Cr(VI) and COD removal from landfill leachate by polyculture constructed wetland at a pilot scale

Four subsurface horizontal-flow constructed wetlands (CWs) at a pilot scale planted with a polyculture of the tropical plants Gynerium sagittatum (Gs), Colocasia esculenta (Ce) and Heliconia psittacorum (He) were evaluated for 7 months. The CW cells with an area of 17.94 m2 and 0.60 m (h) each and 0.5 m of gravel were operated at continuous gravity flow (Q = 0.5 m3 day−1) and a theoretical HRT of 7 days each and treating landfill leachate for the removal of filtered chemical oxygen demand (CODf), BOD5, TKN, NH4 +, NO3 −, PO4 3−–P and Cr(VI). Three CWs were divided into three sections, and each section (5.98 m2) was seeded with 36 cuttings of each species (plant density of six cuttings per square metre). The other unit was planted randomly. The final distributions of plants in the bioreactors were as follows: CW I (He-Ce-Gs), CW II (randomly), CW III (Ce-Gs-He) and CW IV (Gs-He-Ce). The units received effluent from a high-rate anaerobic pond (BLAAT®). The results show a slightly alkaline and anoxic environment in the solid-liquid matrix (pH = 8.0; 0.5–2 mg L−1 dissolved oxygen (DO)). CODf removal was 67 %, BOD5 80 %, and TKN and NH4 + 50–57 %; NO3 − effluents were slightly higher than the influent, PO4 3−–P (38 %) and Cr(VI) between 50 and 58 %. CW IV gave the best performance, indicating that plant distribution may affect the removal capacity of the bioreactors. He and Gs were the plants exhibiting a translocation factor (TF) of Cr(VI) >1. The evaluated plants demonstrated their suitability for phytoremediation of landfill leachate, and all of them can be categorized as Cr(VI) accumulators. The CWs also showed that they could be a low-cost operation as a secondary system for treatment of intermediated landfill leachate (LL).