Anaerobic treatment of municipal organic waste from separate collection : digestate properties and substance flows during two-stage digestion and subsequent aerobic treatment

Abstract

The provision of nutrients and organic matter to arable soils is critical to facilitate an intensive agriculture and secure long-term soil functionality. Municipal organic waste (MOW) is rich in nutrients and organic matter and its recycling onto agricultural lands presents a promising alternative to conventional fertilizers. In comparison to common aerobic treatment, the application of the biogas technology enables the recovery of both energy and soil amendments. The introduction of a mandatory separate collection in Germany in 2015 reflects the political will to increase MOW recovery rates and facilitates its utilization as feedstock in biogas plants. However, MOW is a challenging feedstock as its composition varies and it is often contaminated with impurities such as plastics and metals. The two-stage anaerobic digestion process with dry fermentation is very robust and offers possibilities for process control so that no extensive MOW pretreatment is required. To close nutrient circles, remaining digestates shall be processed to soil amendments, which can be redistributed to arable land. However, less is known about digestate properties from two-stage digestion of MOW and how they are influenced during the treatment process. Furthermore, only scarce information on nutrient recovery rates and the accumulation of elements during processing is available. Therefore, this thesis investigates the development of digestate properties during anaerobic and subsequent aerobic treatment at laboratory and semi-industrial scale. During a first experiment, changes in nutrient and heavy metal concentration in the solid digestate were monitored during anaerobic treatment of MOW in a two-stage laboratory biogas plant. A second investigation related amendment properties of MOW digestate of one origin to different post-treatment procedures. The impact of drying, composting and sieving on final digestate properties and specifically nutrient availability and heavy metal and carbon elution was evaluated. A third experimental approach investigated total material and substance flows during treatment of source-separated MOW in a semi-industrial scale two-stage biogas plant and subsequent digestate composting including impurities removal.