Potential and limits of landfill leachate treatment in a multi-stage subsurface flow constructed wetland - Evaluation of organics and nitrogen removal.

Constructed wetlands have potential of treating wastewater of poor biodegradability. The performance of a multistage sub-surface flow wetland treating municipal landfill leachate was assessed during three years of operation. During the study three research periods with different operation conditions were established. The hydraulic loads, operation mode of vertical flow beds and type of treated wastewater (raw leachate - leachate mixed with municipal wastewater) were changed. Removal of organic matter and nitrogen species was evaluated in each period. The average COD removal efficiency varied from 47.8% to 86.6%. The average total nitrogen removal efficiencies were 98.5%, 68.9% and 79.6% in subsequent research periods. The main problem was too high concentration of recalcitrant organic matter. The labile organic matter was completely removed however the effluent COD remained on relatively high level. Depletion of labile organic matter also limited denitrification resulting in incomplete total nitrogen removal.