Monitoring the anaerobic digestion process

  • In the anaerobic digestion process, microorganisms produce methane and carbon dioxide from organic substrates, either organic waste or renewable primary products. Being a versatile biofuel, biogas can be combusted in a combined heat and power plant to produce electricity and heat or, after purification, fed directly into the natural gas grid as biomethane. Due to the Renewable Energy Sources Act introduced in 1991 in Germany, this process became economically advantageous and led to a boom of biogas plants being built in Germany. A major problem of operating a biogas plant is to monitor an unstable process over time. Parameters like pH or redox potential do not necessarily suffice to estimate the degree of fermentation. At present, the preferred indication parameter are the concentrations of process intermediates, particularly short chain volatile fatty acids. They can be quantified with different gas or liquid chromatographic methods, which requires in-depth knowledge and expensive hardware and is usually carried out by specialized laboratories. Periodically, the digestate is sampled and sent in for analysis. Knowing the absolute concentrations of the different volatile fatty acids can only give a hint about the current fermentation status, though what would be more meaningful would be elucidating the dynamics of generation and degradation of the respective short chain volatile fatty acids. A new online technique using attenuated total-reflectance Fourier-transformed infrared spectroscopy (ATR-MIR-FTIR) was developed, which allows an online monitoring of the concentrations of the different volatile fatty acids in situ. This can give an insight into the dynamics of the anaerobic digestion process. It was adapted to a laboratory scale one-stage biogas plant fed with typically renewable primary products to simulate an agricultural biogas plant. Chemometric models were developed using spiked samples and samples from a real fermentation for acetic, propionic, iso-butyric, butyric, iso-valeric and valeric acid. The methods were evaluated by monitoring the startup phase of the anaerobic digestion of ground wheat in a 10 l continuous-stirred tank reactor. Sample preparation, recording and analysis of IR-spectra of digestate were fully automated. Predictions of the absolute concentration for acetic and propionic acid were reasonable, the existence of other volatile fatty acids could be detected. The developed anaerobic sensor system is able to determine their concentration dynamics and can thereby help to utilize unused potential in biogas plants. Another ascending problem are the substrates being used for the production of biogas. Renewable primary products are in direct rivalry with the agricultural and food industry. For a sustainable future, other biomass sources have to be made accessible for energy production. In contrast to energy crops, especially waste products can be used for energy generation without any concerns. The biogas potential of potential substrates is estimated with parallel running batch experiments. In the process, the minor gas flow rate of these lab-scale fermentations has to be monitored closely and accurately. Especially for this purpose, an easy to build and maintain automated biogas meter was developed to measure the biogas flow in anaerobic digestion experiments. The flow meter is built upon the open-source Arduino platform, and can therefore be easily enhanced or adapted to other environments. Recordings are sent via ethernet to a MySQL database, making the data widely accessible. By combining nine fermenters into an array, triplicate batch experiments according to VDI 4630 with negative control, positive control and the test substrate are monitored effortlessly. Concluding, this thesis focuses on optimizing the biogas production with the development of novel measurement instrumentation. The already well-established process of industrial digestion of energy crops is made transparent with an advanced onlin

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Publishing Institution:IRC-Library, Information Resource Center der Jacobs University Bremen
Granting Institution:Jacobs Univ.
Author:Harry Michael Falk
Referee:Roland Benz, Volker Hass, Thomsen Laurenz, Winterhalter Mathias
Advisor:Roland Benz
Persistent Identifier (URN):urn:nbn:de:101:1-201305294780
Document Type:PhD Thesis
Date of Successful Oral Defense:2011/12/11
Year of Completion:2011
Date of First Publication:2012/09/26
PhD Degree:Biochemical Engineering
School:SES School of Engineering and Science
Library of Congress Classification:T Technology / TP Chemical technology / TP248.13-248.65 Biotechnology / TP248.3 Biochemical engineering. Bioprocess engineering
Call No:Thesis 2011/61

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