The purification of nitrate-rich agricultural runoff by a floating-raft (FR) hydroponic system was investigated at 3-, 2- and 1-d hydraulic retention times (HRTs) with particular emphasis on nitrogen conversion and removal through the system. The FR system has a dissolved oxygen (DO) environment similar to the horizontal subsurface flow system, generally 0.00 mg L-1, that facilitates denitrification. An efficient nitrate-nitrite-nitrogen (NOx-N) removal, 91%, 97% and 71% on average at 3-, 2- and 1-d HRT, respectively, was frequently achieved. The mean retentions were 17-47% for chemical oxygen demand, 31-64% for total nitrogen, and 8-15% for total phosphorus for the FR system. Mass balance analysis implied that the detectable DO concentration in the reactor, as low as 0.7 mg L-1, played a very important role in the conversion and removal of NH3-N and NOx-N, which finally affected the NOx-N removal at 3-d HRT. (C) 2008 Elsevier Ltd. All rights reserved.