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Three rather simple questions.

  1. In absolute terms how much N is preserved per unit of slurry (define as you wish!)? Value and percentage does not allow forward estimates.
  2. Have you considered animal welfare/growth rate? Anecdotally I am aware of use in pig units on slats where the impact of acidification was said to be dramatic on growth rates and welfare.
  3. Have you considered treatment where slurry is separated into liquid and solid? I would expect ammonia loss and ease of treatment is easier in the liquid component.
  4. The cost of acid is also variable. Which acid was used and how concentrated. Silage effluent is likely to be acid: could this be used? If so what proportions?

Thanks for your questions Simon

For a slurry application supplying 150 kg/ha total N ammonia emissions from surface broadcast unacidifed pig slurry were on average 45 kg/ha N; acidification reduced the emissions by around 60% so the saving in N would be around 30 kg/ha of N. 

We weren't able to assess the effect of acidification on pig growth rates etc because  Covid and changes in the pig market meant that the pig management changed significantly at the same time as the acidifcation equipment was introduced at the farm. However the farmer did say that the environment in the house had improved significantly and was more pleasant for people to work in. When discussing with JHAgro a key part of the costs benefit is the improvement in pig production. Its a shame we weren't able to get the data to support this.

During the process the whole slurry is acidified before separation. Analysis of the separated materials suggests that the liquid and solid fractions remain acidified during storage.

The acid used was concentrated sulphuric acid (98%) because it is most available (being a bi product of the petrochemical industry) and requires least volume to reduce pH to target values. Other acids can be used but may have issues relating to availability practicality and cost. There is some interest in using waste products as alternatives as the availability and cost of the acid is key to the process. However further research is needed to identify their effectiveness. Plasma treatment has been shown to be effective at reducing slurry pH sufficiently to reduce ammonia emissions, however the  costs and energy requirements may make the technology unsuitable for some farms



Thanks John

My expectation is that the liquid fraction would contain not only more ammonia but it would be easier to treat. I appreciate that you were working with pig slurry but both dairy slurry and digestate are often separated into solid and liquid fractions. It is much easier to mix acid in solution than with a solid where there is going to be interaction with even more components reducing impact.

Silage effluent can have a pH of four, or even lower, and is available to many AD plant operators and most dairy businesses (although I suspect quantity would be inadequate). 

Ammonia is clearly toxic. The targeting the two elements (liquid and farm acid source) would be a relatively easy policy introduction and would also make a small financial contribution from more N availability.

Every little bit counts!




PS Plasma strikes me as expensive!



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