Effect of outlet water level raising and effluent recirculation on removal efficiency of pilot-scale, horizontal subsurface flow constructed wetlands

Two pilot-scale, horizontal subsurface flow (HSF) constructed wetlands (CWs) were constructed and operated continuously for 3 years, fed with synthetic wastewater. Both wetlands were of rectangular cross-section with dimensions 3.0 m in length and 0.75 in in width. After the first 2 years of operation (period A), two modifications were applied to the wetland units: (1) the outlet water level was raised in the first unit and (2) 50% effluent recirculation was applied to the second unit. Under these two modifications, the two CWs operated for one more year (period B). Water samples were collected every week from the influent, effluent and sampling pipes placed at one-third and two-thirds distance along each unit. The water samples were analysed immediately in the laboratory for BOD5, COD, Total Kjedlahl Nitrogen (TKN), ammonia nitrogen, nitrite, nitrate, total phosphorus (TP) and orthophosphates. The results show that outlet water level raising did not change significantly the performance of the wetland unit, but under this configuration the wetland managed to treat about 15% increased hydraulic and pollutant loads using the same surface area. On the other hand, effluent recirculation negatively affected wetland performance, resulting in a reduction of all pollutant removal rates.