Treatment of dyeing wastewater by MIC anaerobic reactor

•A modified internal circulation anaerobic reactor including an external circulation system only within the 1st UASB was made.•The MIC reactor enhances mass transfer effect relative to the IC reactor.•Granular sludge was used as the seed sludge.•The optimal flow rate of external circulation in MIC reactor was 420 L/h.•Applying the modified Stover–Kincannon model to the MIC reactor.

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.