TY - THES A1 - Knoop, Christine T1 - Anaerobic treatment of municipal organic waste from separate collection: digestate properties and substance flows during two-stage digestion and subsequent aerobic treatment N2 - 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. Results show that MOW composition is rather heterogenic, which is reflected by a high variance of biogas yields and degradability as well as digestate properties. The heterogeneity of nutrient composition is much lower than the heterogeneity of heavy metal contents. A positive correlation was found between impurity contents and the concentration of Cd, Pb and Zn in the samples. The impurity content of investigated MOW samples is up to 12%. A few of the investigates MOW samples already exceed thresholds of BioAbfV for Cd, Pb and Zn. Results underline the influence of feedstock quality on final digestate characteristics. Organic matter degradation during biological treatment leads to further accumulation of unwanted substances. Subsequently, high impurities contamination of feedstocks and heavy metal redistribution during digestion and post-treatment become an exclusion criteria for soil amendments. Therefore, a low feedstock contamination needs to be striven to guarantee the production of marketable, high quality MOW-based soil amendments. Results from the laboratory experiments show that nutrient and heavy metal concentration in the solid digestate is increasing during digestion except for N, P and Mg. The deficit of N, P and Mg amounts to the same extent of up to 45% and a struvite precipitation can be expected. There was also a deficit of Ca, Cd, Cr Ni, Pb and Zn in the solid digestate. This emphasizes that element retention in the digester system has a decisive impact on nutrient contents of digestates. Regular percolation transports mineralized elements from the location of release into storage tanks and the fixed-bed digester, where they may deposit. This distinguishes element distribution in two-stage systems from one-stage systems. In contrast, K is the only investigated element, which was completely rediscovered in the solid digestate during laboratory experiments. K+ is assumed to adsorb to free exchange sites of existing molecular structures of the solid matter of digestate. The total element mineralization was lower during semi-technical scale digestion, caused by shorter retention of MOW, which also resulted in a lower VS (volatile solids) degradation. Release of Mg did not occur to the same extent as for N and P. Consequently, as Mg is the limiting element for struvite precipitation, this effect is assumed to be rather low during semi-technical scale experiments. Furthermore, no K enrichment in the solid digestate was discovered during semi-technical scale experiments. Therefore, it can be concluded that K adsorption occurs only at longer retention times, which determines the rate of degradation, the complexity of the structure of the remaining organic matter and the number of free exchange sites. In comparison to nutrients, total heavy metal loads in the substrate flow were not significantly affected by semi-technical scale treatment. Therefore, the overall share of heavy metals in MOW is assumed to be present in stable compounds. However, results of laboratory digestion with longer retention time revealed a potential heavy metal release from MOW of up to 15% for Cd, Cr, Pb and Zn. The mobility of heavy metals may be promoted by dissolved organic carbon (DOC). When applicated to soil, organometallic complexes can easily be taken up by plants. Consequently, the binding of heavy metals onto soluble organic matter during digestion may lead to an increased heavy metal toxicity in soil after digestate application. To prevent this, impurities need to be removed from MOW in advance of anaerobic treatment. Investigations on different treatment procedures confirm that in comparison to drying, composting of MOW digestates leads to a significant accumulation of K, P, Mg, Ca, Cd and Cr in the digestate. Although the element concentration is higher in composted batches, the water-extractability of nutrients, heavy metals and carbon is significantly lower than from dried batched. A significant correlation was found between the release of Zn, Ni, Ca, Mg and the pH as well as the DOC content of eluates. Results confirm that organic matter stability is a key factor regarding the potential performance of digestates in soil. The elution of DOC may be a good measure to evaluate digestate quality and to decide on treatment measures. Investigated digestates were characterized by a high residual degradation potential. Subsequently, for these digestates composting seems to be the better treatment option as it leads to an improved organic matter stability and higher nutrient concentrations. However, heavy metal accumulation is problematic. Sieving of composted digestates showed that heavy metals are not evenly distributed and that highest heavy metal concentration can be found in the fractions below 1 mm. Consequently, sieving leads to further heavy metal enrichment in the digestate, as fractions with lower contamination are refused in the sieve overflow. Experiments also show that sieving below 10 mm is needed to ensure the legal threshold of 0.5 % impurity content. However, this is related to a mass loss of up to 50%. Again, this emphasizes the importance of feedstock purity. Investigations of semi-industrial scale MOW treatment show that about 60% of nutrients contained in the MOW inflow can be retained in the process outflow during two-stage digestion and subsequent aerobic post-treatment. A higher nutrient release was observed during digestion forced by percolation. Additionally, impurities removal causes a significant reduction of the final nutrient outflow. Based on the results of substance flow analysis, soil amendments from two-stage digestion and subsequent composting of 9 million tons of MOW could provide 1.5% of annual demand for N, 4.1% of annual demand for P and 3.3% of annual demand for K to agriculture. However, investigations have shown that N availability is very low in composted digestates. Consequently, the final output of investigated MOW treatment is most suitable to complement P and K fertilization. Future research is needed to investigate the recovery of nutrients, which are mineralized during anaerobic treatment. Furthermore, the extent of heavy metal mobilisation remains unclear. A deeper understanding of how contaminants behave during biological treatment is needed to estimate the ecotoxicological potential of digestates when applied as soil amendments. Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:co1-opus4-48209 PB - Chair of Geopedology and Landscape Development CY - Cottbus ER -