In this experimental study the treatment efficiency on a milk based synthetic organic waste is examined of a fixed bed reactor with immobilized cells in organic support (straw). The aim of this research effort was to demonstrate an anaerobic treatment system which is flexible enough and can be used widely at small agroindustrial units as a low-energy-demanding treatment system. It can also be possibly used for the stabilization of the organic support which can consequently be incorporated for the faster evolution of the composting phenomenon A significant number of concentrations at the entrance of the fixed bed reactor, with a volume of V = 10.7 l, constructed from plexiglass and downflow fed, has been applied at different hydraulic retention times and at different temperatures (35 degrees, 25 degrees, 15 degrees C). The examined reactor operates in plug-flow mode and the conclusions that were drawn from the experiments, with the above anaerobic treatment system are as follow. The treatment system can treat high and low strength effluents with considerable treatment efficiency. It can treat low strength effluents, with sufficient treatment efficiency, at medium and low temperatures, for anaerobic digestion standards. The time for adaption between different applied loadings at the inlet is quite short, which indicates the considerable flexibility and utility of the system.