Disposal of plastic solid waste is primarily accomplished by landfilling, degradation, and recycling. Degradation is highly justified in some applications. In this work, high impact polystyrene blended with starch (16-32% weight) in a "V" mixer is degraded in concentrated activated sludge for 12 weeks. Samples of 0.75 mm thickness were obtained in ribbon geometry by single screw extrusion. Mechanical degradation was determined by stress-strain tests. Gel permeation chromatography measurements established changes towards lower polymer molecular weights. Scanning electron microscopy examination showed the presence of microorganisms in the polymer samples, as well as changes in polymer morphology in areas near holes produced in samples. Results demonstrate the utility of concentrated activated sludge in polymer degradation and the effectiveness of starch inclusion as a filler to accelerate the structural molecular changes.