Study on the flow characteristics and the wastewater treatment performance in modified internal circulation reactor

•A modified internal circulation reactor with external circulation system was made.•The MIC reactor enhances mass transfer effect relative to the IC reactor.•The performance of MIC reactor was better than IC reactor in different stages.•The hydrodynamics of the MIC reactor was analyzed using CFD method.•The optimal flow rate of external circulation in MIC reactor was 12 L min−1.

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.