This study demonstrates that the degradation of the organics of oil shale processing wastewater is promoted by shale ash. The wastewater TOC was approximately 4800 mg/L with at least 40% being biorefractory compounds. Sampling of effluent from a 1 m column packed with shale ash showed that over 96% of the TOC was removed at an influent flowrate of 40 L/m(2)/d. On-line CO2 measurements showed that 86 +/- 2% of the TOC was biodegraded to CO2. Control experiments included a packed column with glass beads with no TOC adsorption capacity; and stirred sequencing batch reactors with and without 5-10% (w/w) shale ash suspensions. Less than 65% of the TOC was found degraded in the controls and neither extending the hydraulic retention time nor reducing the organic loading enhanced the TOC removal efficiency. The results suggest the optimum degradation environment for the wastewater is within a biofilm with shale ash as a substratum. (C) 2014 Elsevier Ltd. All rights reserved.