cultivation process. This gives a buffering effect as it binds the minerals, so that any slight imbalance can be accommodated. These methods ensured that the pH mostly stayed within the optimal range without any problems. An acidic fertilizer (nitric acid/phosphoric acid) is needed every now and then in order to restore the balance. The desired EC value is 1 to 1.5 mS/cm. The basis for the fertilisation of the duckweed is the liquid fraction of the digestate, because of its favourable N/P ratio. Every month, the results of the irrigation analyses served as the basis for adjustments using liquid fraction, artificial fertilizers, and trace elements. CO2 content and air humidity Bringing the ventilation air from the stables in the greenhouse helped keep the CO2 concentration between 1,500 and 2,000 ppm. That meant that this parameter was non-limiting for the cultivation of duckweed. The relative humidity level (measured 25 cm above the surface of the water) fluctuated mostly between 60 and 80%. Oxygen content in the pond Stagnant water can quickly lose its oxygen, especially if it contains organic particles. Moreover, less oxygen can dissolve in water if the temperature rises (thermodynamic principle). When the liquid fraction of the digestate is used as fertilizer, a small quantity of organic particles is contaminating the water. The breakdown of this organic matter requires oxygen and therefore oxygen has to be added to the water. In anaerobic conditions of the water all kinds of toxic compounds are formed, such as nitrite and sulphite (Schuurman, 2014). This can result in a considerable reduction of growth rate and to the death of the duckweed, leaving rotting material floating on the water and allowing algae to grow in large quantities. The oxygen content of the water appeared to be too low in the spring of 2015. Therefore, a submersible pump and bubble aerators were installed, which provided sufficient oxygen for the duckweed pond. The same effect was achieved with a sprayer and stirrer. Following a minor dip in production, fresh duckweed soon covered the surface of the pond again. Duckweed density and harvest During the growth season, the target for the density of the duckweed on the pond was 400 to 1,000 grams/m². This varies from one species of duckweed to another. In places where it accumulates, the density can be up to 2,000 grams /m². The rule of thumb is that to make the best possible use of incoming sunlight, the entire water surface should be covered with duckweed. However, efforts must be made to prevent the duckweed from piling up, otherwise it could die. Harvesting usually takes place as soon as the duckweed starts to accumulate in multiple locations. Until August 2015, the harvesting process involved the use of a scoop net. After that time, it was carried out mechanically, with the help of a slowly rotating conveyer belt. This worked very well. Timing the harvest is easy by using the light reflection from the pond surface as an indicator for the moment when the harvest should be started and stopped. By allowing the duckweed to dry out for several hours on the floor or on a grate, much of the water is removed. The duckweed is then mixed in with the calves’ 119 Pagina 126
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