Food is great, right? And we get that, mostly, thanks to farms. So, picture this: it’s harvest season, the wheat fields are lush and golden, then the combine-harvester rolls through, reaping, threshing, winnowing, until nothing is left. Nothing? This could not be further from the truth. The ears only represent about 10% of the total weight of a wheat plant, with the largest fractions being the roots and the stems. And those 10% comprise the husks outside the kernels, so in the end, only a small fraction of the wheat plant is useful for food production. Granted, the roots are typically left in the earth to decompose, becoming a good source of nutrients for the future crops, but what about the straw and leaves? It can be baled and sold for purposes such as animal bedding or be composted, but it can also be turned into heating gas and fertilizer, through a pretty simple process! And this goes for almost any kind of organic waste! Manure, food waste, waste streams from the food industry, even glycerine can be converted into useful methane and nutrient-rich digestate!
Digestate, you say?
Yup, this process is called anaerobic digestion, and really, it simply means that small microbes much like your gut microbiota are kept in a big airtight tank and fed any kind of organic matter. For them, anything is food! So, without going into the details, they eat, and when digesting their food, they produce different kinds of gases, liquids and solids (sometimes eaten by other micro-organisms furthering the conversion process). Among those gases is methane, which can be carefully retrieved and put into the gas grid, destined to be used for the same purposes as natural gas, while being a much better deal for the environment!
How so? Isn’t burning gas always the same?
Indeed! In recent years, a surge in biofuels production has begun to mitigate the impact of fuel use on the climate and the environment. But bio-based fuels get burned and release greenhouse gases in a similar fashion to fossil fuels, so how is that any greener?
Well, it all boils down to cycles and timescales. A liter of biodiesel or a cubic meter of biomethane is not particularly sustainable in any way. However, a city producing and relying on biodiesel and biomethane is far more sustainable than a city extracting and burning oil and gas. As you may have noticed, the difference lies in the ways to obtain the fuels, not their use.
A quick primer in biology may remind you of the photosynthesis process: plants containing chlorophyll use sunlight to convert carbon dioxide and water into nutrients and oxygen. They then use these nutrients to grow. Until the combine enters the stage. And then they get replanted.
This is the important part. If a whole field is turned into fuel (which greenhouse gases contents cannot be higher than the contents of the raw plants), when burned, it will release some amount of CO2. And a comparable amount of CO2 will be absorbed by the new field replanted in its place, effectively making the whole cycle more or less carbon neutral.
More or less?
Well, this cycle can never have a 100% efficiency. Not all of the GHG emitted during the cycle is absorbed back by the organisms, especially when considering the emissions linked to operation: harvesting, transportation, conversion… Even if powering your combine with electricity from solar panels, it can be argued that a lot of emissions were required to create those panels. And the combine too! But also, some kinds of biofuels emit compounds that are not typically absorbed by plants.
There is an important caveat to consider, though. The cultivation of energy crops as feedstocks for bioenergy production can have serious adverse effects, including land use change and deforestation, and changes in farmland allocation leading to lower food production. And the use of waste for bioenergy production may incentivize waste generation through its valorization, or divert organic waste streams from other sustainable options such as composting or recycling.
Still, there is no debating that biofuels, imperfect as they are, are a great alternative to fossil fuels, and biogas is one of the most studied and developed forms of bioenergy. So, how does heating your house and cooking your meals with gas made from your crops sound?
Additionally, manure is a significant source of methane and nitrous oxides on farms, usually simply released in the atmosphere when left on site or used as-is for soil amendment, but converting it to biogas allows for the harnessing of that methane, sending it to the grid. Manure indeed has a great biomethane potential, unsurprisingly.
You also mentioned fertilizer? Would it be any better than fresh manure?
Absolutely! The digestate resulting from anaerobic digestion is a stabilized, nutrient-rich material that offers several advantages over fresh manure. For one, it’s safer to use, as the digestion process significantly reduces harmful pathogens. The nutrients it contains, like nitrogen, phosphorus, and potassium, are also more readily available for crops to absorb, making it an effective alternative to synthetic fertilizers.
Digestate also has a less pungent smell compared to untreated manure, which makes handling and application more manageable. Additionally, its use can help mitigate some environmental issues. For instance, digestate when applied appropriately to crops contributes less to greenhouse gas emissions than manure.
By transforming agricultural waste into both renewable energy and a valuable soil amendment, anaerobic digestion not only closes a nutrient loop but also promotes a more sustainable approach to farming and energy production. In this system, waste is a resource, benefiting both the environment and agricultural productivity.
Beyond the direct benefits of biogas and digestate, there are other important takeaways when considering the broader impact of anaerobic digestion. First, this process helps reduce the inflow of waste at treatment plants. Agricultural residues, food waste, and other organic byproducts often end up in landfills or are incinerated, both of which release harmful greenhouse gases and contribute little to the circular economy. By diverting these materials to biogas plants, we not only reduce waste but also maximize resource efficiency.
The potential for energy independence is another notable advantage. Farms and rural communities, in particular, can utilize on-site biogas systems to generate their own heat, electricity, and fuel, reducing reliance on external energy supplies. This can be a game-changer for areas with limited access to affordable energy or where energy security is a concern.
Finally, the flexibility of feedstocks makes anaerobic digestion adaptable to diverse agricultural and industrial contexts. Although it’s not recommended to starkly vary the types of feedstocks used in a specific digester, as a whole they work just as well with crop residues and manure as with food processing byproducts, expired groceries, and even some industrial organic wastes like glycerine. This versatility ensures that no matter the setting, biogas can play a role in closing the loop on organic waste management.
By integrating these systems into farms, municipalities, and industries, we move closer to a circular economy where waste is minimized, resources are maximized, and environmental impacts are significantly reduced. It’s not just about energy or nutrients—it’s about transforming how we view and utilize what was once considered "waste."
If you’re interested in learning more, feel free to reach out to Biogem A/S our affiliated biogas trading company!
How to turn Crop Waste into Energy and Nutrients