Transfers and transformations of zinc in constructed wetlands: Mitigation of a refinery effluent

Two pilot-scale wetlands were constructed to facilitate transfers and transformations of Zn in a secondary refinery wastewater effluent. The wetlands (6.1 x 30.5 m, width x length) were planted with Scirpus californicus and were operated with 24-h nominal hydraulic retention times (HRT). To evaluate wetland performance in terms of Zn removal at two water depths, one wetland was amended with a nominal concentration of 4.0 mg Zn/l as ZnCl2 for 144 days at an operational water depth of 0.3 m, and for an additional 22 days at a water depth of 1 m. The second wetland served as an unamended control. From wetland inflow to outflow, approximately 38% of total recoverable and 65% of soluble Zn was removed during the experiment at the 0.3-m water depth. During the flooded period (1.0-m water depth), approximately 18% of total recoverable and 66% of soluble Zn was removed from the effluent; Toxicity of effluent to Ceriodaphnia dubia Richard and Pimephales promelas Rafinesque decreased from inflow to outflow by similar to 54 and 73%, respectively, at the 0.3-m water depth, and by at least 100% at the 1.0-m water depth. These data illustrate successful construction of wetlands for transfers and transformations of Zn from the water column and for decreases in associated toxicity. (C) 2000 Elsevier Science B.V. All rights reserved.