Investigation of the accumulation of aromatic compounds during biogas production from kitchen waste
Introduction
As two million tonnes of food waste are produced in Germany every year, this presents a tremendous potential of utilisable biomass (Adolph et al., 2004). Because of its high content of water, biogas production from this waste makes more sense than non-biological disposal methods like incineration.
These wastes were mostly disposed of in the past by feeding livestock. However producing biogas from leftover kitchen waste is becoming an ever more important waste disposal alternative as feeding such waste to animals has been prohibited throughout the EU since November 2006 (Regulation (EC) No 1774/2002).
Process stability is important for the operation and economy of a biogas plant. The imbalance often effects the methanogenes and leads to VFA accumulation (Ahring et al., 1995). Process failures are usually the result of higher concentrations of degradation products. Kitchen waste has a high protein and fat content. It is important that some substrate or degradation compounds like inhibitory long-chain fatty acids equally affect all degrading groups in the anaerobic process (Koster and Cramer, 1987). Furthermore the high protein content of food waste can lead to inhibitory effects from toxic ammonia or sulphide concentrations (Braun et al., 2003). For both compounds the inhibitory effect is higher with increasing pH and temperature. Adaption to inhibitory concentration is possible over time and the biogas plant can work with a stable performance but with a lower biogas yield. As a result, the plant is often operated at lower organic loading rates to prevent process failure (Ahring, 2003).
Good process monitoring would, however, make it possible to optimise the operation and to ensure maximum use of the whole plant capacity without having any troubles with process imbalance. However in general there is little information about the condition of the process and few or no devices are in place to inform the operator of the biogas plant when the process is becoming unstable. pH, alkalinity, concentration of VFA, biogas production rate and biogas composition are commonly used indicators for process control. The potential of a single parameter to serve as an indicator depends on the process constitution and the substrate characteristics and cannot be generally applied. Using pH as a process indicator, for example, strongly depends on the buffering capacity. A minor change in carbohydrate content in the substrate can have a tremendous influence on the buffering effect of the system (Björnsson et al., 2000).
In this study, the anaerobic degradation of several types of kitchen waste was investigated in order to analyse changes in the metabolic status of the system depending on the organic loading rate. In addition to the measurement of intermediates like VFA, hydrogen sulphide or ammonium, the analysis of the fermentation media showed early accumulation of further intermediates which originated from the degradation of aromatic amino acids. To ensure that experiments are carried out which are of practical relevance, the investigations are made on two different adapted inocula, one from a biogas plant fed with swine manure and the second from a biogas plant fed with mono-fermented kitchen waste. There has been no publication of the potential use of such aromatic acids as indicator substances for the successful control of the biogas process.
Section snippets
Chemicals
Water (HPLC grade, J.T. Baker, Mallinckrodt B.V./Deventer, The Netherlands), acetonitrile (HPLC grade, Prochem/Weser, Germany), formic acid (50%, HPLC grade, Fluka/Buchs, Switzerland), standards for LC–MS (Fluka/Buchs, Switzerland and Merck/Darmstadt, Germany), NaHSO4, standards for Headspace GC (Fluka/Buchs, Switzerland and Merck/Darmstadt, Germany).
Quasi-continuous experiments
The kitchen waste degradation experiments were carried out in 0.1 L quasi-continuous reactors containing swine manure from a biogas plant
Quasi-continuous experiments with increasing substrate loading
The kitchen waste was first added at 0.85 g L−1 d−1. No metabolites were detected by LC–MS and Headspace GC. After reaching a steady state with a continuous biogas production, the addition of the substrate was gradually increased by 0.1 g L−1 d−1. The biogas-yield increased to a substrate specific maximum. The further increase in kitchen waste concentration resulted in a rapid decrease in biogas production. At the peak biogas-yield, the pH and the concentration of hydrogen sulphide, ammonia and VFA
Conclusions
Principal conclusions derived from this investigation are summarised as follows:
- 1.
The degradation of kitchen waste led to the accumulation of aromatic acids with higher substrate loading.
- 2.
The determination of ammonium or VFA content could not be found to be applicable parameters for successful process control in all experiments.
- 3.
The type of aromatic acids and content of these substances depended less on the biomass used than on the substrate used.
- 4.
The detection of phenylacetic acid was positive in
Acknowledgement
We would like to thank the Ministry of Education for its financial support.
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