Effect of Filter Media and Depth on Hydrodynamics and Treatment Performances of Single-Stage French Vertical Flow Treatment Wetlands Treating Domestic Effluent

International audience

anglais. Treatment wetlands for raw wastewater from small communities (< 2000 p.e.) represent 20% of the wastewater treatment plants in France today. The classical French Vertical Flow Treatment Wetland consists of two stages filter beds intermittently fed with raw wastewater (inflow concentrations TSS 387 +/- 70 mg/L, COD 880 +/- 188 mg/L, TKN 110 +/- 16 mg/L). The system has a good removal performance for TSS and COD with > 90% and TKN of approx. 85% after filter maturation (months 12 to 24 of the trial), but efficiencies can vary between 10 and approx. 20% according to the material type, filter age, and filter depth. This study presents how these systems can be costly optimized when outlet requirements are not too stringent or when carbon is required for a subsequent denitrification step. The study shows the effectiveness of using different gravel depths and types (pea and crushed gravel) over almost 2 years regarding removal performances, carbon source availability, and nitrification. Core samples were taken to evaluate the dry matter accumulation. Tracer tests using fluorescein were performed to assess the internal hydrodynamics related to filter depth. The removal performance was not significantly affected by the filter depth, but the filter filled with 30 cm of pea gravel performed significantly better than the one with 30 cm of crushed gravel: for COD with a removal of 83% compared to 76%, TSS of 87% to 81%, and TKN of 64% to 57%, respectively. This is another indication that the shape of crushed gravel affects biofilm attachment and hydrodynamics in the filters due to irregular compaction.