Let’s lay the debate to rest

14th Jun 16 by Martin Bowman

It might well make sense to turn sewage sludge or manure into energy, but using food waste to make biogas through anaerobic digestion is a different kettle of fish. In short: the energy and resources required to grow, process, package and transport food that ends up wasted can never be recovered through anaerobic digestion.

The Pig Idea continues to advocate the environmental and financial benefits of repurposing food waste to feed livestock. It is important to note that on the subject of feeding pigs, we’re not talking about the kind of supermarket or manufacturing surplus that can go to food banks and feed people, but rather about what most people really would consider waste: restaurant left-overs, plate scrapings that go into your food waste caddy, or manufacturing by-products that have no other purpose.

New Research

In the most comprehensive study to date on the issue, researchers at Cambridge University conclude that turning this kind of food waste into pig feed has significantly more environmental benefit than biogas or composting.  Pig feed – produced in well-regulated, sophisticated waste to feed conversion systems in South Korea – was compared to biogas and compost in terms of 14 health and environmental impact indicators, and pig feed did better than both biogas and compost in all but two:




 From Salemdeeb, zu Ermgassen et al. 2016.

The reason the results are so clear-cut is that the study accounts for reduced demand for soya and other virgin crop ingredients of conventional feed as it is replaced with food waste-based feed. Currently, 88% of UK soya is imported from Brazil where soya production has devastating consequences on biodiverse eco-systems such as savannahs and the Amazon rainforest.

Composting and anaerobic digestion have disproportionate impacts through eutrophication (water pollution caused by a harmful excess of nutrients), environmental toxicity (eg heavy metals) and acidification. With regard to global warming potential, wet pig feed also scores better than biogas and compost. And in spite of the extra energy needed to dry food waste, dry feed still beats compost too.

Most industrial pig farms in the UK currently use dried feeds; wet feeding is more common in other EU countries, such as the Netherlands, where it is favoured because it permits the use of wet agricultural wastes, such as distillery wastes or beet tails and because of reported nutritional benefits of wet feeding. Dry feed or wet is still open for debate, but what is certain is that converting unavoidable food waste into pig feed makes more sense than ever.

We need to challenge any policy or subsidy that supports food waste diversion to biogas in such a way that there is no incentive to avoid waste in the first place. And since Salemdeeb and zu Ermgassen at Cambridge have removed all doubt on the greater benefits of converting unavoidable food waste to pig feed instead of biogas and compost, the path forward for unavoidable waste is clear. So let’s lay that debate to rest once and for all, and concentrate on setting up the right regulations and systems to replicate the South Korean and Japanese examples safely in the UK and Europe.


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