Can floating treatment wetlands improve existing lagoons nitrogen removal through intensifying biofilm activity?

Floating treatment wetlands (FTW) are an emerging Nature Based Solution that have exhibited promising nitrogen removal under different range of effluents pollutant concentrations. Despite the notable increase in the number of FTW studies in the last years, nitrogen (N) removal mechanisms within FTW have not been fully assessed. The present study aims to understand N removal in FTW and investigate their microbial biofilm activity through a pilot scale experiment for agri-food tertiary wastewater treatment. A conventional and a modified (with added cellular concrete (CC)) FTW were monitored over 7 months with respect to two control lagoons (conventional lagoons with or without CC hanging in the water column). Experimental results revealed that the best TN removal was achieved by the lagoon equipped with the modified FTW (20% mean removal increase compared to the conventional lagoon). Biofilm denitrification activity potential was up to 6.7–9.0 times higher within pilot lagoons equipped with FTW than that in control lagoon (LC without FTW), while nitrification activity potential was prominent in biofilms within the control pilot lagoons (exhibiting high dissolved oxygen (DO) concentration). Biofilms from plant roots exhibited the highest overall dissolved inorganic nitrogen treatment followed by the biofilms from sediments and CC material. Plant root biofilms exhibited both significant nitrification and denitrification activity potentials despite the overall low DO and COD concentrations within FTW pilot lagoons' water column. This suggests the existence of micro sites in the roots network which provide adequate aerobic conditions and access to organic carbon most probably through root exudates. Overall, N accumulation in the sediment was a minor removal mechanism for all pilot lagoons. Plant accumulation (accounting for one third of TN removal), nitrification and enhanced denitrification appeared to be the main removal mechanisms in pilot lagoons equipped with FTWs (with or without CC) while nitrification, algal assimilation and NH₄-N volatilization may hav-èe been the major processes driving TN removal in the control pilot lagoons. Hence, FTW could be an interesting retrofit of existing lagoons to promote nitrogen removal through denitrification and plant assimilation, especially in the case of receiving bodies highly sensitive to nitrate input. Further research should address optimizing FTW design to guarantee stable N removal under changing water temperature and mitigate seasonal variations as well as investigate biofilm species for in-depth understanding of N cycle within FTWs.