Methane from cow manure? Just add vegetables

 By Robin Wylie

Cows are infamous for their methane-rich flatulence. But scientists have found an interesting new way to produce gas from the animals’, um, other waste products. It was already well known that cow manure can be used to produce methane. But in a new study, Spanish researchers discovered an interesting new way to boost the methane productivity of bovine waste – just add beets! In the study, the scientists combined different ratios of cow manure and sugar beet strips, before processing the mixtures to produce methane gas. They found that a half-and-half beet-manure mixture provided the best results, boosting methane yields by an impressive 25% compared with cow manure alone. With biofuels becoming big business, these kind of clever tweaks could make fossil-free fuels an even more attractive prospect…

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The humble cow has done a lot for humanity. Its meat and milk help sustain hundreds of millions of people. And, as you may have smelled on the country air, even their – shall we say – less pleasant products have their uses.

Humans have used cow manure as an agricultural fertilizer and a combustible fuel for millennia. But more recently, in the biofuel age, scientists and engineers have been finding increasingly sophisticated ways to extract energy from bovine waste.

Cattle famously emit a lot of methane. But it’s not just their flatulence that gives off this gas. Like most organic material, cow manure can be made to release methane via heating. Many countries are already taking advantage of this plentiful supply of fossil-free hydrocarbon — witness China’s gigantic “methane dairy farm.”

Researchers are always trying to extract more methane out of manure. And one technique they’ve struck on might surprise you: mix in some veg.

Beets, for example, seem to be particularly good at getting the most gas from your dung. In the past decade, studies have shown that combining animal manure with various parts of the beet plant (Beta vulgaris) can significantly increase methane yields.

Photo - Indi Samarajiva

Using beets for this purpose is great from a biofuel perspective. But humans also rely on this vegetable for other products — such as beetroot and beet sugar — making inter-industry competition a potential barrier to using fresh beets to enhance the production of biomethane.

Happily though, science is showing that beets can help us get more methane from manure even after they’ve been used. Exhausted sugar beet cossettes (ESBC), the fibrous residue of beet sugar production, can also be used to enhance manure methane yields.

One of the most successful studies to use ESBC in methane production was published at the end of 2015. A team from the University of Cadiz, Spain, found that by mixing about one part each of cow manure and ESBC, they could boost methane production by around 25% compared to using cow manure alone. This is one of the highest methane yields ever achieved using a manure/beet mixture, even beating some earlier attempts that used fresh beets, rather than ESBC.

Cows get a bad rep for their greenhouse gas emissions, and rightly so; even if we ignore their flatulence, beef is widely considered to be the most carbon-intensive mass-produced meat. But that doesn’t mean that we can’t turn cow waste into something less lowly. With biofuels becoming big business, these kind of clever processes could make fossil-free fuels an even more attractive prospect.

about the author
Robin Wylie
Freelance earth/space science journalist. Currently finishing off a PhD in volcanology at University College London.