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A modified internal circulation (MIC) anaerobic reactor with an external hydraulic circulation system to strengthen mass transfer between sludge and substrates was used to treat dyeing wastewater and its performance was evaluated. During 110 days of operation, the MIC anaerobic reactor underwent three stages: start-up stage, improving stage, and steady stage. At the steady stage, COD removal efficiency, VFA concentration, and biogas production were approximately 85%, 158 mg/L and 1.65 L/L d, respectively. The color removal efficiency was from 77% to 90% in the whole experiment. Compared with the initial granular sludge, the sludge grew bigger with the portion of the granular diameter over 2 mm increasing from 77.9% to 85.5% and the suspended solid (SS) concentration increased from 66.1 to 75.9 g/L. The external circulation system not only shortened the startup time of the IC reactor treating wastewater, but also improved the pollutants removal in anaerobic reactor with a reasonable hydraulic upflow velocity. However, excessive hydraulic upflow velocity (40.9 m/h) caused the granular sludge disintegration and a decline in COD removal. Evaluation of the MIC reactor using the modified Stover–Kincannon model revealed the maximum removal rate constant (Umax) and saturation value constant (KB) of 31.25 g/L d and 34.94 g/L d, respectively.

A modified internal circulation (MIC) reactor with an external circulation system was proposed and the performance of treating dyeing wastewater using both MIC and typical IC reactor were compared. Utilization of the external circulation system in the MIC reactor could dramatically improve the mixing intensity of the biomass with the wastewater and resulted in better performance. The COD removal efficiency, biogas production, volatile fatty acids and effluent color were approximately 87%, 98 L d−1, 180 mg L−1 and 100 times, respectively, in the MIC reactor with a hydraulic retention time of 5 h and organic loading rate of 15 kg COD m−3 d−1. The hydrodynamics of the MIC reactor under different flows rate of external circulation were also analyzed using computational fluid dynamics (CFD) method. The optimal flow rate of external circulation was 12 L min−1, which resulted in a corresponding up-flow velocity of 40 m h−1. The consistency of the result between experiment and simulation validated the scientificity of CFD technique applied to numerical simulation of the MIC reactor.