Re-Evaluation the Parameters in composting and anaerobic digestion

The basic theories of composting and anaerobic digestion (AD) are supposedly well known.However, constant odour problems in the compost after tunnel composting and low energy yields in the dark fermentationprocess may be due to an inadequate understanding of some of their basic parameters. In biochemistry it is taken as readthat an increase in temperature means an increase in biochemical activity as well. Composting, however, is supposed to bean exception: when the process temperature exceeds 60oC, the activity of the micro-organisms is claimed to be inhibited.Estimations of the performance capacity of the microbes carrying out these processes focus on the consumption of theatmospheric oxygen.

The role of bound oxygen as oxidizing agent is all but neglected. The microbes’ utilisation of thecarbon is thought to be selective: lignin is held to be highly resistant while cellulose is held to be highly biodegradable. Inacetogenesis of the AD the contribution of volatile fatty acids (VFAs) is supposed to be based on the acetic acid and higherVFA homologues. Formic acid is not believed to play an important role; indeed, reasons for its non-existence are not evenaddressed.Objective of the work: addressing and answering he following questions: 1) are microbes able to utilise the chemicallybound oxygen in substrates as oxidizing agent, and 2) should lignin be taken as a biodegradable constituent of plantmaterial? Further important questions are 3) does the composting really differ from other biochemical reactions with respectto temperature behaviour, and 4) does the contribution of VFA homologues in AD acetogenesis begin with formic acid?Main results: According to field experiments, it is evident that the efficiency of the composting process increases when thecomposting temperature increases from 60oC to 75oC. Composting thus does not necessarily differ from other biochemicalreactions with respect to temperature behaviour. The important consequences of the mineralisation are: 1) the absolute ashcontent of the substrate does not change, and 2) the relative ash content increases due to the decrease of organic matter.When the relative ash content in aerobic or anaerobic mineralisation increases e.g. from 10% to 20%, the organic mattercontent of the substrate decreases by 55%. The contents of carbon and nitrogen in the residual mass tend to increase slightlywhile the content of hydrogen tends to decrease slightly. Therefore, microbes need to be able to utilise the chemically boundoxygen in the substrate as well. Subsequently respirometric experiments on the efficiency of composting need to be reevaluatedby taking into consideration the role of chemically bound oxygen in the substrate as a microbial source of oxygen,in addition to atmospheric oxygen. The elemental composition and the amount of humic substances should be expressed inreference to ash-free organic matter in order to avoid the suppression by the increasing ash content.The zero-level atmospheric oxygen condition in composting should be described by the word ‘hypoxic’. It indicates thatalthough there is a shortage of atmospheric oxygen, conditions are not anaerobic, considering the possibility of utilizing thebound oxygen. For practical application, it is important to note that too-high intensity aeration (0.5 – 1.9 m3/s) dries out thecompost mass and may trigger the formation of offensive odours which remain in the compost mass after the reactor phase.According to Crawford and Crawford (1976), 47% of the 14C marked natural lignin is degraded by microbes as 14CO2.Lignin should be considered a biodegradable constituent in composting and in AD, like other organic compounds. Its rate ofdegradation may be slower than that of carbohydrates. Overcoming this slow-down is the challenge to practical processing.Formic acid cannot be detected by GC-FID in the presence of other VFA homologues. Theories of AD need to be reevaluatedby considering formic acid as one of the VFA components in the acetogenesis. In order to produce CH4 from CO2,microbes need excessive hydrogen, even up to three times more than the stoichiometry of the reaction CO2 + 4H2 􀃎 CH4 +2H2O would suggest. This restricts the production of hydrogen in dark fermentation instead of methane.Conclusion: Re-evaluating these parameters will broaden and clarify the theoretical and practical understanding of composting and AD.

Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2012 (Juni 2012)
Seiten: 8
Preis inkl. MwSt.: € 8,00
Autor: Kari Hänninen

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