BACKGROUND: This work is focused on the anaerobic biodegradation of wastewater from used industrial oils (UIO) recovery using a bench-scale expanded granular sludge bed reactor (EGSB) at room temperature. RESULTS: Biodegradability tests showed that this wastewater can be partially biodegraded under anaerobic conditions at mesophilic temperature. Low concentrations of wastewater caused an incremented specific activity of the acetoclastic and the hydrogenotrophic methanogens. Anaerobic biodegradation at room temperature is feasible at organic loading rates (OLR) lower than 5.5 g COD L-1 d-1. A further increase of the OLR to around 10 g COD L-1 d-1 had a detrimental effect on the system performance, making it necessary to work at mesophilic conditions. CONCLUSION: Anaerobic treatment using an EGSB reactor is a feasible option for treating UIO wastewater. Long-term treatment caused a specialization of the granular sludge, modifying substantially its microbial composition. Methane production was even stimulated by the addition of UIO wastewater at low concentrations. Copyright (c) 2012 Society of Chemical Industry