A synthetic wastewater was treated in six lab-scale wetland reactors, which had Phragmites australis planted in different types of media: gravel, organic wood mulch, and mixture of gravel-wood mulch. Based on the type of wetland substrates, the wetland reactors were arranged into three parallel systems; each system consisting of a single vertical flow (VF) wetland reactor followed by a horizontal flow (HF) wetland reactor. A comparison of pollutant removal performances among VF wetland reactors indicated higher removal efficiencies in the VF wetland column with organic mulch substrate, as measured by the percentage removal of ammoniacal nitrogen (NH4-N, 99.6%), total nitrogen (TN, 97.8%), total phosphorus (TP, 60.3%) and biochemical oxygen demand (BOD5, 71.3%); this was primarily due to enhanced oxygen transfer and supply of organic carbon from the wood-mulch substrate for heterotrophic denitrification, and adsorption of phosphorus in the humus materials. Among the HF wetland reactors, conventional gravel substrate was most efficient for the removal of ammoniacal nitrogen, total nitrogen, phosphorus, and organic matter, as the percentage removals of ammoniacal nitrogen and organics in this reactor reached 72.0% and 44.0%, respectively. In contrast, the other two HF reactors, which employed wood mulch and gravel-mulch media, caused net increases in organics, phosphorus, and suspended solids (TSS) in the synthetic wastewater. Overall, the results demonstrated the potential of using organic media in vertical flow wetlands to enhance pollutant degradations, but the organic media should not be used in horizontal flow systems. (C) 2011 Elsevier B.V. All rights reserved.