Effluent recirculation was investigated in a vertical flow reed bed system treating an agricultural wastewater. A comparison was made for the efficacy of this system before and after recirculation was adopted. Recirculation considerably improved the removal of BOD5, COD, suspended solids (SS) and NH4-N. The most significant improvement was made for NH4-N, and nitrification of NH4-N into NO2-N and NO3-N only took place when recirculation was employed. The removal Of PO4-P, however, was only slightly improved by recirculation. Generally, the removal rate of NH4-N increased proportionally with its loading onto the system. With recirculation the pH value of the wastewater gradually decreased as a result of alkalinity being consumed during the nitrification process, whereas without recirculation the pH remained virtually unchanged. When recirculation was employed, the ambient temperature did not have a significant influence on the removal of BOD5 but appeared to affect NH4-N removal. (C) 2003 Elsevier Science Ltd. All rights reserved.