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The current experimental work aimed at the investigation of the overall chromium removal capacity of constructed wetlands (CWs) and the chromium fate-distribution within a wetland environment. For this purpose, the experimental setup included the parallel operation and monitoring of two horizontal subsurface flow (HSF) pilot-scale CWs and two vertical flow (VF) pilot-scale CWs treating Cr-bearing wastewater. Samples were collected from the influent, the effluent, the substrate and the plants. Apart from the continuous experiment, batch experiments (kinetics and isotherm) were conducted in order to investigate the chromium adsorption capacity of the substrate material. According to the findings, HSF-CWs demonstrated higher removal capacities in comparison to VF-CWs, while in both types the planted units indicated better performance compared to the unplanted ones. Analysis in various wetland compartments and annual mass balance calculation highlighted the exceptional contribution of substrate to chromium retention, while Cr accumulation in plant was not so high. Finally, experimental data fitted better to the pseudo-second-order and Langmuir models regarding kinetics and isotherm simulation. [ABSTRACT FROM AUTHOR]

Abstract: The performance of natural zeolite is presented in providing additional treatment of the effluent of two pilot-scale, horizontal subsurface flow constructed wetlands. The two constructed wetlands operated initially for 2years without the zeolite-filters (period A). Then, the zeolite-filters (fine- and coarse-grained clinoptilolite) were added at the outflow of each wetland unit, and the two systems operated for an additional year (period B). During period B, the two constructed wetlands showed a comparable performance in organic matter, nitrogen and phosphorus removal to that in period A. The two zeolite-filters removed 60.6% and 63.2% of wetland effluent BOD5 and 52.5% and 62.0% of the wetland effluent COD, respectively. The removal rates of nitrogen pollutants were higher, reaching 75.1% and 83.2% for TKN and 78.3% and 85.8% for . Mean removals of influent ortho-phosphate in the two zeolite-filters were 56.4% and 39.2%, while mean removals of total phosphorus were 56.8% and 40.5%. The comparison of the two working periods (with and without the presence of the zeolite-filters) showed that the addition of the two zeolite-filters improved significantly the effluent quality of the two constructed wetlands, since nearly 95% of the organic matter, 80% of nitrogen and 70% of phosphorus from the wastewater applied to the systems was removed. The performance of the zeolite-filters appeared to be relatively unfluctuating throughout their entire operation period; nevertheless, higher removal rates were observed at higher temperatures. Furthermore, the fine-grained zeolite proved to be more effective in organic matter and nitrogen removal than the coarse-grained one. On the other hand, the coarse-grained zeolite retained phosphorus at a higher level. [Copyright &y& Elsevier]