Developing the Anaerobic Digestion Process through Technology Integration

Process optimization is needed for the development and expansion of the biogas industry and to meet the ever growing demand for methane. This thesis explores process technologies for the development of the anaerobic digestion process and includes pre-treatments, studies on the effects of different mixing modes and evaluation of a water treatment technology. Two pre-treatments were evaluated, mechanical and electroporation, for treatment of ley crop silage. Mechanical treatment included two milling machines designed for recycling of paper, Grubben deflaker and Krima disperser, and showed an increased biogas production of 59 % and 43 % respectively as well as a positive energy balance and economic results.. Electroporation increased the biogas production with 16 %, however, development is needed to increase its energy efficiency. Digester mixing has an effect on the digestion result. The performed review and experiments show that the mixing demand increases with organic loading. Excessive mixing during process start up, instabilities and shock loads leads to increased volatile fatty acid concentrations and process inhibition. Reduction of mixing reduces the effects of process instabilities and periodical mixing with mixing breaks has been shown to be beneficial for biogas production. A high temperature membrane filtration unit was evaluated at 70 °C, 90 °C and 110 °C to determine separation efficiencies, permeation speed when treating process water at a biogas plant. Improved separation can increase the capacity of the substrate pre-processing and reduce process related problems. The results show a total solids separation of 60 %, and an increasing filtration speed with temperature with fluxes of between 113 and 464 L/ h m. The substrate pre-processing could theoretically handle up to 29 % more substrate as a result. Integration of these technologies in a biogas plant show that the pre-treatments studied exhibits a good performance when integrated and that mixing reduction has the potential to lower the process electricity demand by 23 % in the performed case study. However, even though the membrane filtration unit shows promising results it would demand a relatively high energy consumption and lead to limited benefits to a process already at it maximum organic loading. ISBN 978-91-7485-166-3 ISSN 1651-4238 Dedicated to my family!