In flat-panel airlift (FPA) reactors developed by the Fraunhofer IGB even highly shear-sensitive microalgae can be cultivated photoautotrophically by simply using light, CO2 and mineral nutrients. In this project, processes for the repeated fed-batch as well as the continuous culture in FPA reactors were developed for the various filamentous cyanobacteria with proven repellent activity against the cabbage root fly. For the first time filamentous shear-sensitive cyanobacteria were cultivated in FPA reactors and both growth rates and the achievable biomass concentration were optimized. The most important cultivation parameter in this process is the ratio of light input by way of the reactor surface to the cell concentration in the reactor.
The digestion of cow manure and nutrient recovery
The fertilizer pellets were made with digestates from the digestion of cattle manure collected from selected organic farms. The anaerobic digestion of cattle manure to biogas was optimized in a two-step gas-lift reactor with a reactor volume of 2 x 100 liters. With only 14-day hydraulic retention time a biogas production of approx. 300 liters per kg of organic dry mass was achieved. The digestate was dried and the fertilizing qualities of the dried digestate were examined and confirmed in pot experiments with German ryegrass at the Fraunhofer IME. The advantage of ryegrass is that fructification pruning can be carried out three times within a short, three-month period. This way a high level of nutrient depletion by the plant can be attained.
Drying and pelletization
Through cultivating the cyanobacteria in the FPA reactors, we were able to produce a sufficient amount of biomass for two selected strains. These were then dried and combined with the dried manure digestate before pelletization. The cyanobacterial mass and digestate were dried with superheated steam (superheated steam dryer, SHS), developed by the Fraunhofer IGB, to form stable cyanobacterial flakes and digestate under atmospheric pressure (working temperature 120 – 160 °C, retention time 20 – 30 min). The use of superheated steam for drying provides excellent possibilities to optimize the drying process regarding drying time, energy consumption and other parameters such as product quality.