BACKGROUNDBiofilm processes have proven to more efficiently remove xenobiotics than suspended growth systems. They offer higher solid retention times to prevent washout of adapted microorganisms. The main problem with anaerobic filters is clogging when the biofilm develops rapidly or after long operating periods. In this work, an anaerobic filter was operated for 466 days to evaluate the degradation of Direct Blue 2 (DB2) azo dye and the influence of organic loading rate on the hydraulic behaviour and on azo dye removal. Axial Dispersion and Wolf and Resnick models were used for the analysis of tracer curves. RESULTSTracer tests showed that the anaerobic filter behaves predominantly as a plug flow reactor and as the biofilm grows thicker with increasing organic load, the void volume and the hydraulic retention time decrease. In addition, with increasing organic loading rate, the dispersion coefficient increases. The anaerobic biofilm removed up to 60% of the Direct Blue 2 azo dye. CONCLUSIONThe higher the organic load, the better the azo dye was removed. Both the dye Direct Blue 2 and its degradation products (benzidine and 4-aminobiphenyl) can be removed by an anaerobic biofilm. The anaerobic biofilm did not only break the azo bonds but also decomposed benzidine to 4-aminobiphenyl. (c) 2014 Society of Chemical Industry