The public defense of Eva Nordlander´s doctoral theis
Doctoral thesis and Licentiate seminars
The public defense of Eva Nordlander´s doctoral thesis will take place at Mälardalen university in Västerås
The title of the thesis is “System studies of Anaerobi Co-digestion Processes”.
The faculty examiner is Professor Krist Gernaey, Technical University of Denmark; and the examining committee consists of Docent Serna Ahlgren, Swedish University of Agricultural Sciences; Docent Ulf Jeppsson, Lund University; Professor Xiaoyan Ji, Luleå University of Technology.
Reserve: Docent Lena Johansson Westholm, MDH
The doctoral thesis has serial number 237
To achieve the European Union goals of reducing greenhouse gas emissions, increasing the share of renewable energy, and improving energy efficiency, a broad perspective is needed. The production of biogas through anaerobic digestion has the potential to contribute to achieving this goal. It is important that the biogas is produced as efficiently as possible and with low emissions of greenhouse gases. Anaerobic digestion is used in several different systems, both in stand-alone biogas plants primarily intended for biogas production and also in wastewater treatment to treat sewage sludge.
The objective of this thesis is to evaluate different ways of improving processes containing anaerobic digestion. Modelling and simulation are tools that can be used to improve the operation of processes. In this thesis, a full-scale digester has been simulated using two different models, an empirical model and a mechanistic model. Another way of improving the energy balance for such systems is to reconsider the substrates that should be used. This thesis also includes studies of two systems. The first is a life-cycle assessment of a biogas plant in which replacing the ley crop with microalgae is evaluated. The second system studied concerns the inclusion of microalgae in the biological treatment of three wastewater treatment plants. Further, the co-digestion between microalgae and sewage sludge has been simulated to evaluate the effect on biogas and methane yield.
Results show that both empirical and mechanistic model could successfully simulate the raw biogas outflow and the methane content of that biogas. The greatest hindrance in using the models was in measuring parameters related to the characterisation of the substrates. The mechanistic model requires a detailed characterisation of the substrates, which is challenging and not covered by measurements usually made at the biogas plant.
Studying the inclusion of microalgae at the wastewater treatment plant showed that the microalgae had the potential to reduce both energy use and greenhouse gas emissions for the three wastewater treatment plants. However, the land requirement of the current biological treatment needs to be increased several times. At the biogas plant, the co-digestion with microalgae would require greater energy use from a life-cycle perspective than co-digestion with the ley crop silage. On the contrary, the microalgae have the potential to reduce greenhouse gas emissions.
Findings from the simulations of co-digestion with microalgae and sewage sludge showed a reduction in biogas and methane production when replacing part of the incoming waste activated sludge with microalgae.