The aim of this study was to examine the pollutant removal performance of a combined process using horizontal subsurface flow
constructed wetland (HFCW) and an oxidation pond (OP) in the treatment of a decentralized domestic wastewater. Waters along
the water flow and at different depths of the wetland substrate were sampled and analyzed during continuous operation. It was
found that the HFCW-OP combined system presented good organic matter and phosphorus removal capacity. In the first year,
COD and TP outputs were 30.0 and 0.5 mg/L, respectively. The concentrations of NH3-N and TN decreased notably from 6.3 and
16.5 mg/L to approximately 0.2 and 5.0 mg/L after 120 days of operation. This decrease was mainly achieved due to the
formation of an anoxic environment in the wetland substrate. After 3 years of operation, however, the wetland showed different
degrees of substrate clogging and pollutant accumulation. Although the removal of NH3-N and TN were 88% and 90%,
respectively, COD removal decreased from 90% to 78%. By replacing the substrate and vegetation in the last two stages of the
wetland and increasing the depth of the oxidation pond from 1.2 to 1.5 m could improve the COD and TP removal capacity of the
HFCW-OP system effectively.