stroll into the lecture theatre with a big plastic bag full of cows’ organs, lay them carefully on the table and talk through the weird digestive process step-by-step. It goes something like this:

A cow starts by briefly chewing her food (e.g. grass), mixing it with digestive enzymes from her saliva. This passes down the oesophagus into the first two interconnected chambers of her stomach, called the rumen and the reticulum, where it is fermented by specialised bacteria. (Stage 1 on the diagram. The reticulum is a really odd bit of the cow’s system. Larger particles (‘cud’) are burped back into the cow’s mouth so she can chew them a bit more.)

After this reticulo-rumen process, the food goes to the omasum (stage 2 on the diagram) which absorbs all the water. Finally, it passes through the abomasum (stage 3). This digests food in a similar way to a human stomach: sorting it into useful proteins, vitamins, fats etc for absorption into the small intestine. Useless material is sent to the large intestine and ultimately excreted (‘cowpats’).

It's a remarkably efficient system. But it relies on a healthy population of different kinds of specialised bacteria living in the rumen. If the cow is given food that the bacteria uses then its population will grow and the cow will digest more of her food. That's why educated Texan ranchers sometimes say that their job is ‘feeding the rumen bugs,’ not feeding actual cows.

And it’s this insight that leads us back to humans and energy.

You might say that anaerobic digestion (‘AD’) is a way of carrying on the work of a cow’s stomach. It’s about feeding the right kinds of bacteria so that they can turn biomass into something useful. But on an industrial scale.


How anaerobic digestion works

A typical anaerobic digestion process involves feeding manure and other waste matter into a large oxygen-free container (the word ‘anaerobic’ means ‘without oxygen’ ). This stirs the waste around at a warm cow-like temperature so that bacteria can thrive.

These bacteria break down the waste and produce a biogas – primarily methane. This can be piped into a generator (a ‘CHP’ or combined heat and power unit). Here it explodes, turns pistons and creates electricity.

It’s estimated that every tonne of food waste recycled by anaerobic digestion as an alternative to landfill prevents between 0.5 and 1 tonne of CO2 entering the atmosphere.

And what’s more, the digestate (the matter left over from the digestion process) is a nutrient-rich biofertilizer. This is a renewable alternative to commercial fertilizer that’s also more environmentally-friendly than raw manure.


Smarter stomachs

Anaerobic digestion techniques are evolving all the time. And they’re providing new incomes for our farming community.

One of the Bulb generators is Little Pengethley Farm, near Ross-on-Wye. Despite its small size, the farm manages to turn around 3,000 tonnes of poultry litter, 9,000 tonnes of sugar beet and maize, and 1,000 tonnes of vegetable waste into renewable electricity each year. Enough to keep the TVs and kettles on in 500 homes for a year and prevent about 650 tonnes of carbon being released into the atmosphere. Small farm, big impact.

Green gas from the Icknield Farm anaerobic digestion plant

Green gas from the Icknield Farm anaerobic digestion plant


Another is Icknield Farm, where Oxfordshire landowners have transformed 1.8 hectares of farmland into a sophisticated gas-to-grid anaerobic digestion plant. It turns local maize and, erm, wet pig slurry into biomethane gas, which is then injected directly into the gas grid (minus the smell).

Despite all that, digesters are odourless and fit well in the layout of farming businesses.

Anaerobic digestion is getting smarter and the range of waste products we can use is expanding. It promises to be one of the clever ways we can continue to wean ourselves off non-renewable gas and electricity.

Gas in particular has a long way to go. 100% of Bulb’s electricity is renewable but the figure is 10% for the gas we supply. That’s still 100 times higher than the national average, and we’ll continue to source new green gas generators and get that figure up.

But for now, as a Bulb member you can say that at least some of the cups of tea you make are thanks to kettles boiled by a giant cow’s stomach.

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Andrew Nixon