Nitrogen dynamics in a constructed wetland system treating landfill leachate

A pilot scale treatment system was established in 2002 at the Laflèche Landfill in Eastern Ontario, Canada. The system consists of a series of treatment steps: a stabilisation basin (10,000 m³), a woodland peat trickling filter (5,200 m²), a subsurface flow constructed wetland planted in Phragmites sp. (2,600 m²), a surface flow constructed wetland planted in Typha sp. (3,600 m²) and a polishing pond (3,600 m²). The system operates from May to December with leachate being recycled within the landfill during the winter months.

Hydraulic loading was increased three-fold over four operating seasons with nitrogen and organic mass loading increasing six-fold. Excellent removal efficiencies were observed with 93% BOD₅, 90% TKN and 97% NH₄-N removed under the highest loading conditions. Almost complete denitrification was observed throughout the treatment system with NO₃-N concentrations never exceeding 5 mg·L⁻¹. The peat filter reached treatment capacity at a hydraulic loading of 4 cm·d⁻¹ and organic loading rate of 42 kg BOD·ha⁻¹·d⁻¹, which is consistent with design criteria for vertical flow wetland systems and intermittent sand filters.

The first order plug flow kinetic model was effective at describing TKN and ammonium removal in the SSF and FWS wetlands when background concentrations were taken into account. Ammonium removal k-values were consistent with the literature at 52.6 and 57.7 yr⁻¹ for the SSF and FWS wetlands, respectively, while TKN k-values at 6.9 and 7.7 yr⁻¹ were almost an order of magnitude lower than literature values, suggesting that leachate TKN could contain refractory organics not found in domestic wastewater.