The oxidation of dry sewage sludge evolves quite differently depending on the origin of the sludge, even in the case of predominantly municipal origin. The differences are less evident by analyzing the materials with traditional techniques of thermogravimetric analysis suggesting that they derive predominantly from physical, rather than chemical features. This is clearly indicated by thermogravimetric measurements on larger samples (tens of grams), where heat- and mass transfer emerge. Several reaction phases during smouldering are very well identified, operating at sufficiently low heating rate. Mixtures of different sewage sludges average the reactivity, but analytical vs. larger samples suggest different effects on the global reactivity; it appears anticipated in differential scanning calorimetry and reduced on larger samples, presumably because of the ashes that limit the permeation of oxygen. The attenuation of the reactivity is remarkable in the oxidation of char. The ashes produced by the more reactive component, once totally burnt, limit the oxidation of char in the less reactive one, reducing the rate of oxygen transfer. At the same time, ashes provide an additional shield to heat dissipation, allowing the smouldering to progress longer. Ashes showed clear catalytic effects on small samples, but in larger samples the ashes' prevailing effect is a physical, non-chemical process that affect the total smouldering rate, likely to control smouldering at large scale. The large gap between analytical techniques and measurements on larger samples suggests the importance to validate the conversion of unconventional solid fuels at a larger scale, where physical processes of heat- and mass transfer likely to limit the overall rate in real scale applications can be identified. We also isolated intermediate chars showing a different reactivity depending on the origin of the sludge.