Comparison of two hybrid constructed wetland configurations for dairy wastewater treatment

The aim was to evaluate the performance of two hybrid constructed wetlands (HCW) for the treatment of a dairy wastewater at pilot-scale. The HCW1 was composed of vertical flow (VF) and horizontal subsurface flow (HSSF) wetland units while the HCW2 of VF and free water surface (FWS) wetland units. Wastewater, plants, and sediment samples were collected to evaluate the removal efficiency and nutrient accumulation in plant tissues and sediment. Kinetics were evaluated using the volumetric removal rate (k). COD and BOD decrease was significantly higher in HCW1 than in HCW2, due to chemical oxidation prevails in the VF stages and biological processes in the second stage of HCW1, leading to organic matter removal. Total nitrogen (TN) removal was comparable in the two HCW configurations. Nitrogen compounds interconversions were favored in HCW1 due to HSSF wetland conditions and higher plant biomass values in VF stages than in HCW2. Phosphorus removal was favored in FWS. k values showed that HCW1 was kinetically favored. The main strengths of the HSSF wetland were Org-N and NO₃^--N removals, but the weakness was NH₄⁺-N removal under limited nitrification conditions. The HCW1 was efficient for nitrogen interconversions, leading to higher removal efficiencies than HCW2.