The wastewater from a food processing factory, characterised by fluctuations of flow rate, organic strength, and pH, were originally treated by a traditional suspended-biomass digester working at about 25°C. In order to improve the digester efficiency, either in terms of degradation ability or biogas production yield, a set of tests has been carried out on laboratory scale, whose results indicated the way to correctly transform it into an anaerobic hybrid filter. The unacceptable conversion yield of organic substances into biogas observed in the original system has been improved by the presence of the filling medium, due to a marked increase in biomass retention time.The start-up of anaerobic digestion has been studied in this reactor at two different temperatures (25 and 30°C), in order to evaluate the possible advantage of heating the system, simulating continuous variations in feed strength, pH, and composition. IntroductionWastewaters from food processing plants can effectively be treated by anaerobic digestion because of the favourable characteristics of these effluents, such as their content in organic substances, relatively high temperature [1], and good biodegradability [2]. In fact, due to this last peculiarity, their digestion does not require any hydrolytic pre-treatment like that proposed for entirely hemicellulosic or lignocellulosic wastes [3,4], hardly ever the exocellular hydrolysis being the limiting step of the process. Further reasons of the increased interest towards anaerobic digestion of these residues, with respect to traditional aerobic processes, are the formation of valuable products [5] as well as the savings of the high costs of energy to run aerobic alternative process [1].At the moment, most of the organic effluents and wastes coming from industrial, municipal or agricultural activities are treated favourably by anaerobic digestion [6].The traditional suspended-biomass digester, that is in general the optimal solution for the treatment of wastewater with relatively high suspended solids and organic content, working at an average hydraulic residence time of only 72 hours, does not allow the development and the retention of a stable population of low growth rate methanogenic bacteria. So, it is necessary to change the digester configuration, in order to promote the micro-organism retention time and to make it independent of the hydraulic residence time.Among the alternative solutions that have been proposed to this purpose, the fluidised bed reactor is certainly the most attractive, but, at the same time, it is still quite complex for full scale industrial applications. Food processing wastes with high organic content were successfully digested in a fluidised bed reactor jointly with wastewater coming from the washing operations of wine industry [7]. A methane production yield ranging from 98.6 to 85.0% of the maximum theoretical value was in satisfactory agreement with the results obtained by Boening and Larsen for whey [8].A simpler solution is provided by the anaerobic hybrid filter...