Sequential treatment of PTA wastewater in a two-stage UASB process: focusing on p-toluate degradation and microbial distribution.

Two-stage upflow anaerobic sludge blanket (UASB) process was investigated as an efficient process configuration option for the treatment of purified terephthalic acid (PTA) wastewater. To study its feasibility in a defined condition, synthetic wastewater containing only the major pollutants (i.e., acetate, benzoate, terephthalate and p-toluate) was used. By focusing the role of the second stage on the p-toluate degradation, improved overall COD and p-toluate removal capacities of 4.18 and 1.35 g-thCOD/L·day could be achieved together with a complete COD removal efficiency. In this situation, all the pollutants except p-toluate were completely degraded in the first stage while 38 and 62% of p-toluate originally present in the wastewater were consecutively degraded in the individual stages. The concomitant methane production rate in each stage was 0.91 and 0.35 L/L·day respectively, and the methane yield on p-toluate was determined to be 0.12 L/g-thCOD. Batch tests using the granules obtained from each stage revealed that the acidogenic microorganisms enriched in both stages had a universal ability to degrade all aromatic pollutants present in the PTA wastewater. Moreover, image analysis using scanning electron microscope and confocal laser scanning microscopy combined with fluorescence in situ hybridization technique elucidated that the distribution of acidogens and methanogens within the granule was varied in each stage, which influenced the mass transfer regime resulting in the different pollutant degradation rates during the batch tests.