This work aimed at studying the influence of sand layer depth (10, 20 and 40 cm) and percolate impounding regime (batch and permanent impounding) on nitrogen transformation in VFCW treating FS. Based on the results obtained, the following conclusions can be formulated:
- Overall TN mass removal increases with sand layer depth in faecal sludge-fed VFCW. However, TN removal efficiency varies from 87 to 92% and is not significantly influenced by bed configuration nor impounding regime.
- VFCWs, fed once a week with faecal sludge at SLR 250 kgTS/ (m2.year), reach higher nitrate loads in the percolate, i.e. 3–
5 g N/(m2.day) within the first 1–2 days of impounding, irre-
spective of bed configuration or impounding regime. After three days of percolate impounding, the nitrate load in the percolate decreases steadily to reach its initial value. Except for batch-operated beds, all the NOx–N produced in the VFCW beds was denitrified.
- On average, 4.8% of initial TN was lost in batch-impounding beds, and 7.8–12.8% in permanent impounding systems. An increase in sand layer depth in VFCW enhanced N nitrifi- cation and denitrification.
- Impounding periods of six days did not lead to an improved N removal or recovery. Nitrification appears to be the gov- erning factor of N loss. However, nitrification rates in filter media remain unknown.
- Further investigations on NH4 adsorption and nitrification capacity of filter material are necessary to improve bed configuration and operating conditions for enhanced nitrogen recovery or removal.