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Biogas GCSE – BBC Bitsize Teaching Errors?

Image about Biogas GCSE.
It is questionable whether it is useful to teach UK students about a buried chamber type biogas plant type (see image), which they will never see in the UK.

Did you know that biogas GSCE content is being taught in UK schools?  Yes, it is.

The rapid assimilation of the concept of biogas as a resource, and in particular the ability of methane to provide renewable fuel, is evidenced by the fact that the United Kingdom GCSE Course in Biology (B3) includes information on biogas and the anaerobic digestion process (for example from exam board Edexcel).

Biogas GCSE Students Learn About Anaerobic Decay, Methane Gas and Biogas Generators

This is a good foundation for later when students may continue to learn about the anaerobic digestion process and biogas production. The facts given are generally good, but I think there are mistakes.

My concerns are as follows:

1. Students are Told About Overwatering Houseplants and the Decay When Plant Owners Flood the Roots –  But is This Really A Correct Example of Anaerobic Decay (Digestion)?

They are told about the process of anaerobic decay which occurs when “bacteria and fungi break down dead matter without oxygen”, in a way which I find confusing.

“This can happen naturally in some soils, particularly water logged soils, and in lakes and marshes.

When people overwater their houseplants and flood the roots, anaerobic decay can occur, which can kill them.”

As an anaerobic digestion expert I wonder just how correct it is to refer to water logged (waterlogged is one word?) soils, lakes and marshes, where there is undoubtedly anaerobic digestion taking place as being equivalent to houseplant waterlogging.

I would not classify root decay from “overwatering houseplants” as anaerobic decay (digestion).

By their own definition (within the same paragraph) anaerobic decay occurs “without oxygen”.

It is also well-established in the research literature that biogas is only created by methanogenic organisms. These only become predominant once all available oxygen has been removed (from all pore spaces and from chemical oxygen present).

It would seem highly unlikely that all available oxygen would be depleted sufficiently in houseplant pots, due to their small size and the ability of oxygen to diffuse the relatively short distance into the soil.

Rather more likely is, that plants which don’t normally grow in waterlogged ground, which find themselves with roots in fully saturated soils suffer, fungal and bacterial decay from aerobic and facultative anaerobes in such cases.

Rotting roots simply occur because the plant is not well-adapted to continually saturated conditions.

2. The Use of the Term “Biogas Generators” Leads to Confusion

“Biogas generators are large vessels in  in which animal waste or specially grown crops such as maize are allowed to anaerobically digest.” via

The use of the term Biogas Generators in this way is contrary to the usage applied by the biogas industry.

To a biogas expert a “biogas generator” is the electrical power generator set which creates electricity by burning biogas as its fuel.

It’s not anything to do with the vessel in which biogas is produced. That is called either the “biogas reactor”, or the “digester/ fermenter”.

The BBC Bitesize website is not the only educational resource making this error. Amazon has several ebooks aimed at school students which include biogas subjects, and a GCSE exam paper available from shows a question about a biogas generator.

The above shows that there is a danger of confusing students by what they are being told on the BBC Bitesize GCSE AQA page about Decomposition

3. Is the Biogas GCSE Teaching the Right Information for UK Students?

BC Bitesize GCSE AQA page about Decomposition at, mixes up two different anaerobic digestion plant processes in the illustrations. There is a diagram directly above a photograph of a confusingly different type of digester.

The Diagram

The diagram shows a type of buried digester used solely in the warm countries of the developing nations which is not used in the UK. It shows the biogas being used for cooking.

This appears to be a buried masonry dome type anaerobic digestion system of a type developed historically in China, and India. It is a type of digester no contractor would offer to build as it would not be a feasible form of construction to comply with the EU rules for the protection of groundwater (EU Groundwater Directive). It is also a design which has no method of stirring/ mixing.

The Picture Below

Below it is an image showing a modern Continuously Stirred Tank Reactor (CSTR) which is commonly used in the UK, but not for provision of cooking gas.

In fact the biogas plant shown in the image is most likely that there is an electrical generator installed out of sight of the photograph, behind the CSTR.

One wonders why UK kids are being shown an above-ground (so called “industrial biogas generator”)  in the GCSE training, when there are so many dissimilarities with the diagram?

The type of digester construction shown in the diagram is also too prone to cooling in the winter to be viable in the UK.


It would be better to show the BiteSize Student consistent examples of one type of AD Plant only.

That plant should be a heat insulated and warmed AD Plant in both the process diagram and the image. Both should be of the of the commercial biogas plant type most often being built in the UK.

Even the use of the description by the educationalists of a “biogas” plant, rather than the more correct English usage of an “anaerobic digestion plant”, is off-message for the UK audience.

Readers should just try to remember that the renewable energy plant process is anaerobic digestion and the output is biogas!

Oh! Yes.

And, when did you last see dung being collected for an AD Plant feedstock in the UK! Our teachers seem to be getting carried away by the use of the internet as an educational resource. They should remember that international practice may not be applicable to Great Britain.

Am I the only person that would say that they should be more critical and apply UK information when teaching our children, rather than the methods which work in Pakistan where their agricultural circumstances and climate are quite different?

But, this is just whingeing! All those that believe that the AD process and the production of biogas has huge potential and should be being better promoted and to an extent more heavily subsidized by various governments including the UK coalition government (2010 to 2015), will be at least pleased that the biogas GCSE includes this training.

This example should give them cheer, as this is surely another indication that the message about biogas, and the fact that it is highly sustainable and indeed renewable, is getting across to the educational establishment.


Let’s hope that the UK will not have to wait until the current generation grows up and gets into power in government before applying a commonsense level of government subsidy.

Then they should aim at ensuring the financial viability of AD technology for farms is real, and is available for on-farm biogas plants is demonstrable.

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    • DereGist
    • May 22, 2019

    Biogas lessons. Not bad. Good graders.

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