[Residence time distributions and spatial variation of N, P in the subsurface-flow constructed wetlands for purification of eutrophic aquaculture water].

Hydraulic residence time distributions (RTD) and spatial variations of N, P were studied in a small-scale horizontal subsurface-flow constructed wetlands (HSFCWs) planted with Cyperous alternifolius and Typha angustifolia respectively for purification of eutrophic aquaculture water. The results show that the residence time distribution curves of the investigated HSFCWs lie between plug-flow and completely mixed model with characteristic values (sigma2) of 0.3246 and 0.4108, respectively. Compared with Typha angustifolia, Cyperous alternifolius wetland shows fine flow pattern with characteristics of smoother RTD curve and weaker vertical mixed flow. Total nitrogen (TN) and ammonia nitrogen (NH4+-N) show stratified distributions in the two HSFCWs, especially in the front end of the wetland beds. TN in the lower layer is higher than that in the upper, while NH4+-N in the middle layer is the lowest in all the sampling layers. Total phosphorus (TP) and phosphate (PO4(3-)-P) increases with sampling depth. Differences in TP and PO4(3-)-P between the layers decrease gradually along distance. Cyperous alternifolius wetland shows better stratification distributions of N, P, as compared with Typha angustifolia, which is mainly contributed to the difference in flow patterns between the two HSFCWs. On average, concentrations of TN and TP in the rear end of the Cyperous alternifolius wetland are 19.3% and 12.5% lower, respectively, as compared to the Typha angustifolia wetland, suggesting that removal efficiencies of the Cyperous alternifolius wetland for purification of eutrophic aquaculture water is higher than those of the Typha angustifolia.