Performance of carbon and nitrogen removal in a system combining an aerobic trickling filter followed by two stages of vertical flow treatment wetland

Treatment wetland has become a reference in wastewater treatment, particularly for the treatment of domestic or agricultural effluent in rural areas. The AZOE process developed by the company SCIRPE is an optimised system of water treatment based on a conventional two-stage treatment wetlands with vertical hydraulic flow. A trickling filter has been added at the head of the system and filtration stages have been partially saturated to promote anoxic conditions. This study presents the monitoring of a complete pilot-scale AZOE system receiving real and continuous effluents. Effluents from two periods of the year are studied: autumn and spring when the organic load is higher. The performance of each treatment unit, as well as the total treatment unit is presented in this study. The main results show that i) the trickling filter consumes about half of the carbon load whatever the season but that nitrification is lower when the incoming organic load is higher ii) the first stage of filtration contributes a lot to denitrification thanks to the anoxic zones iii) the contribution of the second stage is lower but can increase in case of higher incoming load highlighting a safety role of the treatment. The continuous ammonium and nitrate data at the outlet of the first and second stages show a very characteristic dynamic of this system during the feeding period: a well-defined nitrate peak at the outlet of the first stage which is found on the second stage. The lack of carbon is pointed out as the most limiting factor to denitrification on the second stage. • TF performance variability strongly influences first stage nitrogen removal • First stage achieves most of the denitrification • A peak of nitrates concentration is observed at first stage outlet shortly after feeding • Second stage guarantees high COD and TKN removal rates • Organic carbon availability in second stage is the main limitation to denitrification