Novel insight of spatial mass transfer conditions of upflow anaerobic reactor

Refractory organics in industrial wastewater are difficult to treat and pose a serious threat to the environment. Upflow anaerobic reactors such as upflow anaerobic sludge blanket (UASB) and internal circulation (IC) reactors are most widely used in the world due to high efficiency, but the inside process remains unknown like a black box. In this study, spatial mass transfer conditions of a pilot-scale UASB reactor, being decomposed as the bottom, middle and top zones, treating pharmaceutical wastewater at organic loading rates (OLR) of 4.32-8.30 kg COD/(m(3).d) were investigated. In addition, the roles of external and internal factors such as organic loading rate and granule properties including size, density, porosity and permeability on mass transfer were investigated for the first time. The average molecular diffusion rates (F-MD) of reactor were 7.43 x 10(-8),1.4 x 10(-7), 2.0 x 10(-7) and 2.73 x 10(-7) mg/s respectively, while, average convective diffusion rates (F-CD) were 5.39 x 10(-1), 1.09, 1.32 and 1.42 mg/s at OLRs of 4.32, 5.70, 6.92 and 8.30 kg COD/(m(3).d) respectively. Interestingly, mass transfer rates increased with the increase of OLR and decreased with a height of the reactor, also, it is directly dependent on granule size. The novel results reveal that the bottom zone is of the fastest mass transfer as compared to middle and top zones of the reactor; furthermore, it is the main degradation zone due to not only high mass transfer but also low feed/sludge and high granule level. The performance of upflow reactors could be enhanced for higher biogas production by installation of biofilm zone in the top zone to replace granules of that zone. The results could facilitate understanding bioprocess in upflow reactors. (C) 2018 Elsevier Ltd. All rights reserved.