Wednesday, July 17, 2024

Anaerobic Digestors without the Hot Air

Extracts from the Feedback Report, Green Gas Without the Hot Air, 2020

“As countries and companies commit to net zero greenhouse gas (GHG) targets of varying ambition, anaerobic digestion (AD) has been framed as an environmental silver bullet, a form of renewable energy to rival wind and solar in its desirability and environmental credentials. AD is the process of taking organic materials, known as ‘feedstocks’, both purpose-grown, like maize and other crops, and waste streams, like food waste and manure, and breaking them down using micro-organisms in the absence of air. This produces methane-rich biogas, which can be used to generate heat or electricity, and nutrient-rich digestate, which can be used as a fertiliser. In the UK, the AD industry portrays itself as both a panacea for difficult-to-decarbonise sectors like heating and transport – by providing a sustainable source of power – and a solution to organic waste management – for multiple sectors from livestock farming to retail.” 

Typically 60% of the AD feedstock comes from maize or grass or other biomass and 40% slurry from farm animals.

To date, the AD industry’s claims have largely gone unchallenged. However, by comparing AD to proven climate change mitigation policies, this report shows that the benefits of AD have been overstated. Worse, the industry’s ambitions are crowding out better environmental alternatives. “This report uses the results of a life cycle assessment (LCA) conducted in collaboration with researchers at Bangor University to shed some much-needed light on the limitations of AD, and show what role there is (and is not) for AD in a sustainable future.”  You can read the full report here.

Is Anaerobic Digestion an Efficient way to produce energy?

Solar Photovoltaics (PV) generates 12–18 times more energy per hectare than the energy generated from the anaerobic digestion of maize or grass per hectare.

Is using land to produce feedstocks for Anaerobic Digestion the best way to reduce GHG emissions?

AD produces methane, which displaces fossil natural gas.  But if instead of using land to provide feedstocks for AD we used it to grow forests instead we could sequester far more green house gases than the methane displaced by AD.  “Growing forests on land currently used to grow AD feedstocks would achieve between 2.6 times (vs. maize biogas) and 11.5 times (vs. grass biogas) more net GHG mitigation.”

Food Waste – A ‘Prevention First’ Approach

“The report finds that far from only dealing with ‘unavoidable waste’, when AD subsidies are set very high, as the AD industry is calling for, AD can actively hinder waste prevention, particularly when paired with a lack of regulation and funding for the better alternatives. In the current context, preventing food waste results in direct emissions savings over nine times higher than sending food waste to AD (per tonne of food waste).”

If the grassland used to produce the meat and dairy that ends up as waste is instead afforested, emissions savings are on average 40 times higher than sending the same volume of food waste to AD.

Industrial Livestock Production

“The use of manure and slurries from livestock for AD shows the highest potential for emissions savings – mainly because of the staggering volumes produced by the intensive livestock sector. However, processing slurries may not be economically viable without huge subsidies, because slurries and manure have a very low energy density per tonne, which is why they are usually digested in combination with purpose-grown crops – which, as previously discussed, have questionable sustainability.

Again, there is a better alternative to AD – preventing the manure and slurries from being produced in the first place (plus all the other emissions impacts of intensive livestock production), through reduced meat and dairy production and consumption. This would reduce emissions substantially more than the mitigation potential offered by AD, and also has the potential to free up vast quantities of land for tree planting and additional carbon sequestration. The emissions mitigation from processing manure also significantly declines in future decarbonisation contexts because emissions from slurry storage and fertiliser production are projected to decline anyway.”

“A report commissioned by the Committee on Climate Change (CCC) estimates that a 50% reduction in the UK’s beef, lamb and dairy consumption could result in a 37% reduction in the total UK agricultural sector’s domestic emissions. It would also free up an estimated 4.2 to 6.9 million hectares of grassland. If trees were planted on 4.2 million hectares, this would result in an estimated 54 million tonnes CO2eq annual average carbon sequestration by 2032, which (assuming UK agriculture’s emissions fall by 37%) would be enough to offset remaining UK domestic agricultural emissions nearly twice over. Dietary shifts away from chicken and pork are also very effective – on average, switching from poultry meat to tofu results in reductions of 65% in emissions and 69% in land use (Poore and Nemecek, 2018).”

VSM Conclusion

Overall, as all the above arguments clearly show, AD completely fails on multiple sustainability indicators.  Furthermore, it locks in fossil foods (meat, dairy and fish) production, encourages industrial farming which prevents land being restored to precious ecosystems and undermines food security. It’s a lose-lose in many different ways. 

Feedback is a (non-vegan) UK- and Netherlands-based environmental campaign group working for food that is good for the planet and its people. To do this they challenge power, catalyse action and empower people to achieve positive change.


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