Two anaerobic filters were subjected to shock treatments in similar conditions, by suddenly lowering the temperature of the reactors by 10 degrees C in one case and by suddenly stopping the addition of alkali to the feed in a second experiment. In both cases the reactor packed with polyurethane sponge showed no variations in the different analysed parameters with respect to the values of the steady-state of reference, while the filter packed with rocks was more affected by the temperature shock than by the decrease of the pH of the feed.