Th is was very limited in 2015, because of the small surface area of the duckweed, but it could be improved if the area were to be enlarged. a biobed. Th is sharply increases the CO2 content in the greenhouse. Th e urine from the calves and the liquid fraction separated from the digestate contain nutrients needed for the cultivation of the duckweed. Th ey are diluted using clean well water, in order to obtain the correct concentration of minerals. Residual heat from the CHP unit is stored in water used by the farm and that of the neighbours. Th is heat is therefore not available for cultivating the duckweed. Th e diagram below shows a simplifi ed version of the system. Cultivating duckweed Th e experiences associated with the cultivation of duckweed are described in Chapter 2. Th e cultivation of duckweed was started in 2014, in the loft of the new calf building. Th e loft is actually a greenhouse of around 4,400 m², which has been built on the stable in which 1,600 calves are housed. Th e roof is made largely of transparent polycarbonate sheets. Th e heat from the calves radiates towards the concrete fl oor of the loft; this is direct fl oor heating. In addition, some of the ventilation air from the calves is directed through the greenhouse, after the ammonia and particles have been removed in 117 Figure: Scheme of the stable and the greenhouse with the fl ows of urine, liquid fraction digestate and duckweed as cattle feed. In the fi rst year, the cultivation process took place in small basins (50 m²), made of agricultural plastic. Various modifi cations have been made to the cultivation system, on the basis of these small-scale experiments. Th is laid the foundation for the cultivation of duckweed in 2015. Th e most important modifi cations were actively adding oxygen to the water, making improvements to control the pH, spray cooling of the air in the greenhouse, and adding humic acid as a mineral buff er to the duckweed pond. Th e experiences are described in detail in the report entitled ‘Kroosteelt 2014’ (see Appendix 1). In 2015, the cultivation of the duckweed was well under control. Th e growth fi rst became visible in the second half of February, and the harvesting of the duckweed started in March. Fresh duckweed was fed to the calves until November as part of their mixed diet. Th roughout this time, data on the cultivation conditions and the production were collected on a frequent basis. A duckweed growth model was developed and validated. Niek van den Top, a student at Wageningen UR, received the Shell thesis prize for his work on developing the model. Pagina 124
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