Abstract The main objectives of the present investigations were to determine the maximum extent of anaerobic biological degradation of the soluble, colloidal and Coarse Suspended Solids fractions of slaughterhouse wastewater, in order to elucidate the mechanisms involved in the removal of these organic pollutant fractions and to determine the rate-limiting steps in the degradation of each fraction of the wastewater, and to assess the effects of the fractions on the methanogenic activity of the sludge. The experiments were performed with membrane-filtered wastewater (wastewater mf), paper-filtered wastewater (wastewater pf) and the whole (total) wastewater. The experimental arrangements consisted of a column containing granular sludge, through which the test wastewater was recirculated. The biodegradability tests with the Coarse Suspended Solids fraction of the wastewater were performed with granular and flocculent sludge in conventional stirred batch digesters. All experiments were performed at 30°C and 20°C. The maximum biodegradabilities (i.e. conversion into methane) found at 30°C were 75% for wastewater mf, 61% for wastewater pf and 67% for wastewater total, while at 20°C these values were 72%, 49% and 51%, respectively. The maximum biodegradability of the Coarse Suspended Solids fraction of the waste was 50% at 30°C and 45% at 20°C. The results indicate that an important mechanism in the removal of the soluble, and especially the colloidal, fractions of the wastewater is adsorption. Particularly at imposed high sludge loads of approximately 0·17 g colloidal-COD g −1 VSS the limiting rate of liquefaction of the absorbed compounds may result in a serious drop in the methanogenic activity of the sludge. The extent, as well as the rate, of the liquefaction of the adsorbed insoluble substrate material of the wastewater are the controlling factors with respect to loading potentials of the process, and consequently temperature is a factor of predominant importance. The methanogenic activity of the granular sludge deteriorates because of the relatively high degree of adsorption of the colloidal fraction of the wastewater to the surface of the sludge and its high fat content.