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Friday February 25 2022

Why are Biogas Generators Built Underground?

In the UK, biogas plants are a familiar sight. Often placed in rural areas such as on farms or in open fields, the round reactor bins with their domed roofs are relatively common and recognisable to most people.

However, did you know that in many developing nations, AD plants are often built underground?

This may seem odd to many of those working in the UK sector, but it often makes a great deal of sense.

In this article, we look at why this is the case, and what advantages it brings.

Where are AD plants built underground?

Before we look at why AD plants are built underground, it’s interesting to look at where this technique is employed.

Because of the relative ease of production, and because of the variety of feedstocks that can be used to create biogas, particularly animal dung, anaerobic digestion has become a very popular method of producing heat and power in small rural communities in developing nations.

In India, for example, it is common in rural areas for families to have individual biogas reactors that process cattle dung, other farm waste, kitchen waste and even human waste. From this, they can create power for cooking and lighting, and the digestate can be used as a fertiliser, reducing their reliance on manufactured fertilisers.

These small, biogas reactors also improve local sanitation as they can be directly linked to toilets.

Their use is also increasing in Africa, where they follow the similar trend of building small family or community AD plants below ground level.

So, what are the advantages of doing this?

Space

Most of the people employing underground AD reactors are subsistence farmers who grow crops and raise livestock mainly to feed their own families and to trade or barter any leftover produce for other items they need.

As a result, they tend to own relatively small patches of land which they have to make the most of to make ends meet.

Putting a biogas reactor underground is the most efficiency use of space, ensuring that they can produce power and heat without hindering their farming operations.

Building materials

One of the reasons AD plants in developing nations are built underground is because they don’t have access to the type and quality of building materials we have in the UK.

In Britain, biogas reactors are manufactured offsite out of high-quality steel to specifications able to withstand the rigours of the anaerobic digestion process.

However, in less affluent countries, local communities are unable to afford such materials and as a result, are forced to improvise.

Many biogas reactors in these regions are built from traditional masonry walls. And whereas these are fine for housing above ground, when it comes to a vessel for housing large volumes of water or digestate, they run the risk of bursting due to internal pressure.

Building them underground with the masonry walls against the earth adds the extra layer of support needed to ensure they contain the feedstocks without any risk of rupture.

Insulation

Another reason biogas plants are built underground in developing nations is for insulation.

In the UK, we have sophisticated heating systems to ensure an AD reactor stays at the optimum temperature to produce biogas. This is around 35oC for a mesophilic digester and any departure away from this can lead to a significant drop in plant efficiency.

Indeed, the rate of growth and activity of the bacteria powering the digester reaction drops by 50 per cent for every 10oC fall in temperature. It is for this reason, temperature is constantly monitored in AD plants, to keep them running in optimal condition.

Those building and running plants in developing nations often don’t have access to sophisticated temperature control systems, either due to the lack of expertise, resources, or materials. So, instead, they build biogas plants underground.

The earth is a fantastic insulator capable of keeping the AD reactor at temperatures close to those needed for the efficient production of biogas for long periods of time. This would not be possible by building them above ground in many developing countries.

Sealing the reactor

Unsurprisingly given its name, anaerobic digestion takes place under anaerobic conditions. That is, without the presence of oxygen. Therefore, it is absolutely imperative that no air can get into the biogas reactor and disrupt the process.

Again, as many developing nations don’t have access to the quality of materials that the US, Europe and other more affluent continents do, placing a biogas reactor unground makes perfect sense. By surrounding the reactor walls with earth, and sealing the top with it, the anaerobic environment can be successfully created and sustained.

With the technology currently available to subsistence farmers, this would be difficult to consistently achieve with above-ground bioreactors.

Protection from physical damage

Another reason for building biogas reactors underground is to protect them from damage, and there are a lot of things that could lead to damage in remote, rural communities.

Livestock are a great example. If AD plants were built above ground from masonry walls, livestock would be likely to shelter against them, rub themselves against the walls and even crash into them while fighting, running, or during mating. This, combined with the internal pressure from the feedstock, would likely rupture the walls and completely ruin the digestor.

In certain regions, the same can be said of wildlife. Large fauna might well use above ground digesters for sheltering from the weather or crash into them for other reasons, leaving families without power and heat.

And then of course there is the weather itself. High winds, rain, storms, snow, and other weather events could lead an above ground AD digestor to fail, leaving communities without power and heat at the time they most need it.

Unsurprisingly, therefore, building AD plants underground makes a lot of sense in developing nations.

Biogas in developing nations

What is interesting about all of this is not so much why people in developing nations build biogas reactors underground, but that they build them at all.

What it demonstrates is how valuable the production of biogas is both as a cheap and simple solution to the provision of heat and power in remote rural areas and as a sustainable solution to the production of green gas in developed nations, to help reduce our reliance on fossil fuels.

The fact subsistence farmers build them underground is testament to their ingenuity of getting the best out of the materials they have at their disposal. As these nations become richer, it is these people who will be able to supply the innovation and expertise to help with scaling up energy production for future generations.

If you’re in the UK, however, and you have any service, maintenance or operational requirements for an AD plant or CHP unit, don’t hesitate to get in touch.