Increasing chemical oxygen demand and nitrogen removal efficiencies of surface-flow constructed wetlands in macrophyte-dominant seasons by adding artificial macrophytes

[Display omitted] •Adding artificial macrophytes (AMs) significantly improved COD and N removal in SFCWs.•AMs did not significantly affect emergent macrophyte (EM) biomass or root activity.•AMs reduced the secondary pollutants released from decayed EMs.•EMs benefitted the AM-attached biofilm functions of chemoheterotrophy and the N cycle.•Adding AMs is a useful strategy for COD and N removal in SFCWs in cold seasons. The objective was to test the effectiveness of a new strategy, i.e., adding artificial macrophytes (AMs) to surface-flow constructed wetlands (SFCWs) planted with emergent macrophytes (EMs), to improve COD and N removal efficiencies (REs) at the pilot scale. During EM-dominant periods, the SFCWs with only EMs had REs of 78.2%, 59.8%, 50.8% and −54.2% for NH4+-N, NO3–-N, TN, and COD, respectively, while the SFCWs with both AMs and EMs increased the REs to 85.1%, 72.2%, 73.8%, and 2.0%. The addition of AMs to SFCWs had no significant effect on EM growth (biomass or root activity). Mutual benefit was found between EMs and AMs, i.e., AMs reduced the secondary pollution caused by withered EMs, while EMs improved AM-attached biofilm functions of chemoheterotrophy, nitrate_reduction, and nitrification. Therefore, AM addition is a useful strategy to improve COD and N REs during EM-dominant periods when pollutant removal is a challenge for SFCWs.