| Abstract | Among the conventional treatment processes, the constructed wetlands (CW) are found to be efficient in treating industrial wastewater containing chromium (Cr) due to various treatment mechanisms including biological, physical and chemical interactions of the soilplant-microorganisms. At the relatively high pollutant loading, the vertical flow CW system apparently shows the promising treatment efficiencies. The aim of this study was to determine the effects of bed media on Cr removal efficiencies in the vertical flow CW units as well as to observe the Cr toxicity on nitrogen (N) and COD removals. Four laboratory-scale vertical flow CW units, each with dimensions of 750 x 635 m (diameter x height), were installed with different bed media: CWc2 with 200 100, 100 and 150 mm of charcoal, sand, small gravel and large gravel; CWc1 with 100, 200, 100 and 150 mm of charcoal sand, small gravel and large gravel; CWco and CWcontol with 200, 200 and 150 mm of sand, small gravel and large gravel. Typha Angustifolia (Cattails) were planted at initial density of 50 no./m2 in CWc1, CWc2 and CWco units. The experimental units were fed with AIT domestic wastewater spiked with Cr at the concentrations ranging from 10 to 100 mg/L at the Hydraulic Retention Time (HRT) of 4 days. Samples from CW units were taken once in three days throughout 115 days of operation for analysis of COD, total Cr, nitrogen, Oxidation reduction potential, sulfate reduction and pH. In addition, Cr and N accumulated in the bed media and plant biomass (roots and shoots) were analyzed to determine mass balance in the CW units. Experimental data revealed that the CW units could obtain mass Cr removal efficiencies of 99 - 100%, resulting which the effluent concentrations below the industrial effluent standards of 2 mg/L (Bosnic et al.,2000) even feeding at Cr concentration as high as 30 mg/L in the influent. Beyond the influent Cr concentration of 30 mg/L, it is apparent that only the effluent of CW unit with thick charcoal layer could meet the industrial effluent standard, even at the influent Cr concentration of 100 mg/L. Based on the Cr mass balance and statistical analysis, it could reveal that bed media (both sand and charcoal) was predominant in removing Cr (86% form the total Cr input). It could also illustrate that the Cr plant uptake of 95 % was accumulated in the plant root systems. In addition, it was found that CW units with charcoal layers could result in COD and N removals of 82 - 92 and 57 - 87%, respectively. Possibly due to the inhibitory effect of Cr on N plant uptake, the N removal in CW units were decreased rapidly with increasing Cr loading. On the contrary, the increase in Cr loading did not affect the COD removal efficiencies in CW units. Based on the 115-day experimental results from CW units feeding with high Cr concentrations, it can be concluded that CW unit with thick charcoal layer can enhance the Cr removal and minimizing the effects on N removals. |