Effects of a saturated layer and recirculation on nitrogen treatment performances of a single stage Vertical Flow Constructed Wetland (VFCW)

Upgrades to enhance nitrogen removal were tested in a 2 year old pilot vertical flow constructed wetland in spring and summer periods. The effects of a saturated layer and of recirculation were tested in particular. Two pilots (L = 2 m, W = 1.25 m, H = 1.2 m), filled with expanded schist (Mayennite^®), were designed with hydraulic saturated layers of 20 and 40 cm at the bottom. Each pilot was fed with raw domestic wastewater under field conditions according to a hydraulic load of 15–38 cm d⁻¹ (i.e. 158–401 g COD (chemical oxygen demand) m⁻² d⁻¹) and to recirculation rates ranging from 0% up to 150%. The initial load during the first 2 years of operation resulted in an incomplete mineralized accumulated sludge leading to total suspended solids (TSS), COD and biochemical oxygen demand (BOD₅) release. A 40 cm hydraulic saturated layer enabled an increase of 5–10% total nitrogen (TN) removal compared to a 20 cm saturated layer. Recirculation allowed the dilution of raw wastewater and enhanced nitrification in a single stage. A design of 1.8 m² pe⁻¹ (48 cm d⁻¹, 191 g COD m⁻² d⁻¹) with a 40 cm saturated layer and 100% recirculation enabled the French standard D4 (35 mg TSS L⁻¹, 125 mg COD L⁻¹, 25 mg BOD₅ L⁻¹), nitrogen concentrations below 20 mg TKN (total Kjeldahl nitrogen) L⁻¹ and 50 mg TN L⁻¹, to be met.