The feasibility was studied of anaerobic treatment of wastewater generated during purified terephthalic acid (PTA) production in two-stage upflow anaerobic sludge blanket (UASB) reactor system. The artificial influent of the system contained the main organic substrates of PTA-wastewater: acetate, benzoate, and terephthalate. Three parallel operated reactors were used for the second stage, and seeded with a suspended terephthalate degrading culture, with and without additional methanogenic granular sludge (two different types). The first stage UASB-reactor was seeded with methanogenic granular sludge. Reactors were operated at 37 degrees C and pH 7. During the first 300 days of operation a clear distinction between the biomass grown in both reactor stages was obtained. In the first stage, acetate and benzoate were degraded at a volumetric loading rate of 40 g-COD/L (.) day at a COD-removal efficiency of 95% within the first 25 days of operation. No degradation of terephthalate was obtained in the first stage during the first 300 days of operation despite operation of the reactor at a decreased volumetric loading rate with acetate and benzoate of 9 g-COD/L (.) day from day 150. Batch incubation of biomass from the reactor with terephthalate showed that the lag-phase prior to terephthalate degradation remained largely unchanged, indicating that no net growth of terephthalate degrading biomass occurred in the first stage reactor. From day 300, however, terephthalate degradation was observed in the first stage, and the biomass in this reactor could successfully be enriched with terephthalate degrading biomass, resulting in terephthalate removal capacities of 15 g-COD/L (.) day. Even though no single reason could be identified why (suddenly) terephthalate degradation was obtained after such a long period of operation, it is suggested that the solid retention time as well the prevailing reactor concentrations acetate and benzoate may have played an important role. From day 1 of operation, terephthalate was degraded in the second stage. In presence of methanogenic granular biomass, high terephthalate removal capacities were obtained in these reactors (15 g-COD/L (.) day) after approximately 125 days of operation. From the results obtained it is concluded that terephthalate degradation is the bottleneck during anaerobic treatment of PTA-wastewater. Pre-removal of acetate and benzoate in staged bioreactor reduces the lag-phase prior to terephthalate degradation in latter stages, and enables high rate treatment of PTA-wastewater. (c) 2005 Wiley Periodicals, Inc.