Water and mass budgets of a vertical-flow constructed wetland used for wastewater treatment

To estimate the nutrient and organic matter (Biological Oxygen Demand (BODs) and Chemical Oxygen Demand (COD)) removal capacity of a constructed vertical-flow wetland in The Netherlands, a water and nutrient budget study was conducted. Also bacterial water quality enhancement was measured. The system had a Phragmites australis vegetation and comprised four parallel compartments of 0.25 ha each, which were loaded sequentially with sewage from recreational facilities. Annual loading rates were moderate and were estimated to be 16700 kg COD ha^-1; 6700 kg BOD₅ ha^-1; 2400 kg N ha^-1; and 335 kg P ha^-1. The removal efficiencies for COD (81%) and BODs (96%) were high. Almost all Escherichia coli and F-specific RNA bacteriophages (> 99%) were removed from the wastewater during transport. The removal efficiencies for nitrogen (30%) and phosphorus (24%) were much lower. Nutrient removal was the result of plant uptake and harvesting (15% of total N input, 10% of total P input), denitrification (8% of total N input), sedimentation and accumulation of organic matter in the soil (7% of N total input, 14% of total P input). Removal efficiencies for N and P could be increased by harvesting the Phragmites vegetation in October, rather than the current harvesting practice in December. This vertical-flow wetland appeared to be P-saturated after 15 years of operation. The use of sandy sediments with better P-adsorbing properties is advocated as a critical issue for the design of these systems. Further, the groundwater eutrophication resulting from the poor functioning of the drainage systems has shown that a good hydraulic separation of vertical-flow systems from the subsurface is an important prerequisite for their proper functioning.