Effects of Connection Mode and Hydraulic Retention Time on Wastewater Pollutants Removal in Constructed Wetland Microcosms

A series of multi-stage connected constructed wetland (CW) microcosms were studied at different hydraulic retention times (HRTs) to determine the effects of the wetland connection mode and HRT on wastewater pollutants removal efficiency. The connection modes included series (Ser), parallel (Par), Ser–Par, and Par–Ser, while HRTs of 1, 2, 3, 4, and 5 h were studied. By integrating three stages of vertical- and horizontal-flow CWs, the pollutant removal processes were separated into different steps. The results showed that the Ser–Par mode had the highest total nitrogen removal efficiency (45.2%) at an HRT of 3 h and the highest total phosphorus removal efficiency (74.6%) at HRT = 5 h. In terms of chemical oxygen demand (COD), the Par system had the highest removal efficiency (75.9%) at HRT = 4 h. The highest removal efficiency for NH4+-N (68.4%) was observed in the Ser system at HRT = 4 h. Overall, coupled systems at the 4 h HRT showed optimal removal efficiency where 75.9% of COD and 68.4% of NH4+-N were removed, respectively. Our results suggest multi-stage coupled CW with appropriate HRT can be a comprehensive and effective method for urban wastewater treatment.