On-site single-stage constructed wetland fed by raw wastewater: performances and resilience of the system

On-site sanitation systems in Europe are evaluated through a CE marked procedure done on a platform test under a specific schedule of loads. Nevertheless, the test procedure conditions do not represent the real conditions of treatment systems in terms of wastewater characteristics and loads. On another angle, in France, systems implemented for capacities above 20 p.e. do not need the CE marked procedure but have to comply with performance requirements. French on-site treatment regulations lead to a paradoxical situation where constructed wetlands (CW) designed for 21 p.e. can be more compact than for 15 p.e. Here we focus on a single-stage vertical flow CW treating raw wastewater from a six-person house. Working with a (compact) community CW design, the objectives were to evaluate, in real-world conditions, the limits of the system and its ability to handle the high hydraulic and organic load variations found in on-site sanitation. Concentrations and fluxes showed high inter-day and intra-day variability, confirming the necessity for treatment systems to be robust enough for on-site sanitation. The compact CW appeared very efficient and stable for organic pollutants and nitrification (average removal rates of more than 98%, 99%, 94% and 97% for TSS, BOD₅, COD and TKN, respectively). Denitrification has been optimized to reach 70% of TN removal, but seems unable to go higher due to a lack of carbon.