Browsing by Subject "Mechanical treatment"

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Optimierung der primären Gärung bei zweistufigen Biogasanlagen
(2016) Lindner, Jonas Philipp; Jungbluth, Thomas
The microbial conversion of biomass into biogas generally comprises several steps. These steps, are divided in accordance to the involved microorganisms and are often referred to as primary fermentation, secondary fermentation and methane formation. In contrast to single-stage, two-stage biogas system performs primary fermentation spatially separated from the methanogenesis in order to provide optimal milieu conditions for each group of microorganisms. There are many different reactor settings outlined in scientific literature for two-stage biogas production. For the digestion of energy crops or biowaste, discontinuously charged leach-bed reactors are often combined with anaerobic filters. The main disadvantage of this setup is the impossibility of regulating the pH-value in the first step, thus leading to fluctuating acid and gas production rates. To avoid this, new approaches aim to use continuous flow stirred-tank reactors for the process of primary fermentation, using chemical additives for the pH regulation. In the framework of this research, a process automation for a continuous two-stage system was developed and implemented in two lab scale plants at the State Institute of Agricultural Engineering and Bioenergy. Each laboratory plant comprised of a continuous stirred-tank reactor with an integrated filtration for solid-liquid separation and an anaerobic filter. In the primary fermentation stage, the adjustment of the pH-value was made by an indicatorbased return of alkaline effluent from the anaerobic filter. In order to evaluate and optimize the newly developed and completely automated pH-regulation system, this study investigated the (I) influence of the substrate characteristics on the degree of degradation and the biogas yields, (II) optimal pH-value for biomass degradation in the primary fermentation and (III) the possibility of enhancing methane yields by combined mechanical and enzymatic treatment of digestates with a subsequent refeeding into the process. The results of the investigations clearly showed the suitability of the system for a highprecision pH-regulation in primary fermentation for the tested pH-values 5.5, 6.0, 7.0 and 7.5. This unique technique enabled the continuous formation of organic acids and biogenic gases. Hay/straw, maize silage and sugar beet silage were digested at a pH-value of 5.75 in order to investigate the influence of different substrates on the two-stage system performance. Compared to the determined potential biogas yields, the recorded methane yields were 70.6 % lower for the hay/straw substrate and 31.3 % lower for maize silage in the two-stage system. Contrary to this, for sugar beet silage no difference in the gas yields between the batchtest and the two-stage system could be detected. Further investigations on the influence of pH-value on the degradation rate of lignocellulosic substrates showed an optimum pH between 7 and 8. The mechanical treatment of the digestates with the ball mill exhibited no losses of volatile solids through warming. The application of this procedure enhanced the specific methane yield from 9 to 17 % for maize silage digestate and hay/straw digestate respectively from the described laboratory plant. The treatment of the digestate obtained from a full-scale plant permitted a triplication of the specific methane yield at very low level. The combined mechanical and enzymatic treatment through the aerobic fungi “lentinula edodes” resulted in losses of volatile solids between 58.2 and 86.4 % for the hay/straw digestate and between 10.8 and 18.4 % for the substrate from the full-scale biogas plant depending on the incubation time. Furthermore, the investigations determined an increase in the lignin content of the hay/straw digestate by the combined treatment. Overall, using the digestate of the two-stage system, the mechanical/enzymatic treatment attributed to a methane yield loss by 86.4 %. In contrast, an increase of methane production by 134.5 % was observed with the full-scale digestates. This study has revealed that two-stage biogas systems are favorable only for easily degradable substrates. The phase separation was not beneficial for fiber rich substrates. Based on the results, a reactor cascade consisting of a continuous stirred-tank reactor and an anaerobic filter with similar milieu conditions seems to be well suited for the digestion of organic wastes. Due to the extraordinary high process stability, flexibility and high load capacity this system is very suitable for the treatment of substrates with extremely varying compositions. Moreover, within the study essential basics for the application of innovative fermentation procedures (e.g. pressure fermentation) were investigated. The combined mechanical and enzymatic treatment of digestates seems to be an interesting alternative to the established substrate pretreatment systems and it can be recommended also for single-stage biogas plants.