The startup of a hybrid system consisting of an upflow anaerobic sludge blanket (UASB) and an anaerobic fixing filter (AFF) with internal hydraulic circulation and external sludge circulation was investigated. The reactor was rapidly cultivated using municipal sludge as a seed 38 d after a failed startup. During the operation, the average size of the granular sludge increased from 111 mu m to 264 mu m, and the sludge was uniformly distributed in the reacting region. Efficient performance was attributed to good hydraulic contact between the substrate and sludge and the low loss of sludge. However, excessive hydraulic circulation resulted in a sharp decline in the effectiveness. After a pause in the operation, a second startup was rapidly completed in 15 d, during which time the organic load reached 15.4 COD/(m(3) d). (C) 2009 Elsevier B.V. All rights reserved.