Anaerobic digestion of organic wastes – clean, safe and environmentally friendly?

The majority of problems resulted from digestion of protein- rich substrates, such as slaughterhouse wastes, kitchen and dairy wastes. Complaints also occurred in two cases during application of the fermentation-residues on farmland. Analysis of process data of selected plants (especially of volatile fatty acids (VAT)), as well as the results of a countrywide survey of 31 biogas plants, with a wide range of input materials were reviewed. Both show that the fermentation process can become unstable and the biodegradation of the substrates incomplete when a critical threshold level of more than 20-30 % of protein rich substrates is exceeded. High concentrations of volatile fatty acids, among them very malodorous substances like i-butyric acid, and other products of incomplete protein degradation like p-cresol are formed. These conclusions of our investigation correspond with results from recent research conducted on a laboratory scale in Germany.