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Abstract :
[en] The European Union has been striving to become the first climate-neutral continent by 2050. This implicates an intensified transition towards sustainability. The most applied renewable energy sources are the sun and wind, which are intermittent. Thus, great fluctuating shares in the energy network are expected within the next years. Consequently, there might occur periods of no congruence between energy demand and energy supply leading to destabilization of the electricity grid. Therefore, an urgency to overcome the intermittency arises. One feasible option is to use a third renewable energy source, biomass, which can be produced demand-oriented. Hence, a flexible biogas plant running on a two-stage mode, where the first stage would serve as a storage for liquid intermediates, could be a viable option to create demand-driven and need-orientated electricity. Since vast amounts of food waste are thrown away each year (in 2015 they amounted 88 mio. tones within the EU-28, accounting for ca. 93 TWh of energy), one could energetically recover this substrate in the above-described process. This is a promising concept, however, not widely applicable as it faces many challenges, such as technical and economical. Additionally, food waste is inhomogeneous, and its composition is country- and collecting season dependent.
The motivation of this work was to contribute to a vaster understanding of the two-stage anaerobic digestion process by using food waste as a major substrate. At first, an innovative substitute for a heterogeny food waste was introduced and examined at two different loadings and temperature modes. It was proven that the Model Kitchen Waste (MKW) was comparable to the real Kitchen Waste (KW), at mesophilic and thermophilic mode for an organic loading in accordance with the guideline VDI 4630 (2016). For an “extreme” loading, and mesophilic mode, the MKW generated similar biogas, methane, and volatile fatty acid (VFA) patterns as well. Furthermore, another two MKW versions were developed, allowing covering a variety of different organic wastes and analyzing the impact of fat content on the biogas production.
Afterwards, a semi-continuous one-stage experiment of 122 days was conducted. It was followed by an extensive semi-continuous two-stage study of almost 1.5-year runtime. Different loadings and hydraulic retention times were investigated in order to optimize this challenging process. Additionally, the impact of co-digestion of lignocellulose substrate was analyzed. It was concluded that two-stage mode led to a higher biogas and methane yield than the one-stage. However, the former posed challenges related to the stability and the process maintenance. Additionally, it was found that co-digestion of food waste and maize silage results in methane yield, atypical for the acidic stage.
Apart from the experiments, the Anaerobic Digestion Model No. 1 (ADM1), originally developed for wastewater, was modified so that it would suit the anaerobic digestion of food waste of different fat contents, at batch and semi-continuous mode consisting of one- and two-stages. The goodness of fit was assessed by the Normalized Root Mean Square Error (NRMSE) and coefficient of efficiency (CE). For the batch mode, two temperature modes could be properly simulated at loadings conform and nonconform to the VDI 4630 (2016). For each mode, two different sets of parameters were introduced, namely for substrates of low-fat content and for substrates of middle/high fat content (ArSo LF and ArSo MF, with LF standing for low fat and MF for middle fat). The models could be further validated in another experiment, also using a co-digestion of lignocellulose substances. Further, parameters estimated for the batch mode, were applied for the semi-continuous experiment. It proved successful, however, due to a high amount of butyrate (HBu) and valerate (HVa), the model underwent calibration so that it could better predict the acids (model developed for one-stage semi-continuous experiment was called: ArSo M LF*). This could be validated on another semi-continuous reactor running on one-stage. Finally, the acidic-stage of the two-stage mode was analyzed. The model applied for one-stage fitted the data of the two-stage mode as far as the VFA are concerned. Nevertheless, due to a vast amount of acids, it was adjusted and called ArSo M LF**.
