Toxicity of Chromium on Nitrogen and Biodegradable Compound Removal in Vertical Flow Constructed Wetlands

The objective of this study is to investigate the removal efficiencies of Chromium (Cr), Nitrogen and biodegradable organic compounds, as well as its interference of each other in vertical flow constructed wetlands with different bed media. Four laboratory-scale vertical flow constructed wetland (CW) units, each with dimensions of 750 x 635 m (diameter x height), were installed with different bed media: CWc2 with 200 100, lOOand 150 mm of charcoal, sand, small gravel and large gravel; CWci with 100, 200,100 and 150 mm of charcoal, sand, small gravel and large gravel; CWco and CWControl with 200, 200 and 150mm of sand, small gravel and large gravel. Typha Angustifolia (Cattails) were planted at initial density of 50 shoots./m2 in CWci, CWc2 and CWco units and fed with 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 3 days throughout 115 days of operation for analysis of total Cr, nitrogen, pH. In addition, N accumulated in the plant biomass (roots and shoots) were analysed to identify the Cr toxicity for plant uptake of Nitrogen. For influent concentrations up to 30 mg/L, it was found that Cr mass removal efficiencies for all CW units were 99-100 %, resulting effluent concentrations below the industrial effluent standard which is 2 mg/L. Beyond 30 mg/L influent Cr concentrations, it was apparent that the CW unit with thick charcoal layer met the industrial effluent standard, even at an influent Cr concentration of 100 mg/L. Experimental data revealed that the 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.