Green fertilizer based on pig manure or digestate

It has been stated that the process of nitrogen reduction in manure processing by means of blowing air currents through the manure is a waste of energy and financial resources. However, the DIGESMART PROJECT is proving that the solution could be feasible for certain locations where transportation of pig manure or digestate from biogas plants involves great distances.

The DIGESMART project is focused on lowering the costs for processing, storage and spreading of the digestate, while the project partners also want to create a higher value for digestate derivatives. For this reason, the project partners want to attain greater efficiency in the process of recovering these products.

Two routes are being explored by the project partners. On the one hand, work is being carried out on a system that recovers ammonium from manure and digestate. At the same time, a solar installation is being tested as a drying system, through the application of heat to the digestate (currently pilot tests), with the ultimate aim being the potential use of the digestate subsequent to stripping. DIGESMART is not just a research project. For that reason, it aims to introduce the technology in at least seven countries, namely Belgium, Italy, Spain, The Netherlands, Germany, Denmark and France.

Nutrient recovery process through stripping

Several physicochemical processes are involved in the system. In particular, the thin fraction from manure or digestate separation is used as raw input material. Depending on the configuration, 50% to 95% of the nitrogen is recovered by means of stripping the ammonium and scrubbing it with nitric acid to form ammonium nitrate (NH4NO3).

This technique is not new but it has been optimised in terms of energy efficiency for the type of input material. In the initial stages of the project, this was evaluated by means of computer modelling and pilot testing. Now, the experiment is being carried out through the installation of the DIGESMART project with a process capacity of 25 tonnes a day. The process can deliver a product with a fixed composition of 51% – 53% ammonium nitrate, which is the equivalent of an 18% nitrogen concentration. The resulting solution is transparent and particle-free.

The process starts by adding calcium oxide to the liquid in order to increase the pH value. A valuable side effect is the formation of calcium phosphate that can be recovered by precipitation and settling. A lower phosphate content is important in terms of staying within the limits required for the fertiliser produced. Calcium phosphate can be added to the compost to ameliorate the quality.

Nitrogen levels are reduced to 0.5 – 1 kg N/ton to maintain the energy balance of the system. Phosphorus levels are reduced to 0.1 kg P/ton, which is sufficient for subsequent application to land or similar uses.

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Paz Gómez (AINIA), Remigio Berruto (DEIAFA-UNITO), Stefano Bechis (UNITO), Denis De Wilde (DETRICON), Jonathan De Mey (BIOGAS-E), Paolo Rendina (SATA).


Article published in: FuturENVIRO #32 July-August 2016