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Valuation of fermentation residues from biogas stations

ABSTRACT The recent decade of unprecedented support for renewable energy in Central Europe has led to a boom in the construction of biogas plants (among others). As renewables are coming close to being price competitive, this trend is passing onto a global scale and it is the developing countries in particular that are interested in technology transfer. Regardless of whether the biogas plant processes biowaste or purpose-grown phytomass, nowadays both are interlinked with farming practice, because it generates voluminous quantities of highly-diluted fermentation residues (FRs). These are routinely plowed into local arable land, however, there are uncertainties regarding their agrochemical and economic value. The market price of the FRs has stabilized under one third of what was expected at the beginning of the biogas fever. This has caused unexpected cash-flow problems to some business projects. Analyses revealed multiple reasons for the initial overestimation. It is indicated that the most overrated factor is the quality of organic matter in the FRs. Its low biodegradability limits the metabolism of nitrogen-mineralizing soil biota, which consequently slows the transformation of organic nitrogen into mineral forms that are accepted by plants. To make matters worse, the slow decomposition of hardly biodegradable organic matter forms preconditions that lead to worse water-retention soil characteristics.