The effect of three different types of glycerol on the performance of up- flow anaerobic sludge blanket ( UASB) reactors treating potato processing wastewater was investigated. High COD removal efficiencies were obtained in both control and supplemented UASB reactors ( around 85%). By adding 2 ml glycerol product per liter of raw wastewater, the biogas production could be increased by 0.74 l biogas ml(-1) glycerol product, which leads to energy values in the range of 810 - 1270 kWh(electric) per m 3 product. Moreover, a better in- reactor biomass yield was observed for the supplemented UASB reactor ( 0.012 g VSS g(-1) CODremoved) compared to the UASB control ( 0.002 g VSS g(-1) CODremoved), which suggests a positive effect of glycerol on the sludge blanket growth.