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 28 m2, each receiving 9.5 m3 d−1 of pre-treated sewage corresponding to a hydraulic loading rate of 0.34 m(m2 d)−1 and a COD loading rate of 0.15 kg (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 57 mm d−1, respectively, for Arundo and Phragmites, while evaporation did not exceed 7 mm d−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 9 h for the three filters (13 h 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 0.904 d−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 130 mg 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.

The paper shows that SSF-h CW could be adopted as a post-treatment unit for effluent pre-treated (or primarily) in anaerobic reactors. Regarding the performance achieved, the SSF-h CW was able to reduce further the level of the main physicochemical pollution parameters. The faecal coliforms content was also downed to acceptable levels for restricted reuse in irrigation i.e. category B following WHO guidelines [11]. Based on the results, the answer to the famous question: “to plant or not to plant a gravel bed?” is definitely yes. The plants do play an important role in the treatment. Two drawbacks however for the SSF-h CW system: the water loss by evapotranspiration, which reached 11 and 17% of the applied flow rates, respectively, for Arundo donax and Phragmites australis, and the very limited effect on phosphorus and on nitrogen removal probably because of a low oxygen availability for nitrogen oxidation in this system.