The contribution of natural zeolite to the enhancement of the anaerobic digestion processes of piggery waste and synthetic wastewater with similar strengths was studied. Natural zeolite of up to 1 mm particle size was used in batch experiments. The influence of the particle size of natural zeolite on the anaerobic digestion processes of these substrates was also studied. The anaerobic process was favoured by the addition of natural zeolite at doses of between 0.05 and 0.30 g/g of volatile suspended solids (VSS), the optimum value being 0.10 g/g VSS. A first-order kinetic model was used to fit the experimental data with a probability level of 95% (P less than or equal to 0.05). Values of the kinetic constants were determined to be dependent on the zeolite doses, achieving a maximum value (0.218/d) at a dose of 0.10 g zeolite/g VSS and a minimum value (0.145/d) at a dose of 0.40 g zeolite/g VSS. Zeolite and sand as supports for the immobilization of microorganisms were compared using a synthetic substrate and significant differences in TCOD removal efficiencies and methane production were not found between these two supports. It was found that, under the experimental conditions evaluated, the main mechanism of anaerobic process enhancement, in the case of piggery waste, was the high capacity of zeolite for microorganisms immobilization while the capacity of this support for ammonia nitrogen reduction was not relevant. However, in the case of synthetic waste, both the capacity for microorganisms immobilization and for reducing the concentration of toxic nitrogen (NH3) by the zeolite were relevant. (C) 2004 Elsevier Ltd. All rights reserved.