Subsurface-horizontal flow constructed wetland for sewage treatment under Moroccan climate conditions

Performance and behaviour of a subsurface-horizontal flow constructed wetlands (SSF-h CW) used for sewage post-treatment behind an upflow anaerobic reactor under Moroccan climate conditions are reported. The experimental setup included three parallel beds of 28m2, each receiving 9.5m3d−1 of pre-treated sewage corresponding to a hydraulic loading rate of 0.34m(m2 d)−1 and a COD loading rate of 0.15kg (m2 d)−1. Beds 1 and 2 were planted with Phragmites australis and Arundo donax, respectively, and bed 3 was kept unplanted (control). Average data reported here coincided with a plant age going from 12 to 18 months and covered an entire cold season and the early part of the hot season. Average water losses by evapotranspiration amounted 40 and 57mm d−1, respectively, for Arundo and Phragmites, while evaporation did not exceed 7mm −1 for the unplanted bed. Percentage of losses given as the ratio of lost flow/applied flow, amounted 11, 17 and 2%, respectively, for Arundo, Phragmites and the control.A potassium bromide (KBr) tracer study was performed on the three filters simultaneously. The actual hydraulic retention time (HRT) was found to be around 9h for the three filters (13h for the theoretical HRT, V/Q), indicating the occurrence of a dead zone volume of 30% in each filter, a result explained by the fact that the three filters had the same design and filling medium. Kinetic first order constant, K (20°C) for BOD5 removal was calculated using the tracer study results. K values were 1.384, 1.284 and 0.904d−1, respectively, for Arundo, Phragmites and for the control. Compared to the control, K value increased by 53 and 42% for Arundo and Phragmites, respectively, clearly showing the impact of planting the beds. In addition, some effluent surfacing occurred for the control but not for the planted beds.A satisfactory COD removal pattern was achieved. We found 70, 85 and 130mg L−1, respectively, for the effluents of Arundo, Phragmites and the control. However, no similar removal pattern was found for nitrogen and for phosphorus. In addition, faecal coliforms removal rate was small and did not exceed 1 Log Unit in the best case.