Drying of biogas digestate in a hybrid waste-heat/solar dryer and the effect on nitrogen emissions and fertilizer quality

As the number of biogas plants expanded in recent years significantly, the supply of nutrients per hectare in livestock intensive regions has also increased. Hence, nutrient and digestate management is becoming more important for the biogas plants operators. Therefore, the objectives of the present thesis were to investigate the technical applicability of solar drying systems with the additional use of waste-heat from biogas cogeneration plants to improve the efficiency of the drying line and to concentrate the nutrients in digestate. Additionally, this thesis focused on the volatilization of nitrogen during digestate drying and further to investigate the short-term plant uptake of nitrogen and phosphorus for differently processed digestate. The technical applicability was tested in trials in a large-scale hybrid waste-heat/solar dryer to study the influence of additional waste-heat utilization from a combined heat and power unit and a micro turbine on the drying process. Water evaporation and energy consumption of the drying process were measured to determine the drying performance and the drying behavior under different operating modes. The nitrogen emissions in the exhaust air of the large-scale hybrid waste-heat/solar dryer were detected by Fourier transform infrared spectroscopy. With the results of the measurements, the volatilization of nitrogen in the form of ammonia was calculated to quantify the emission rate and the nitrogen loss during the drying process for untreated digestate and the separated solid fraction of digestate. The short-term fertilizer quality of digestate was investigated in pot experiments with barley plants. The nitrogen and phosphorus uptake of the plants was determined for solid dried digestate, solid and liquid fractions of separated digestate and untreated digestate. Special focus was on the influence of substrate treatment on the nutrient uptake of the organic fertilizer. The study demonstrated that a hybrid waste-heat solar dryer presents a suitable low-maintenance and low-workload option. The drying of digestate leads to a concentration of phosphorus and organically bounded nitrogen.