SAFT2cattle

Separation and acidification after precipitation of liquid manure from cattle barns

Based on the results of the SAFT-project, which significantly reduces ammonia and methane emissions from a fattening pig barn by acidifying slurry with sulphuric acid, a technology is being developed which is also suitable for cattle farming. The technology is to be optimised in order to reduce acid consumption, which will increase cost and resource efficiency. By using calcium additives, the carbonate buffer, which is responsible for most of the amount of acid during acidification, is eliminated and precipitated in advance. Separation of the slurry into a liquid and a solid phase further reduces the amount of acid required and leads to nutrient removal from the liquid phase.

Considering these factors, alternatives to sulphuric acid are being searched for. This makes the process also applicable in organic farming, the risk of sulphur overfertilisation after slurry application is eliminated, the problem of concrete corrosion in slurry channels and storage tanks is minimised and the methane gas yield when using slurry in anaerobic digestion is increased.

In laboratory and technical scale (500 l), the required amount of calcium additive for the precipitation of the carbonate buffer as well as the influence of a separation of the slurry on the amount of acid and the subsequent pH development will be investigated. The amount of acid for acidification of the slurry with different acids is also to be quantified. During the subsequent storage, the NH3 and CH4 emissions from the acidified liquid phase will be investigated in addition to the pH value. In addition, biogas potential determinations of the different slurry variants will be carried out.

The already established acidification technology is extended by a carbonate precipitation module and a separation unit. An alternative pH determination instead of pH sensors is being investigated. Since the pH value in slurry is strongly dependent on the carbonate buffer, a technology for volumetric carbonate content determination is to be developed which will automatically provide information on the pH value. As soon as the technology has been tested on a technical scale, it will be incorporated into the acidification system for the practical tests.

For the practical trials, the acidification system and two storage tanks (trial variant with acidification, control variant without acidification) are set up near a dairy barn. In the batch process, part of the fresh dairy cattle slurry is first separated. Calcium additives are then added to the liquid phase in the process tank. This is followed by a second separation. With the volumetric carbonate content determination, the pH value is calculated so that the appropriate amount of sulphuric acid can be added to reduce the slurry pH value to 5.5. The acidified liquid phase is then stored in the external double-walled storage tank. During storage, NH3 and CH4 emissions as well as odour measurements are carried out on the surface of the slurry.

Finally, the system is evaluated economically, taking into account the fertiliser balance, the emission reduction as well as the biogas potential.