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Reduced nitrogen emission by bio-addification of cattle slurry

Sugar in your slurry, anyone?

Ammonia emissions from manure are one of the largest sources of air pollution in Denmark. The nutrients harm the environment. Farmers therefore try to limit the evaporation by, for example, applying the slurry to fields using trailing hoses.

Agriculture can probably halve emissions of harmful ammonia by adding sugar to the slurry. New research from Aarhus University suggests that sugar can replace sulphuric acid as a method
of reducing ammonia emissions - to the benefit of
organic farming and biogas production.

It may sound a bit illogical that something becomes more acidic if you add sugar to it, but this is nevertheless true of slurry.

And this does in fact have a large potential. Acidic slurry (i.e. slurry with a low pH) does not emit nearly as much ammonia as neutral slurry. Acidification of slurry can actually reduce evaporation of ammonia by up to 70 percent. An increasing number of Danish farmers have therefore, over the past 10 years, started adding sulphuric acid to their slurry. In 2014, 12 percent of Danish slurry was thus acidified.

Acidification of slurry with sulphuric acid is an effective alternative to other methods that farmers use to reduce ammonia emissions, but it is not a universal tool. Organic farmers may not use it, and biogas reactors should preferably not receive more than 10 percent of their feedstock as acidified slurry – if the concentration is higher it negatively affects the production of biogas.

Now researchers from Aarhus University in partnership with the commercial sector have found out that sugar is just as effective as sulphuric acid at acidifying the slurry. And sugar does not pose a problem for organic farmers or biogas producers.

Why does slurry need to be so sour?
Ammonia emission from manure is one of the largest sources of air pollution in Denmark. Not only do emissions burden the environment with surplus nutrients to the detriment of a number of ammonia-sensitive habitats such as certain types of woodland, bogs, moors and lakes, they also impair the health of thousands of people.
Neither is ammonia good business for the farmer who would prefer instead to put the nutrients to better use in the field.

So there are good reasons why the authorities would like agriculture to reduce emissions. In Denmark, the requirements are much tougher than in most of the EU, which is probably why the technology for slurry acidification has not yet caught on outside Denmark.

Researchers track sugar alternatives
But how can sugar replace sulphuric acid?

“The sugar acts as a substrate – in other words a feed – for the bacteria that produce lactic acid. And the lactic acid has the same effect on ammonia as sulphuric acid,” explains Assistant Professor Maibritt Hjorth.

She explains how the research has been conducted:

“Initially, we tested how the slurry reacted when we added a combination of lactic acid bacteria and sugar. It turned out to work very well, and we could even make the pH drop even further than was necessary. But we have since found that you do not need to add microorganisms because the organisms already present in the manure can easily manage the job themselves if you make sure to give them the right growing conditions. And this is done by adding sugar,” says Maibritt Hjorth.

Sugar should here be taken in its wider sense; we are not going see tractors queuing up at the supermarket for sugar because soya molasses, beet molasses, cheese whey, maize silage, straw and fodder beet can probably be used as alternatives. The extent to which they can replace sugar in this context is what Maibritt Hjorth and her colleague, Senior Researcher Anders Peter Adamsen, are now in the process of investigating.

“The big advantage is that the individual farmer will to a certain extent be able to use the by-products that are produced continuously on the farm. And the sugars will be easier to handle than sulphuric acid which has to be procured from a factory,” she adds.

Basis for new markets
The development of the new acidification method was done in a collaboration between Aarhus University, AgroTech, SEGES (which is the new name for the erstwhile Knowledge Centre for Agriculture and Pig Research Centre), the biotech company Lallemand and JH Agro A/S, one of the world’s leading suppliers of manure acidification systems.

Maibritt Hjorth estimates that the new method is quite close to being market-ready.

“We have yet to get a handle on the delicate balances: what is broken down at what speed and in what order, and how do we achieve the desired pH values without spending too much money? We will conclude soon with some laboratory tests and a pilot experiment by AgroTech, and then JH Agro can upscale and sell the technology,” says Maibritt Hjorth.