Manure Management – waste not, want not

By Christine Parry, AB Agri

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In a Net Zero farming eco system, one of the core ideas is that absolutely everything is captured and nothing goes to waste. By doing this we’ll make the whole farm system as circular as possible.  

Circularity is by no means a new idea when it comes to manure, but technology can help us to make even better use of this fantastic natural resource. There are various technologies in development, from adding bacterial products to the manure to break it down, to adding nitrification inhibitors to prevent bacteria in the soil from converting ammonium into nitrate. 

What is ‘acidification’ and does it work?  

Acidification is one of the most widely used approaches to manure management. It works by adding acid to lower the pH of the manure. The lower the pH, the more nitrogen is kept in the liquid as ammonium, so there's less 'free' ammonia escaping into the environment. It works, but there are a few potential drawbacks to consider.

First of all, there’s no return on investment for the farmer; acidification as a process simply adds an ongoing cost that doesn‘t generate any additional value. Also, we don’t fully understand the impact of putting acidified material onto land. It’s possible that, further down the line, this could lead to problems with increases in Hydrogen Sulphide (H₂S) and Nitrogen Dioxide (NO₂), far outweighing any benefits you are getting from adding the acids to stop the ammonia and methane being produced.  

What are the alternatives?  

Anaerobic Digestion (AD) presents an exciting alternative for farmers, not least because AD also generates additional revenue. In an anaerobic digestion system, bacteria break down the manure to produce both a natural gas and a natural fertiliser, known as ‘digestate’. The great benefit here is the ability to sell or use the gas - with many using it as a heat source or selling it into the grid. There have also been massive leaps around the technology supporting biogas/methane fuelled tractors and farm equipment, and this could well be the obvious power source for farm equipment in the future.  

It is of course the digestate from AD plants that plays a vital role in this circular story, and there are new approaches to how this digestate can be optimised as a fertiliser. For example, a lot of farm-based systems can separate the digestate into solid and liquid factions. The liquid faction is rich in nitrogen, while the solids contain phosphorus. These two factions can be used in various ways, enabling low-tech ways of being able to apply fertiliser in a relatively simple precision agriculture approach. At the moment, the liquid fraction is relatively dilute and that means multiple passes over crop land to avoid overwatering. Alternatively it is possible to concentrate the material, allowing for just one pass over the land, reducing potential for soil compaction. 

Net Zero is not just about new technologies, it is about innovating and changing practices on farm. For example, it is not just putting slurry through a spreader, but applying it in a more efficient way. By separating the phosphorus and applying liquid nitrogen in a precise way through dribble bars right at the crop, you will lose much less of this nitrogen into the air. The phosphorus can then be applied when you want it.  

When it comes to manure from cows and pigs in particular, AD offers a very simple way to make use of waste, maximise efficiencies and help towards Net Zero goals.  

Will AD work for all livestock? 

AD should be a very effective solution for cows and pigs, but the very high nitrogen content in chicken litter makes it more difficult to anaerobically digest. This high nitrogen content is often better applied directly as a fertiliser. Producers are also looking at how they can burn that material, but it is very much case specific.   

If, for example, you have a mixed farm with an arable operation, it would absolutely make sense to use the litter as fertiliser, but there are options to improve emissions regardless of which manure we look at.  

With investment in AD technology on farm, the potential reductions in farm energy costs and use of synthetic fertilisers are hugely significant. 

Christine Parry is AB Agri’s Head of Development and Innovation. As co-creator of Amur, AB Agri’s anaerobic digestion business, Christine has been instrumental in delivering the associated product portfolio for the AD market.