Influence of sand layer depth and percolate impounding regime on nitrogen transformation in vertical-flow constructed wetlands treating faecal sludge

Four laboratory-scale units of vertical-flow constructed wetlands (VFCW) were fed once a week with faecal sludge (FS) at a constant solids loading rate (SLR) of 250 kg TS/(m2.year) (equivalent to 260–300 g N/(m2.week)) for a period of 12 weeks to study: i) the nitrification and denitrification potential of the sand layer of VFCWs and ii) the effect of percolate impounding regime (permanent or batch-impounding) on nitrogen transformation. The TN content of raw FS was characterised by 65% org-N, 34% NH4–N and 1% NOx–N. After FS application and a six-day impounding period, 8–13% TN were recovered in the percolate exhibiting the following composition: 70–80% NH4–N, 25–30% org-N and <1% NOx–N. A large fraction of the influent organic N (55%) was filtered in the bed and 24–29% of initial NH4–N were lost due to nitrification and volatilisation. In permanent impounding systems, 8–11% TN were recovered in the percolate versus 13% in batch-operated beds. N loss was increased with sand layer depth (20–40 cm) under permanent impounding regimes.