Trash to Treasure… Or Something Like That, Pt. 2

By Heather Parker

This is part two of my three-part mini-series about my work as a Peace Corps volunteer in waste management/upcycling:

BIODIGESTERS

What would you say if I told you that your cooking could be powered by guinea pig manure? 

Well, in fact, in my community we are trying to do just this. Over the course of the last several months, community members and I have constructed two biodigesters. Though biodigesters  come in all shapes and sizes, ours are lovingly dubbed the salchicha (hotdog) model and take the form of a long tubular bag made of very heavy-duty black plastic that tapers down to a narrow sleeve on each end. Inserted in each of these sleeves is a tube – an entrance and an exit – and on the top of the bag is a system of tubes and valves – and a few filters to remove unwanted gases like water vapor and hydrogen sulfide – that ultimately connects to a stove burner. 

At this point, you may be thinking, well, I understand what it looks like, but what does it do? The short of it is that a biodigester can produce gas for cooking (and for other purposes), but let me explain this all in a bit more detail.

A biodigester functions much like a human stomach. It must be fed on a daily basis – it’s preferred meal is a 3 to 1 mixture of water to animal (e.g. guinea pig, which is considered a delicacy in Ecuador) manure – and the material it’s fed is then digested through an anaerobic process, meaning that the biological reaction that breaks down this “food” takes place within the biodigester in the absence of oxygen. And just like a human stomach, a biodigester ultimately produces three byproducts: a solid, a liquid, and a gas. 

Unlike in the human digestive system, a biodigester’s solid and liquid byproducts leave the system together, through the same exit (and in this case a PVC tube), in the form of a thick sludge-like substance. This output is a very high-grade organic fertilizer that can be further separated into its liquid and solid components if desired. In Spanish, the liquid component of this fertilizer is called biol, and, when mixed with other ingredients such as garlic, onions, or ají (a hot pepper), it can be used as a natural alternative to chemical pesticides and fungicides. The solid component is called humus, and can be spread around the base of plants or tilled into soil much like other organic fertilizers like cow manure (or human manure from dry toilets!). 

The gas produced by a biodigester is perhaps the most useful end product and, in my opinion, the most exciting. This gas, often christened “biogas” in the literature about biodigesters, is composed principally of methane, carbon dioxide, and several other gases including water vapor and hydrogen sulfide. For our purposes, the most important component of this biogas is methane because this methane, which accumulates within the biodigester and is released through the tubes and valves on the top of the system, can be used for cooking, heating, and even electricity generation. Biogas can therefore be used in rural communities as an alternative to firewood – thus not only slowing the rate of deforestation but also reducing wood smoke, which can be very detrimental to respiratory health – and in more industrialized areas as an alternative energy source.  And so, ultimately, poop becomes electricity. How cool is that?

The biodigesters that I have constructed in my community are quite small – about five meters in length and a meter and a half in diameter – but biodigesters can be built on much larger scales. And, given that the anaerobic reaction that produces biogas will readily take place if the biodigester is fed human waste, biodigesters can be used to manage sewage, too. And, in fact, some towns and cities have caught on to the enormous potential of biodigesters and are doing just this. 

Are there downsides to biodigesters? Well, sure. If the sludge – the mixture of humus and biol – that leaves the system is not handled correctly, it can potentially pollute waterways. And given that methane is highly flammable, if the biodigester is allowed to fall into a state of disrepair and develop leaks, there are obvious safety hazards. But these disadvantages are few and can be easily managed by proper use and maintenance of the systems. The advantages, among them better management of biological waste and the subsequent improvement of water quality, the production of high-grade organic fertilizer that can be used as an agricultural alternative to chemicals, and the creation of biogas that can be used in place of firewood or non-renewable energy sources, greatly outweigh these downsides. 

The biodigesters that we have built in my community are not yet fully functioning due to some challenges we encountered during their construction, so I can’t yet attest to the viability of the specific model that we have adapted to fit the climate and situation in my community. But biodigesters certainly do work, and a quick Google search for “biodigesters” will reveal only a few examples of the many success stories documented in countries all over the globe. Biodigesters as an alternative waste management technology have enormous potential, and it seems that many people – myself included – are beginning to realize this. And on that note, I’ll leave you with this question: Wouldn’t it be so cool to see a city, as an alternative to letting it’s sewage run into and pollute its waterways, instead harness this waste to produce electricity for its citizens?