Fate of phosphorus in the surface sludge deposit of a vertical flow constructed wetland

International audiencePhosphorus is a critical environmental issue due to the risks of eutrophication associated to its release into surface water and the depletion of natural resources. For domestic wastewater treatment Vertical-flow constructed wetlands (VFCW) have proven their good performances in nitrification and organic matter removal from domestic wastewaters of small communities. However, total nitrogen (TN) removal and phosphorus (P) retention are still a matter of research and development. A particularity of these VFCW is the formation of a surface deposit usually called sludge by accumulation of the suspended solids at the surface of the 1st stage. The layer’s thickness grows at a rate of about 2 cm/year. The sludge thus accumulated is planned to be removed by dredging every 10 to 15 years. Her implication in the performance of the system was poorly known and has to be studied. In addition, phosphorus speciation in this sludge is also a key factor in the availability of phosphorus to crops in the perspective of using the sludge as a soil conditioner in agriculture.This study concerns the AZOE® process patented by SCIRPE company in which the treatment line combines (i) a biological aerobic trickling filter, (ii) ferric chloride (FeCl3) addition for dissolved phosphate removal and (iii) two stages of partially flooded VFCWs. Through an experimental approach conducted at different scales this study was interested to (i) the characterization of the sludge composition using several complementary, with regard on the effect of aging on the evolution of phosphorus speciation in the sludge, (ii) the influence of extreme pH and redox condition on the release of elements from sludge.Results showed a mineralization of the surface sludge deposit (Kim, 2013). Phosphorus appears to be mainly retain with mineral part of the sludge with an effect of aging on the speciation (Kim, 2015a). Moreover, this sludge has a relatively high buffering capacity when submitted to extreme conditions of pH and redox (Kim, 2015b)