In order to improve the performance of internal circulation(IC) reactor in which granular sludge had formed

an additional external circulation device was added to the reactor. The operation stability of this system was investigated at various reflux ratios of 0

1.0

2.0

3.0 and 4.0. Compared with without external circulation situation

additional device do not indeed damage anaerobic conditions inside the IC reactor and the reactor operation was stable at given range of reflux ratio. When chemical oxygen demand(COD) of influent was 6000 mg·L-1

hydraulic retention time(HRT) about 10h

and volume loading 14 kg·m-3·d-1

the MLSS of system (mixed liquid suspended solids) and the removal rate of COD increased with the growth of reflux ratio

and could reach the maximum 97.3% and the lowest COD of effluent 160 mg·L-1 respectively. With increase of reflux ratio

the total quantity of biogas produced increased to 171.2 L·d-1. The methane production increased at the beginning and then maintained stabilization

and was 91.7 L·d-1 at reflux ratio 2.0. Remarkably

biodiversity in granular sludge had been improved due to additional external circulation. After the system with additional external circulation operated for a period of time

the dominant bacteria for methane production changed from Methanobacterium to Methanococcus. After comprehensive consideration of energy consumption and COD removal rate

the optimal reflux ratio would be 2.0.