120 daily rations. The proportion of dry matter is determined by using an infrared dryer set at 105°C. Dry matter content varied from 5 to 7% of the weight of the fresh product. Results A total of 14,430 kg of fresh duckweed was harvested in 2015. This is 11,480 kg of dry matter per hectare. Because the other growth factors (CO2, nutrients, and pH) are non-limiting, the only measurements carried out are related to the duration and intensity of the solar radiation (just above the duckweed) and to the water temperature (just under the duckweed). The water temperature and solar radiation were measured continuously between May and October. The results are shown in the Figure below. Figure: Photoperiod and water temperature of the duckweed pond, May to October 2015. The optimal levels for the photoperiod and water temperature are 13 [hours] and 26 [°C] respectively. The Figure above suggests that the photoperiod never rose above the saturation level of 13 hours. This means that light was a limiting factor. It is also clear that the water in the pond was effectively cooled during the summer months by means of a spray installation. This prevented the duckweed from dying from overheating. During the oxygen shortage as a result of the high temperatures in May and June 2015, the dominant duckweed species in the pond became Spirodela sp, instead of Lemma Minor. Little is known in literature about the effect of a lack of oxygen on different species of duckweed. Spirodela sp. is better able to cope with extreme temperatures. Homogeneity of duckweed cover The results suggest that the distribution of the duckweed on the surface of the pond was not homogenous (with an average density of 350g /m² and a standard deviation of 50g /m²). The design of the pond is therefore not optimal. For example, the duckweed accumulated before the corners, whereas in other parts, especially close to the pumps the density of the duckweed was low. The uneven distribution of the duckweed on the pond surface makes it difficult to determine the right time to harvest. Model calculations The continuously measured water temperature and light intensity are the most significant input variables for the dynamic growth model of duckweed (Van den Top, 2014). On the basis of these growth factors, the model calculates a production of 23,160 kg of fresh duckweed. Converted into dry matter yield, the model calculated a yield of Pagina 127
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